Technical Field

The invention relates to a device, method, and system for wirelessly broadcasting information pertaining to an aircraft for traffic awareness and collision avoidance.

Background Art

Every year, a substantial number of VFR (visual flight rules) aircraft are involved in mid-air collisions. Unfortunately, half of these incidents are fatal. Surprisingly, most of these accidents happen in good visibility and during daylight conditions.

Accident investigations have shown that the VFR-principle of “see and avoid” is insufficient, as it is often nearly impossible to see the other aircraft. The human visual system is not well suited for objects on a collision course, because these are on a fixed vector from the aircraft, i.e. the image of a foreign aircraft does not “move” on the pilot’s retina. Other biological, physiological, and psychological effects (“human factors”) as well as limited cockpit visibility in a typical General Aviation aircraft further decrease the chance of seeing the other aircraft early enough to avoid a mid-air collision.

Traditional FLARM as well as newer PowerFLARM devices (see, m/wp-content/uploads/man/FLARM-General-EN.pdf as accessed on 2021-02-05) work by calculating and broadcasting their own future flight paths to nearby aircraft together with a unique identifier. At the same time, they receive the future flight paths from surrounding aircraft. All these values are stored in broadcasted data packets. Then, an intelligent motion prediction algorithm calculates a collision risk for each aircraft. When a collision probability exceeds a threshold, the pilots are alerted with the relative position of the other aircraft, enabling them to avoid the collision.

Besides issuing collision warnings, FLARM devices can also show nearby aircraft on an overview screen showing surrounding airspace. This helps pilots to detect the other aircraft and perform an evasive maneuver before a collision warning becomes necessary. VARGA MIHALY et al.: “ADS-B based real-time air traffic monitoring system”, 38 ^th International Conference on Telecommunications and Signal Processing (TSP), IEEE, 9 July 2015, pages 215-219 discloses an implementation of an Automatic Dependent Surveillance-Broadcast (ADS-B) based real time air traffic monitoring and tracking system.

US 2021/0035454 Al discloses a remote identification device and method for unmanned aircrafts.

US 2013/0036238 Al discloses methods and systems for compressing location data of a mobile radio for over-the-air transmission to a stationary receiver.

However, the amount of broadcasted information is restricted with these prior art devices.

Disclosure of the Invention

The problem to be solved by the present invention is therefore to at least in part overcome these shortcomings of prior art solutions.

This problem is solved by the devices and methods of the independent claims.

Accordingly, a broadcast device for wirelessly broadcasting information pertaining to a first aircraft comprises a positioning device (e.g. a GNSS receiver such as a GPS receiver, a GLONASS receiver, and/or a Galileo receiver) configured to determine position data PD1 which is indicative of a position Pl of the broadcast device or - if the broadcast device is mounted to, affixed at, or situated in or at the aircraft or a pilot onboard the aircraft - of the aircraft. The position data PD1 advantageously comprises latitude data PD1 LAT indicative of a latitude, longitude data PD1 LON indicative of a longitude and advantageously altitude data PD1 ALT indicative of an altitude of the broadcast device. The position data PD1 can optionally at least in part be determined and/or enhanced from on-board navigational systems of the aircraft such as a barometric pressure sensor, a magnetic sensor, an acceleration sensor, an inertial navigation system, etc., e.g. to increase the altitude precision which can be rather poor for typical GNSS devices without correction. In such a case, the combined GNSS receiver together with the additional sensor/system as well as any data fusion logics involved qualifies as “positioning device” according to the invention.

The broadcast device further comprises a control unit (such as a microcontroller with a memory) which is configured to receive the position data PD1 from the positioning device, preferably via an internal bus such as a serial or an I2C bus, and advantageously store it in the memory (typically as short-term storage in a volatile memory such as RAM). Thus, the control unit can further process the position data PD1, e.g. use it for determining truncated position data PDF which is later added to a to-be-broadcasted data packet (see below).

The position data PD1 and/or the truncated position data PDF and/or the pair cl is advantageously encoded as an integral data type (e.g. with a value 5312) indicative of a multiple of a specific fraction of a degree (e.g. 1E-7 ^O ), e.g., received from the positioning device (e.g. 5312 * 1E-7 ^O ). Thus, computation is simplified.

In addition, the control unit is configured to (advantageously repeatedly for each data packet, see below) determine the truncated position data PDF (i.e. position data with discarded information compared to the original untruncated position data) using at least a part of the received position data PD 1. A bit width S F of the truncated position data PDF is thereby smaller than a bit width SI of the position data PD1. As an example, the truncated position data PDF has a bit width (size) of 20 bits for latitude and longitude each (e.g. after a reduction of resolution and range, see below) while the original untruncated position data PD1 has a bit width (size) of 32 bits for each dimension. Thus, bandwidth usage can be reduced when the truncated position data PDF is later broadcasted (see below). This enables the broadcasting of more information and/or at a higher update rate.

According to the invention, the control unit is further configured to generate the data packet DI based on the truncated position data PDF and based on an identifier ID1 of the broadcast device. The term “based on” is to be understood in such a way that the mentioned information or values indicative thereof are comprised in the data packet D 1.

Note that further information can optionally be determined by the positioning device and/ or generated by the control unit, e.g. ground speed, course/track, climb rate, acceleration, turn rate, movement mode, horizontal position accuracy, vertical position accuracy, velocity accuracy, a future flight trajectory, e.g. as computed from the current position Pl and velocity and/ or acceleration and/ or wind vectors, etc. This further information or values indicative thereof can then be added to the data packet DI in truncated or untruncated form which improves the calculation of collision probabilities and/or situational awareness.

According to the invention, the broadcast device further comprises a radio transmitter which is configured to receive the generated data packet DI from the control unit, e.g. via an internal bus such as an I2C or a serial bus. The data packet DI is indicative of the to-be-broadcasted information pertaining to the first aircraft (at least when the broadcast device is mounted to, affixed at, or situated in or at the aircraft or a pilot onboard the aircraft) and it is wirelessly broadcasted by the radio transmitter, e.g. to ground based receiver stations and/ or to adjacent aircraft. Thus, a receiver of the data packet DI can reconstruct the position Pl and - using these values - e.g. calculate a collision probability and - based thereon - putatively issue a warning to the pilot. This enhances the safety of the aircraft(s) and/or overall situational awareness.

According to the invention, the broadcast device is further configured to generate the data packet DI in such a way that it comprises a pair cl = (el, ml) with an exponent el being a natural number and with a mantissa ml being a natural number. This pair or code point cl is indicative of a value vl (e.g. including a rounding of the value vl), which can be a floating point or a natural number. The mantissa ml has a bit width of N _m j (e.g. 7) and the exponent el has a bit width of N _e j (e.g. 2). Then, vl = 2 ^e ^ * (2Nml + _m i) > 2^ml According to the invention, the bit widths N _m j and N _e j are selected such that a total bit width Nl=N _e i+N _mi of the pair cl is smaller than a total bit width of the value vl. A linear scaling factor Al representing the physical unit/ resolution for the encoded numerical value vl can also be used, see chapter 2.1 for the AMP protocol description below for details. Thus, bandwidth is saved while a wide range of values vl can be encoded.

Advantageously, the broadcast device is configured to generate the data packet DI in such a way that the pair cl is indicative of velocity data VD1 of the first aircraft. In particular the value vl is indicative of a velocity vector magnitude of the first aircraft (i.e. an absolute value of the aircraft’s velocity). Thus, bandwidth is saved while a wide range of velocity vector magnitudes (e.g., ranging from a hobbyist UAV to a military jetplane) can be encoded.

In an advantageous embodiment, the broadcast device is configured to determine the truncated position data PDF using

- the latitude data PD1 LAT such that the truncated position data PDF comprises truncated latitude data PDF LAT and/or

- the longitude data PD1 LON such that the truncated position data PDF comprises truncated longitude data PDF LON.

Both the truncated latitude data PDF LAT and the truncated longitude data PDF LON are smaller in their bit widths than their original untruncated counterparts PD1 LAT and PD1 LON, respectively. Thus, size reduction can be performed independently from each other for the latitude data and the longitude data.

In particular, the broadcast device is configured to determine the truncated position data PDF such that the truncated position data comprises the altitude data PD1 ALT or a value indicative thereof (e.g. with an offset), advantageously in an untruncated form.

In another advantageous embodiment, the broadcast device is configured to, for determining the truncated position data PDF, reduce a resolution (i.e. the smallest encodable values/ value differences become coarser) of at least a part of the original untruncated position data PD1, in particular of the latitude data PD1 LAT and/or of the longitude data PD1 LON. This helps to save bandwidth. The resolution reduction is advantageously performed by truncating, e.g. the original latitude data PD1 LAT and/or the original longitude data PD1 LON in a binary representation by a first number of trailing bits (i.e. least significant bits). As an example, the original untruncated latitude/longitude data is each stored as a signed 32-bit integer, from which 6 trailing bits are truncated each, thereby effectively reducing the resolution. Thus, a computationally efficient resolution reduction is achieved.

Then, advantageously, the broadcast device is configured to, prior to or together with reducing the resolution as described above, round the original untruncated latitude/longitude data. As an example, the value of the most significant to-be-truncated bit (i.e. 1/2 of the value of the least significant not-to-be-truncated bit) can be added to the original value before truncation. Thus, it is ensured that a proper mapping results between untruncated and truncated position data.

In another advantageous embodiment, the broadcast device is configured to, for determining the truncated position data PDF, reduce an encodable value range for at least a part of the original untruncated position data PD1, in particular for the latitude data PD1 LAT and/or for the longitude data PD1 LON. Thus, the truncated latitude data PDF LAT and the truncated longitude data PDF LON are obtained. This helps to save bandwidth. The encodable value range reduction is advantageously performed by truncating, e.g. the original latitude data PD1 LAT and/or the original longitude data PD1 LON in a binary representation by a second number of leading bits (i.e. most significant bits). Due to how signed numbers are represented (two’s complement), this approach works independently of the sign. As an example, the original untruncated latitude/longitude data is each stored as a signed 32-bit integer, from which 6 leading bits are truncated each, thereby effectively reducing the range of encodable values (disregarding the scaling factor) from -2,147,483,648. . .2,147,483,647 (2 ³ ) to 0. . .67,108,863 (2 ²⁶ -l) for unsigned truncated latitude/longitude data. Thus, a computationally efficient range reduction is achieved.

It can be imagined that the described reduction in range corresponds to relating the truncated position data PDF, in particular the truncated latitude data PDl’ LAT and/or the truncated longitude data PDl’ LON to an origin of a local grid cell around the sender/receiver and transmitting only a relative position to the origin of said grid cell. As an illustrative example (disregarding the 1E-7 ^O scaling), the untruncated position 47.37647448696338 N, 8.559292331307462 E (https://w3w.co/spiele.bemerkte.handlung) could be broadcasted as 37647448696338 N, 559292331307462 E, thus truncating or neglecting the integral parts of the coordinates above (47° N/ 8° E) and only broadcasting the fractional part. Such an approach introduces ambiguity, however, because the position could be in the city of Zurich (for the true 47° N/ 8° E), in Altstatten SG (for 47° N/ 9° E), in Schwetzingen near Heidelberg (for 49° N/ 8° E), and so on. However, when a receiver knows that the sender must be located close to the city of Zurich (e.g. because it cannot receive radio signals from Altstatten SG, from Schwetzingen, etc.), this ambiguity can later be resolved. The size of such imaginary grid cells is therefore advantageously set to be larger than the maximum possible radio range of the broadcast device. Then, during later position reconstruction, e.g. the original latitude data PD1 LAT and/or the original longitude data PD1 LON can be unambiguously reconstructed by using the principle of locality (see below).

Then, the broadcast device is advantageously configured to set the encodable value range such that a maximum encodable longitudinal separation (i.e. a longitudinal extent of the grid cell represented in km) and a maximum encodable latitudinal separation (i.e. a latitudinal extent of the grid cell represented in km) are both larger than a radio range of the (radio transmitter of the) broadcast device, in particular by a factor of 2 or more. These “longitudinal extents” and “latitudinal extents” of the grid cells can be set by selecting the number of leading bits to be truncated as explained above. By setting these encodable value ranges as explained, the introduced ambiguity can be later removed due to the fact that the radio signals must necessarily be local and that the receiving broadcast device knows its own unambiguous position by means of its own positioning device. Advantageously, then, the broadcast device is configured to set the encodable longitudinal separation depending on the latitude data PD1 LAT of the position data PD1. In particular, with an increasing abs(PDl LAT), a decreasing number of leading bits is truncated from PD1 LON. Thus, it can be ensured that the “longitudinal extents” of the grid cells always stay above the radio range of the radio transmitter, even when the broadcast device approaches the North or South Pole.

Advantageously, both operations (i.e. trailing bit truncation for resolution reduction and leading bit truncation for value-range reduction) are performed at the same time. Thus, even more bandwidth is saved. Then, it is advantageous that a constant total number of bits is truncated, but the block of untruncated bits contributing to the truncated longitude data PDl’ LON is gradually shifted to the left as the latitude data PDI LAT increases, i.e. as the broadcast device approaches the poles. In other words, the longitude grid parameters are adapted to the meridians as they converge towards the poles, i.e. with increasing latitude.

In another preferred embodiment of the invention, the broadcast device further comprises a radio receiver (or a combined radio transceiver for broadcasting and receiving data packets) which is configured to receive a foreign data packet D2 as broadcasted from a foreign broadcast device. The foreign data packet D2 is, similarly to the first data packet DI as discussed above, indicative of information pertaining to a second aircraft, at least when mounted to, affixed at, or situated in or at the second aircraft or a pilot onboard the second aircraft. Specifically, the foreign data packet D2 comprises foreign truncated position data PD2’, in particular foreign truncated latitude data PD2’_LAT and/or foreign truncated longitude data PD2’_LON. Furthermore, the foreign data packet D2 comprises a pair c2 = (e2, m2) with an exponent e2 being a natural number and with a mantissa m2 being a natural number, wherein the pair c2 is indicative of a value v2, wherein the mantissa m2 has a bit width of N _m 2 and wherein the exponent e2 has a bit width of N _e 2, wherein v2 = 2 ^e2 and wherein the bit widths N _m 2 and N _e 2 are selected such that a total bit width N2=N _e 2+N _m 2 of the pair c2 is smaller than a total bit width of the value v2. A linear scaling factor A2 representing the physical unit/ resolution for the encoded numerical value v2 can also be used, see chapter 2. 1 for the AMP protocol description below for details. Then, the broadcast device is configured to compute (decode) the value v2 using the pair c2 as received in the data packet D2. Thus, bandwidth is saved while a wide range of values v2 can be transferred.

Furthermore, advantageously, the broadcast device is configured to calculate a collision probability between the first aircraft and the second aircraft and/or provide information improving situational awareness, e.g. by taking the aircraft positions as comprised in the first and second data packets into account. In general, the situation is assessed based on the information pertaining to the first aircraft which is available to the broadcast device (own information) and based on the received information pertaining to the second aircraft (foreign information). Preferably, a collision warning is then issued to the pilot when the collision probability exceeds a certain threshold which helps to decreases the risk of a mid-air collision.

For this, advantageously, the broadcast device is configured to disambiguate the foreign truncated position data PD2’, in particular the foreign truncated latitude data PD2’_LAT and/or the foreign truncated longitude data PD2’ LON. This is achieved using its own position data PD1, in particular using its own latitude data PD1 LAT and its own longitude data PD1 LON. The term “disambiguate” relates to a removal of the ambiguities introduced by the “grid celling of the coordinate space”, i.e. the encodable value range reduction as discussed above. Specifically, the receiving broadcast device determines the original foreign position data PD2 without reduced value ranges (but with reduced resolution and rounding, if any) taking into account its own untruncated position. In the example above, the broadcast device would know that it can only receive data packets from the region around the city of Zurich and add 47° N/ 8° E to the received foreign truncated position data PD2’ taking into account the fact that both broadcast devices must be located in or near the city of Zurich due to radio range considerations. The foreign position candidate that results in the lowest distance between the sending and the receiving broadcast devices is considered to be correct. Thus, ambiguities in the foreign truncated position data PD2’ are resolved.

In addition, the signal strength (e.g. RSSI) and/or directional characteristics (e.g. from a receiver antenna array) of the received foreign data packet D2 can be taken into account for disambiguation.

Advantageously, the pair c2 as comprised in the foreign data packet D2 is indicative of foreign velocity data VD2 of the second aircraft. In particular the value v2 is indicative of a velocity vector magnitude of the second aircraft (i.e. an absolute value of the aircraft’s velocity). Then, the broadcast device is configured to compute (decode) the foreign velocity data VD2 using the received pair c2, in particular to compute the velocity vector magnitude v2 using the pair c2. Thus, bandwidth is saved while a wide range of values v2 (e.g., velocity vector magnitudes ranging from a hobbyist UAV to a military jetplane) can be transferred.

Preferably, the broadcast device is configured to generate the data packet DI in such a way that it comprises a header section and a payload section. In particular the header section is non-encrypted and/or the payload section is encrypted, e.g. by means of a symmetric or an asymmetric (e.g. public/private key) cryptographic algorithm. Thus, parts of the data packet DI can be received and read by anyone while other parts of the packet can only be read by authorized receivers. This enhances security.

Then, advantageously, the payload section of the data packet DI is encrypted by means of a symmetric cryptographic algorithm (e.g. AES with a key size of, e.g. 128 bits) and, in particular, the broadcast device is configured to use a cryptographic nonce based on the header section of the data packet DI, based on a time stamp, and based on a secret constant for encrypting the payload section of the data packet DI. Thus, security is further enhanced, because, e.g. the cryptographic nonce contains the changing time stamp and the variable data packet header which renders replay attacks not feasible.

In yet another preferred embodiment, the broadcast device is configured to generate the data packet D 1 in such a way that it comprises at least one of

- a timestamp, in particular in the (e.g. encrypted) payload section of the data packet,

- a packet protocol version, in particular in the (e.g. non-encrypted) header section of the data packet, and

- a maximum supported packet protocol version, in particular in the (e.g. non-encrypted) header section of the data packet.

This makes it possible to implement additional features, e.g. for enhancing protocol compatibility between different devices with putatively varying computational resources. In an advantageous embodiment of the invention, the broadcast device is configured to,

- repeatedly determine updated position data PD1 indicative of an updated position Pl of the broadcast device by means of the positioning device, in particular comprising updated longitude data PD1 LON and/or updated latitude data PD1 LAT,

- repeatedly determine updated truncated position data PDF using at least a part of the updated position data PD1, in particular the updated longitude data PD1 LON such that the updated truncated position data PDF comprises updated truncated longitude data PDF LON and/or updated latitude data PD1 LAT such that the updated truncated position data PDF comprises updated truncated latitude data PDF LAT, and

- repeatedly generate and broadcast an updated data packet DI based on the updated truncated position data PDF, in particular the updated truncated longitude data PDF LON and/or the updated truncated latitude data PDF LAT, and based on the identifier ID1 of the broadcast device.

In particular, any time interval between two of such consecutive updates (i.e. determining the updated position data PD1, truncating at least a part of it to determine the updated truncated position data PDF, and generating and broadcasting the updated data packet DI) is between 0. 1 s and 5 s, in particular is between 0.5 s and 1 s, and in particular is 1 s.

Advantageously, the broadcast device is configured to, at a predefined interval of, e.g. 30 s,

- repeatedly determine updated position data PD1 indicative of an updated position Pl of the broadcast device by means of the positioning device, in particular comprising updated longitude data PD1 LON and/or updated latitude data PD1 LAT,

- repeatedly generate and broadcast an updated data packet DI based on the updated position data PD1, in particular the updated longitude data PD1 LON and/or the updated latitude data PD1 LAT, and based on the identifier ID1 of the broadcast device.

This way, intermingled in the truncated position data packets as described above, untruncated position data can be broadcasted with a rather slow update rate, which helps, e.g. stationary receiving broadcast devices with putatively larger radio ranges to initialize a tracking mechanism which helps to disambiguate signals that are received from a distance exceeding the grid size. As another aspect of the invention, a receiver device comprises:

- a radio receiver configured to receive a foreign data packet D2 as broadcasted from a foreign broadcast device, in particular as discussed above with regard to the first aspect of the invention, wherein the foreign data packet D2 is indicative of information pertaining to a second aircraft.

The foreign data packet D2 comprises foreign truncated position data PD2’, in particular foreign truncated latitude data PD2’_LAT and foreign truncated longitude data PD2’_LON. A bit width S2’ of the foreign truncated position data PD2’ is smaller than a bit width S2 of foreign position data PD2 being indicative of an untruncated position P2 of the foreign broadcast device.

Further, the foreign data packet D2 comprises a pair c2 = (e2, m2) with an exponent e2 being a natural number and with a mantissa m2 being a natural number, wherein the pair c2 is indicative of a value v2, wherein the mantissa m2 has a bit width of N _m 2 and wherein the exponent e2 has a bit width of N _e 2, wherein v2 = 2 ^e2 and wherein the bit widths N _m 2 and N _e 2 are selected such that a total bit width N2=N _e 2+N _m 2 of the pair c2 is smaller than a total bit width of the value v2. A linear scaling factor A2 representing the physical unit/ resolution for the encoded numerical value v2 can also be used, see chapter 2. 1 for the AMP protocol description below for details.

The receiver device further comprises

- a control unit which is configured to:

* receive position data PD1 indicative of a position Pl of the receiver device. This position Pl can be fixed, e.g. for a stationary receiver station on the ground or it can be variable, e.g. for a receiving only device mounted in a car or a “receiving only” aircraft. In the first case, the position Pl can e.g. be hardcoded in firmware and read out/received by the control unit, in the second case, the position Pl can be determined by a positioning device such as a GNSS receiver of the receiver device and received by the control unit, similarly to the case discussed above with regard to the combined transmitting and receiving broadcast device.

Further, the control unit is configured to:

* receive the foreign data packet D2 as received by the radio receiver, * disambiguate the foreign truncated position data PD2’, in particular the foreign truncated latitude data PD2’_LAT and the foreign truncated longitude data PD2’_LON using its own position data PD1, and

* compute (decode) the value v2 using the pair c2 as received in the data packet D2. Thus, bandwidth is saved while a wide range of values v2 can be transferred.

Please note in this regard that all the technical effects and advantages as described above with the regard to the transmitting broadcast device similarly apply here for the receiving only device and are not repeated for reasons of clarity, as the devices complement each other and rely on the same inventive concept.

Advantageously, the pair c2 as comprised in the foreign data packet D2 is indicative of foreign velocity data VD2 of the second aircraft. In particular the value v2 is indicative of a velocity vector magnitude of the second aircraft (i.e. an absolute value of the aircraft’s velocity). Then, the receiver device is configured to compute (decode) the foreign velocity data VD2 using the received pair c2, in particular to compute the velocity vector magnitude v2 using the pair c2. Thus, bandwidth is saved while a wide range of values v2 (e.g., velocity vector magnitudes ranging from a hobbyist UAV to a military jetplane) can be transferred.

As yet another aspect of the invention, a method for, by means of a broadcast device, in particular as discussed above with regard to the first aspect of the invention, wirelessly broadcasting information pertaining to a first aircraft comprises steps of

- providing the broadcast device comprising a positioning device, a control unit, and a radio transmitter, advantageously mounted or mountable to, affixed or affixable at, or situated or situatable in or at the first aircraft or a pilot onboard the first aircraft, and

- by means of the positioning device (e.g. a GNSS receiver such as a GPS receiver, a GLONASS receiver, a Galileo receiver and/or a combined positioning device taking into account information from onboard navigational systems, see above) determining position data PD1 indicative of a position Pl of the broadcast device. The position data PD1 comprises latitude data PD1 LAT indicative of a latitude of the broadcast device and longitude data PD1 LON indicative of a longitude of the positioning device. Altitude data PD1 ALT is advantageously also comprised. The method comprises a further step of, by means of the control unit, receiving the position data PD1 as determined by the positioning device, preferably via an internal bus such as a serial or an I2C bus. Then, the position data is advantageously stored in the memory (typically as short-term storage in a volatile memory such as RAM). Subsequently, the control unit determines truncated position data PDF (i.e. position data with discarded information compared to the original untruncated position data), in particular truncated latitude data PDl’ LAT using the original untruncated latitude data PD1 LAT and truncated longitude data PD1 LON using the original untruncated longitude data PD1 LON. Thereby, a bit width SI’ of the truncated position data PDF is smaller than a bit width SI of the position data PD1. As an example, the truncated position data PDF has a bit width (size) of 20 bits for latitude and longitude each (e.g. after a reduction of resolution and range) while the original untruncated position data PD1 has a bit width (size) of 32 bits each. Thus, bandwidth usage can be reduced when the truncated position data PDF is later broadcasted (see below). This enables the broadcasting of more information and/or at a higher update rate.

According to the invention, the method comprises a further step of

- generating a data packet DI by means of the control unit based on the truncated position data PD1 and based on an identifier ID1 of the broadcast device. The term “based on” is to-be-understood in such a way that the mentioned information or values indicative thereof are comprised in the data packet DI .

Please note that further information can optionally be determined by the positioning device and/ or generated by the control unit, e.g. ground speed, course/track, climb rate, acceleration, turn rate, movement mode, horizontal position accuracy, vertical position accuracy, velocity accuracy, a future flight trajectory as computed from the current position Pl and velocity and/or acceleration and/ or wind vectors, etc. These further information or values indicative thereof can then be added to the data packet DI in truncated or untruncated form which improves the calculation of collision probabilities and/or situational awareness.

According to the invention, the method comprises a further step of

- by means of the radio transmitter receiving the generated data packet DI (e.g. via an internal bus) and wirelessly broadcasting the received data packet DI, e.g. to ground based receiver stations and/or to adjacent aircraft. The broadcasted data packet DI is indicative of the to-be-broadcasted information pertaining to the first aircraft. Thus, a receiver of the data packet DI can reconstruct the position Pl and - using these values - e.g. calculate a collision probability and - based thereon - putatively issue a warning to the pilot. This enhances the safety of the aircrafts) and/or overall situational awareness.

According to the invention, the method comprises a further step of generating the data packet DI in such a way that it comprises a pair cl = (el, ml) with an exponent el being a natural number and with a mantissa ml being a natural number. This pair or code point cl is indicative of a value vl (e.g. including a rounding of the value vl), which can be a floating point or a natural number. The mantissa ml has a bit width of N _m j (e.g. 7) and the exponent el has a bit width of N _e j (e.g. 2). Then, vl = 2 ^e ' * (2Nml + ml) - According to the invention, the bit widths N _m j and N _e j are selected such that a total bit width N I=N _e |+N _m | of the pair cl is smaller than a total bit width of the value vl. A linear scaling factor Al representing the physical unit/ resolution for the encoded numerical value vl can also be used, see chapter 2.1 for the AMP protocol description below for details. Thus, bandwidth is saved while a wide range of values vl can be encoded.

Advantageously, the data packet DI is generated in such a way (or in other words, the method comprises a step of generating the data packet DI in such a way) that the pair cl is indicative of velocity data VD1 of the first aircraft. In particular the value vl is indicative of a velocity vector magnitude of the first aircraft (i.e. an absolute value of the aircraft’s velocity). Thus, bandwidth is saved while a wide range of velocity vector magnitudes (e.g., ranging from a hobbyist UAV to a military jetplane) can be encoded.

In an advantageous embodiment, the method comprises a further step of

- for determining the truncated position data PDF, reducing a resolution (i.e. the smallest encodable values/ value differences become coarser) of at least a part of the original untruncated position data PD1, in particular of the latitude data PD1 LAT and/or of the longitude data PD1 LON. This is advantageously performed by truncating, e.g. the original latitude data PDI LAT and/or the original longitude data PD1 LON in a binary representation by a first number of trailing bits (i.e. least significant bits). As an example, the original untruncated latitude/longitude data is each stored as a signed 32-bit integer, from which 6 trailing bits are truncated each, thereby effectively reducing the resolution. Thus, a computationally efficient resolution reduction is achieved.

Advantageously, the method comprises a further step of - prior to reducing the resolution as described above, rounding the relevant part of the original untruncated position data, e.g. the latitude/longitude data. As an example, the value of the most significant to-be-truncated bit can be added to the original value before truncation. Thus, it is ensured that a proper mapping results between untruncated and truncated position data.

As an alternative or in addition to reducing the resolution, the method advantageously comprises a further step of

- for determining the truncated position data PDF, reducing an encodable value range for at least a part of the original untruncated position data PD1, in particular for the latitude data PD1 LAT and/or for the longitude data PD1 LON. Thus, the truncated latitude data PDl’ LAT and the truncated longitude data PDl’ LON are obtained. This helps to save bandwidth. The encodable value range reduction is advantageously performed by truncating, e.g. the original latitude data PD1 LAT and/or the original longitude data PD1 LON in a binary representation by a second number of leading bits (i.e. most significant bits). As an example, the original untruncated latitude/longitude data is each stored as a signed 32-bit integer, from which 6 leading bits are truncated each, thereby effectively reducing the range of encodable values. Thus, a computationally efficient range reduction is achieved.

Then, the method advantageously comprises a further step of

- setting the encodable value range such that a maximum encodable longitudinal separation and a maximum encodable latitudinal separation are both larger than a radio range of the broadcast device, in particular by a factor of 2 or more. The maximum encodable longitudinal/latitudinal separation can be set by selecting the number of leading bits to be truncated as explained above. By setting these values as explained, introduced ambiguities can be later removed due to the fact that the radio signals must be local and that a receiving broadcast device knows its own unambiguous position by means of its own positioning device.

Advantageously, then, the method comprises a further step of

- setting the maximum encodable longitudinal separation depending on the latitude data PD1 LAT of the position data PD1. In particular, with an increasing abs(PDl LAT), a decreasing number of leading bits is truncated from PD1 LON. Thus, it can be ensured that the “longitudinal extents” of the “grid cells” always stay above the radio range of the radio transmitter, even when the broadcast device approaches the poles. In another preferred embodiment of the invention, the broadcast device further comprises a radio receiver (or a combined radio transceiver for broadcasting and receiving data packets).

The method comprises a step of receiving, by means of the radio receiver, a foreign data packet D2 as broadcasted from a foreign broadcast device. The foreign data packet D2 is, similarly to the first data packet DI as discussed above, indicative of information pertaining to a second aircraft, at least when mounted to, affixed at, or situated in or at the second aircraft or a pilot onboard the second aircraft. Specifically, the foreign data packet D2 comprises foreign truncated position data PD2’, in particular with foreign truncated latitude data PD2’_LAT and/or foreign truncated longitude data PD2’_LON.

The method comprises a further step of

- disambiguating the foreign truncated position data PD2’, in particular the foreign truncated latitude data PD2’_LAT and the foreign truncated longitude data PD2’_LON using its own position data PD1, in particular using its own latitude data PD1 LAT and its own longitude data PD1 LON. Thus, the receiving broadcast device can determine the untruncated (i.e. without reduced value range, not without reduced resolution) foreign position data PD2 that results in the lowest distance to the sending broadcast device. By the principle of locality, this must then be the true solution, since other solutions are not physically possible due to the radio range. Thus, ambiguities in the foreign truncated position data PD2’ are resolved.

Furthermore, the foreign data packet D2 comprises a pair c2 = (e2, m2) with an exponent e2 being a natural number and with a mantissa m2 being a natural number, wherein the pair c2 is indicative of a value v2, wherein the mantissa m2 has a bit width of N _m 2 and wherein the exponent e2 has a bit width of N _e 2, wherein v2 = 2 ^e2 and wherein the bit widths N _m 2 and N _e 2 are selected such that a total bit width N2=N _e 2+N _m 2 of the pair c2 is smaller than a total bit width of the value v2. A linear scaling factor A2 representing the physical unit/ resolution for the encoded numerical value v2 can also be used, see chapter 2. 1 for the AMP protocol description below for details.

Then, the method comprises a further step of computing (decoding) the value v2 using the pair c2 as received in the data packet D2. Thus, bandwidth is saved while a wide range of values v2 can be transferred. Furthermore, advantageously, the method comprises a step of

- calculating a collision probability between the first aircraft and the second aircraft and/or providing information improving situational awareness, e.g. by taking the aircraft positions as comprised in the first and second data packets into account. In general, the situation is assessed based on the information pertaining to the first aircraft which is available to the broadcast device (own information) and based on the received information pertaining to the second aircraft (foreign information). Preferably, a collision warning is then issued to the pilot when the collision probability exceeds a certain threshold which helps to decreases the risk of a mid-air collision.

Advantageously, the pair c2 as comprised in the foreign data packet D2 is indicative of foreign velocity data VD2 of the second aircraft. In particular the value v2 is indicative of a velocity vector magnitude of the second aircraft (i.e. an absolute value of the aircraft’s velocity). Then, the method comprises a further step of computing (decoding) the foreign velocity data VD2 using the received pair c2, in particular computing the velocity vector magnitude v2 using the pair c2.

Thus, bandwidth is saved while a wide range of values v2 (e.g., velocity vector magnitudes ranging from a hobbyist UAV to a military jetplane) can be transferred.

As yet another aspect of the invention, a method for, by means of a receiver device, in particular as discussed above with regard to the second aspect of the invention, wirelessly receiving information pertaining to a second aircraft comprises steps of

- providing the receiver device comprising a control unit and a radio receiver,

- by means of the radio receiver receiving a foreign data packet D2 as broadcasted from a foreign broadcast device, in particular according to the first aspect of the invention, the foreign data packet D2 being indicative of the information pertaining to the second aircraft.

The foreign data packet D2 comprises foreign truncated position data PD2’, in particular foreign truncated latitude data PD2’_LAT and foreign truncated longitude data PD2’_LON, wherein a bit width S2’ of the foreign truncated position data PD2’ is smaller than a bit width S2 of foreign position data PD2 being indicative of an untruncated position P2 of the foreign broadcast device. Further, the foreign data packet D2 comprises a pair c2 = (e2, m2) with an exponent e2 being a natural number and with a mantissa m2 being a natural number, wherein the pair c2 is indicative of a value v2, wherein the mantissa m2 has a bit width of N _m 2 and wherein the exponent e2 has a bit width of N _e 2, wherein v2 = 2 ^e2 and wherein the bit widths N _m 2 and N _e 2 are selected such that a total bit width N2=N _e 2+N _m 2 of the pair c2 is smaller than a total bit width of the value v2. A linear scaling factor A2 representing the physical unit/ resolution for the encoded numerical value v2 can also be used, see chapter 2. 1 for the AMP protocol description below for details.

The method comprises a further step of, by means of the control unit:

* receiving position data PD1 indicative of a position Pl of the receiver device. This position Pl can be fixed, e.g. for a stationary receiver station on the ground or it can be variable, e.g. for a receiving only device mounted in a car or a “receiving only” aircraft. In the first case, the position Pl can e.g. be hardcoded in firmware and read out/received by the control unit, in the second case, the position Pl can be determined by a positioning device such as a GNSS receiver of the receiver device and received by the control unit, similarly to the case discussed above with regard to the combined transmitting and receiving broadcast device.

* receiving the foreign data packet D2 as received by the radio receiver,

* disambiguating the foreign truncated position data PD2’, in particular the foreign truncated latitude data PD2’_LAT and the foreign truncated longitude data PD2’_LON using its own position data PD1, and

* computing (decoding) the value v2 using the pair c2 as received in the data packet D2. Thus, bandwidth is saved while a wide range of values v2 can be transferred.

Please note in this regard that all the technical effects and advantages as described above with the regard to the transmitting method similarly apply here for the receiving only method and are not repeated for reasons of clarity, as the devices complement each other and rely on the same inventive concept. Advantageously, the pair c2 as comprised in the foreign data packet D2 is indicative of foreign velocity data VD2 of the second aircraft. In particular the value v2 is indicative of a velocity vector magnitude of the second aircraft (i.e. an absolute value of the aircraft’s velocity). Then, the method comprises a further step of computing (decoding) the foreign velocity data VD2 using the received pair c2, in particular computing the velocity vector magnitude v2 using the pair c2. Thus, bandwidth is saved while a wide range of values v2 (e.g., velocity vector magnitudes ranging from a hobbyist UAV to a military jetplane) can be transferred.

As yet another aspect of the invention, a computer program product comprises instructions to cause a device as described above with regard to the first aspect of the invention to execute the steps of a method as described above with regard to the second aspect of the invention.

This computer-program product is - according to another aspect of the invention - stored on a computer-readable medium. It can then be read by a device as discussed above with regard to the first aspect of the invention and it can cause the device to execute the steps of a method as described above with regard to the second aspect of the invention.

As another aspect of the invention, a use of a broadcast device as discussed above with regard to the first aspect of the invention at a first aircraft (or a pilot onboard the first aircraft such as a paraglider pilot wearing a variometer/RCDI device implementing the functionality as discussed above with regard to the first and second aspects of the invention) for wirelessly broadcasting information pertaining to the first aircraft is disclosed, in particular for collision avoidance and/or improved situational awareness. This improves compatibility thus enabling efficient collision avoidance and/or situational awareness functionality.

As yet another aspect of the invention, a system for aircraft collision avoidance comprises

- a first broadcast device as discussed above with regard to the first aspect of the invention at a first aircraft (or pilot) for wirelessly broadcasting information pertaining to the first aircraft, and

- a second broadcast device as discussed above with regard to the first aspect of the invention at a second aircraft (or pilot) for wirelessly broadcasting information pertaining to the second aircraft. Thus a collision probability between the first aircraft and the second aircraft is easier to derive, e.g. by taking the information pertaining to the first and second aircraft into account. Preferably, a collision warning is then issued to the pilot when the collision probability exceeds a certain threshold which helps to decreases the risk of a mid-air collision. This improves overall safety and/or situational awareness.

Brief Description of the Drawings

The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:

Figs, la-lc show the principle of Extended Range Encoding (ERC), Figs. 2 - 4 show the principle of Adaptive Coordinate Truncation (ACT),

Fig. 5 shows a broadcast device 10 according to an embodiment of the invention and a display unit 18, the broadcast device 10 comprising a positioning device 11, a control unit 12, and a radio transceiver 13, 14,

Fig. 6 shows a system for aircraft collision avoidance comprising a first broadcast device 10 at a first aircraft 1, a second broadcast device 20 at a second aircraft 2, a third broadcast device 30 at a paraglider pilot, and a ground based receiver station 40,

Fig. 7 shows a diagram visualizing determination of truncated position data PDl’ LAT and reconstruction of untruncated position data as used in the broadcast device 10/ receiver device 100 of Figs. 5, 6, 8 and 9,

Fig. 8 shows a broadcast device 10 according to an embodiment the invention and a display unit 18, the broadcast device 10 comprising a positioning device 11, a control unit 12, and a radio transceiver 13, 14, and

Fig. 9 shows a receiver device 100 according to an embodiment of the invention and a display unit 18, the receiver device 100 comprising a positioning device 11, a control unit 12, and a radio receiver 14. Modes for Carrying Out the Invention

Figs, la-lc show the principle of Extended Range Encoding (ERC) as used in an embodiment of the invention. Specifically, in Fig. la, it is shown that the encodable value range increases from 0. . .511 for a 9 bit integer and for a linear mapping with 2^-1 (x-axis) to 0. .. 1912 for a 9 bit ERC pair with N _e = 2, N _m = 7 (y- axis, v _max =23(2^ + 127) - 2^ = 1912 with e = 2^-1 and m = 2^-1). In Fig. lb, it is shown that the absolute resolution decreases (i.e. the quantization step increases) at the values v = 128, v =128+256 = 384, and v = 128+256+512 = 896 (x-axis), respectively. However, as it is shown in Fig. 1c, the relative quantization error (y-axis, the quantization step as shown in Fig. lb at a given value divided by the value itself) decreases with higher ERC encoded values v (x-axis). See the chapter “AMP Protocol Description”, section 2. 1 for details.

Figs. 2 - 4 show the principle of Adaptive Coordinate Truncation (ACT) as used in the invention, in which a broadcast device 10 transmits its truncated position PDF in a data packet DI relative to a local grid cell (rectangles) origin. In Fig. 2, a receiver (“x”) (e.g. a second broadcast device 20) of a data packet DI determines the correct position Pl (black solid dot) of the broadcast device 10 by taking proximity considerations due to limited radio range into account. Due to properties of the grid, any of the open dot positions are also mathematically correct and can only be discarded due to the inherent physical proximity of the sender and the receiver. In Fig. 3, the principle of creating the grid by binary truncation is explained. Six leading and six trailing bits (white on black) are removed from a base 32-bit signed integer, and the center 20 bits are transmitted (black on white) as an unsigned integer. The number of deleted trailing bits determines the extent to which resolution is lost, while the number of deleted leading bits determines the grid size or encodable separation due to a reduced encodable value range. The deletion of the trailing bits is performed after rounding to the closest admissible integer. In Fig. 4, the non-uniform grid of ACT is visualized: If a static bit truncation was used, the resulting grid size would contract towards the poles of the earth for the longitude dimension. As a consequence, the longitudinal extent would drop below the radio range at some point; a receiver could then no longer unambiguously determine the sender’s position. Conversely, to maintain a sufficient grid size, longitude would require more bits in the transmission. ACT addresses this by dynamically adapting the longitude grid size with higher/ lower latitudes. The following table (see also Section 2.2 of the chapter “AMP Protocol Description”) provides the rule for constructing the longitude grid in an embodiment of the invention:

Latitude range # leading # trailing Longitude bits truncated bits truncated Resolution

|lat| < 36° 6 6 0.0000064°

36° <=|lat| < 66° 5 7 0.0000128°

66° <=|lat| < 78° 4 8 0.0000256°

78° <=|lat| < 84° 3 9 0.0000512°

84° <=|lat| 2 10 0.0001024°

The resulting resolution and grid sizes are shown in Fig. 4: In the top part (panel a), the number of truncated leading bits is shown as a function of the latitude in degrees; the closer the broadcast device gets to the poles (i.e. the more the latitude increases), the less leading bits are truncated from the longitude data PD1 LON. In the middle part (panel b) of Fig. 4, the resulting longitudinal grid size in km as a function of the latitude in degrees is shown, and in the lower part (panel c) the longitudinal resolution in m as a function of the latitude in degrees.

Fig. 5 shows a schematic view of a broadcast device 10 according to an embodiment of the invention as well as a display unit 18. The broadcast device 10 comprises a positioning device 11, a control unit 12 with memory, and a radio transceiver 13, 14. The broadcast device 10 is mounted at a first aircraft 1 (not shown) and receives pressure altitude (ALT) and heading (HDG) data from onboard navigation systems. The positioning device 11 (GPS) is configured to determine three dimensional position data PD1 indicative of a three-dimensional position Pl of the broadcast device/ of the aircraft 1. The control unit 12 (CPU) is configured to receive (via an internal serial bus) the position data PD1 as determined by the positioning device 11, store it in its memory and fuse the received GPS altitude data with the received pressure-based altitude data to improve altitude precision. Specifically, latitude data PDI LAT and longitude data PD1 LON are stored as signed 32-bit integers.

Further, the control unit 12 determines truncated position data PDF comprising truncated latitude data PDl’ LAT and truncated longitude data PDl’ LON, each having a bit width of 20 bits. The original altitude data P1 ALT is also comprised in the truncated position data PDF with an offset of -1000 m above the geoid. To generate the truncated latitude/longitude data from the original untrun- cated latitude/longitude data, a rounding is performed by adding 25 to the original lat- itude/longitude values and then truncating 6 least significant, i.e. trailing bits (i.e. from Thus, a resolution is reduced. These values are used for PD1 LAT and for PD1 LON with |PD1_LAT| < 36° and they are adapted for higher latitudes (i.e. positions towards the poles, see table above).

Then, a “grid celling” of the coordinate space is performed as described above with regards to Figs. 2-4 which effectively reduces the encodable value range. For this, specifically, 6 most significant, i.e. leading bits (i.e. from 2^6 ..231) are truncated. These values are used for PD1 LAT and for PD1 LON with |PD1_LAT| < 36° and they are adapted for higher latitudes (i.e. positions towards the poles, see table above). Thus, it is ensured that a longitudinal extent and a latitudinal extent of the local grid cells are larger than a radio range of the radio transmitter. By means of truncation, memory space and bandwidth during later broadcasts is saved.

As a next step, the to-be-broadcasted data packet DI is generated based on the truncated position data PDF, heading HDG, and an identifier ID1 of the broadcast device stored in non-volatile memory.

In the described embodiment, this is all done in software (i.e. as a computer program product stored in a flash memory of the control unit) running on the control unit 12, although outsourcing certain operations to dedicated hardware units (e.g. for encryption/decryption) is possible as well. Acceleration data SD originating from an accelerometer 17 of the broadcast device 10 serves to augment the position data PD1. The data packet DI is then sent via another internal serial bus to the radio transceiver 13 (RF) which wirelessly broadcasts the received data packet DI (undirected transmission, non-connection based). The data packet DI is indicative of the to-be-broadcasted information (ID1, truncated latitude, truncated longitude, altitude, aircraft type, ground track/ heading, ground speed as calculated from position updates, climb rate as calculated from altitude updates, turn rate as calculated from heading updates, movement mode, time and other, see sections 3.1.1 and 3. 1.2 of the “AMP Protocol Description” for a list).

Updated position data PD1 (plus heading, speed, etc.) is determined at an update frequency of 1 Hz, i.e. a time frame duration is 1 s. In each of these time frames, two data packets DI are broadcasted with the same information, one in each transmit-window. The nominal transmit/update rate is thus 2 data packets per second.

The data packet DI comprises a header section and a payload section, wherein the header section is non-encrypted and wherein the payload section is encrypted by the AES algorithm with a key size of 128 bits (see the chapter “AMP Protocol Description” for details). In addition to broadcasting the data packets DI, the radio transceiver 13, 14 (RF) also acts as a radio receiver 14 for receiving foreign data packets D2, D3 as broadcasted from the foreign broadcast devices 20, 30 (see Fig. 6). Such foreign data packets D2, D3 are indicative of information pertaining to second/third aircraft 2, 3 (see Fig. 6), and the broadcast device 10 is configured to calculate a collision probability between the first aircraft 1 and the second/third aircraft 2, 3 based on the information pertaining to the first aircraft 1 and based on the received information pertaining to the second/third aircraft 2, 3.

Specifically, the foreign data packets D2, D3 comprise foreign truncated position data PD2’, PD3’ with foreign truncated latitude data PD2’_LAT, PD3’_LAT, foreign truncated longitude data PD2’_LON, PD3’_LAT, and foreign altitude data PD2 ALT, PD3 ALT.

After reception of a foreign data packet D2, D3, the foreign truncated position data PD2’, PD3’, in particular the foreign truncated latitude data PD2’_LAT, PD3’_LAT and the foreign truncated longitude data PD2’_LON, PD3’_LON is disambiguated in the following way:

1. Latitude data PD2 LAT, PD3 LAT is disambiguated or reconstructed from PD2’_LAT, PD3’_LAT. This can be done due to the fact that the latitude grid (i.e. the latitudinal extent of the local grid cells) is uniform and that the position data PD1 (and thus PD1 LAT) is known.

2. The longitude grid parameters are determined based on the known position data PD2 LAT, PD3 LAT as computed in step 1 and based on the table above with regard to Figs. 2-4.

3. Longitude data PD2 LON, PD3 LON is disambiguated or reconstructed from PD2’_LON, PD3’_LON, the known grid parameters from step 2, and PD1 LON.

In these disambiguation step, it is checked whether a mathematically possible position in the same local grid cell or in adjacent local grid cells results in a lower distance. The solution with the lowest distance is taken as the correct one.

If the collision probability exceeds a certain threshold, a collision warning (“TRAFFIC WARNING”) is issued to the pilot by means of an audiovisual display 15 of the broadcast device 10. This enhances the safety. A separate display unit 18 helps to improve the pilot’s situational awareness by displaying the first (“own”) aircraft 1 in the center of three circles and the second/third (“foreign”) aircraft 2, 3 with their courses and velocities (arrow lengths, not to scale), also see Fig. 6. The display unit 18 can also be part of the broadcast device 10 (not shown). This enables the use of the broadcast device 10 for collision avoidance with an improved situational awareness as well as the creation of a system for aircraft collision avoidance comprising a first broadcast device 10 at a first aircraft 1, a second broadcast device 20 at a second aircraft 2, and a third broadcast device 30 at a paraglider pilot. Such a system is shown in Fig. 6. The first aircraft 1 at position Pl is equipped with a broadcast device 10 as described above. The second aircraft 2 at position P2 is equipped with a broadcast device 20 which is - except for a different identifier ID2 - the same as the broadcast device 10 as described above. The third aircraft 3 (paraglider and pilot) at position P3 is equipped with a broadcast device 30 which is similar to the broadcast devices 10 and 20 as described above. As a difference to these, however, this foreign broadcast device 30 cannot receive data packets.

A non-sending/receiving-only ground-based receiver station 40 forwards received data packets DI, D2, and D3 to the internet/air traffic control (see Fig. 9 for such a receiver device 100).

The first broadcast device 10 wirelessly broadcasts information pertaining to the first aircraft 1 in the form of data packets DI. The second broadcast device 20 wirelessly broadcasts information pertaining to the second aircraft 2 in the form of data packets D2. The third broadcast device 30 wirelessly broadcasts information pertaining to the third aircraft 3 in the form of data packets D3.

Because the first broadcast device 10 receives the data packets D2, D3 as broadcasted from the second and third broadcast devices 20, 30 (and vice versa, except for the third broadcast device 30), a collision probability is easier to derive by taking the information pertaining to the first, second, and third aircraft 1, 2, 3 into account. This enhances safety and the pilots’ situational awareness. Due to the invention with its broadcasting of truncated position data PDF, PD2’, and PD3’, bandwidth is saved and additional information can be broadcasted.

Fig. 7 shows a diagram visualizing determination of and reconstruction from truncated position data PDF as used in the broadcast device 10/receiver device 100 of Figs. 5, 6, 8 and 9. The 32 bit integer from Fig. 3 with its bit values of 2^...2^1 is shown in the top panel. An example latitude data PD1 LAT is shown as input value in panel a in integer encoding. Because 6 trailing bits are to be truncated for resolution reduction, 2 is added prior to truncation for rounding purposes (panel b). Then, the value in panel c) results. This value is then right-shifted by 6 bits (»6), thus truncating the 6 trailing bits (reducing resolution) and resulting in the new value in panel d. As a last step, a mask (panel e) keeping the least significant 20 bits is ANDed (&), thus creating the truncated latitude data PDF LAT shown in panel f in its full and in panel g in its truncated, 20-bit wide form. This truncated form of panel g is then broadcasted in the data packet DI, thus saving 12 bits of bandwidth compared to the untruncated PD1 LAT of panel a. However, ambiguity is introduced by truncating, e.g., the bit 30 which is set to 1 in PD1 LAT (panel a).

A receiving broadcast device 20 knows its own untruncated latitude data PD2 LAT in the vicinity to PD1 LAT (e.g. bit 30 is set to 1 both for PD1 LAT and PD2 LAT) as shown in panel h. It receives the truncated latitude data PDl’ LAT shown in panel g. The received truncated position data PDl’ LAT is shifted by 6 trailing bits to the left (panel i) to compensate for the (lost) resolution reduction. Then, a mask defined as # bits in truncated data + # trailing bits for resolution reduction (panel j) is ANDed which gives the result in panel k. To compensate for the range reduction, the inversed mask ~j of panel j is ANDed with the known full latitude data PD2 LAT of the receiving broadcast device (panel h), thus yielding the result in panel 1. This result is then ORed with the result in panel k, thus yielding the full reconstruction of the original untruncated latitude data PD1 LAT in panel m (except for the rounding and the resolution reduction, which is lost). Please note here that an additional search (not shown) is performed in neighboring grid cells and the result with the closest distance is taken as the correct solution.

Fig. 8 shows a broadcast device 10 according to an embodiment the invention and a display unit 18, the broadcast device 10 comprising a positioning device 11, a control unit 12, and a radio transceiver 13, 14. The broadcast device 10 is mostly identical to the one shown in Fig. 5 described above with the following differences: The broadcast device 10 is configured to generate the data packet DI in such a way that the data packet DI comprises a pair cl = (el, ml) with an exponent el being a natural number and with a mantissa ml being a natural number. The pair cl is indicative of velocity data VD1 of the first aircraft 1, specifically a value vl is indicative of a velocity vector magnitude of the first aircraft. This velocity vector magnitude can be received by the control unit 12 together with ALT and HDG from onboard navigation systems and/or it can be calculated from position updates of the aircraft. The mantissa m has a bit width of N _m j = 7 bits and the exponent e has a bit width of N _e j = 2 bits, wherein ml) - 2 ^Nml .

The bit widths N _m j and N _e j are selected such that a total bit width Nl=N _e i+N _mi of the pair cl is smaller than a total bit width of the value vl. A linear scaling factor Al representing the physical unit “knots” is used for computing vl according to A * vl = v _aircra ft. Please see chapter 2.1 of the “AMP protocol description” as well as Figs, la-lc for details.

The same applies mutatis mutandis for the foreign broadcast devices 20 and 30 as well as for the foreign data packets D2, D3 indicative of information pertaining to second/third aircraft 2, 3, respectively.

In other words, the foreign data packets D2, D3 each comprise a pair c2,3 = (e2,3, m2, 3) with an exponent e2,3 being a natural number and with a mantissa m2, 3 being a natural number. The pair c2,3 is indicative of a value v2,3, wherein the mantissa m2, 3 has a bit width of N _m 2 3 and wherein the exponent e2,3 has a bit width of N _e 2 3, and wherein The bit widths N _m 2 3 ^an d N _e 2 3 are selected such that a total bit width N2,3=N _e 2 3 ⁺ N _m 2 3 of the pair c2,3 is smaller than a total bit width of the value v2,3. A linear scaling factor A2,3 representing the physical unit/ resolution for the encoded numerical value v2,3 is also used as discussed above. The pair c2,3 as comprised in the foreign data packet D2,3 is indicative of foreign velocity data VD2,3 of the second/third aircraft 2,3, specifically the value v2,3 is indicative of a velocity vector magnitude of the second/third aircraft 2,3 (i.e. an absolute value of the aircraft’s velocity).

Then, the broadcast device 10 is configured to compute (decode) the foreign velocity data VD2,3 using the received pair c2,3, specifically to compute the velocity vector magnitude v2,3 of the second/third aircraft 2,3 using the pair c2,3 as received in the foreign data packets D2,3. Thus, bandwidth is saved while a wide range of values v2,3 (i.e., velocity vector magnitudes) can be transferred.

Fig. 9 shows a receiver device 100 according to an embodiment of the invention and a display unit 18, the receiver device 100 comprising a positioning device 11, a control unit 12, and a radio receiver 14. The receiver device 100 is mostly identical to the broadcast device 10 shown in Fig. 8 with the following differences: Instead of a radio transceiver 13, 14, the receiver device 100 comprises a radio receiver 14 only and has therefore no capabilities to transmit data packets. Also no ALT, HDG, vl, and SD data is fed to the control unit 12 because no data packets are sent from the receiver device 100, but data packets DI, D2, and D3 are received by radio receiver 14. The receiver device 100 can therefore be used as a non-sending/re- ceiving-only ground-based receiver station 40 as shown in Fig. 6.

The data packets DI, D2, and D3 as transmitted by the respective broadcast devices 10, 20, 30 are each indicative of information pertaining to a first/second/third aircraft 1, 2, 3 as discussed above (ID 1,2, 3, truncated latitude_l,2,3, truncated longitude_l,2,3, altitude_l,2,3, aircraft type_l,2,3, ground track/ head- ing_l,2,3, ground speed_l,2,3, climb rate_l,2,3, turn rate_l,2,3, movement mode_l,2,3, time_l,2,3 and other, see sections 3.1.1 and 3.1.2 of the “AMP Protocol Description” for a list).

Because the receiver device 100 is configured to compute (decode) the foreign velocity data VD1,2,3 using the received pairs cl, 2, 3 in the data packets D1,D2,D3, specifically to compute the velocity vector magnitude vl,2,3 of the first/second/third aircraft 1,2,3 using the pair cl, 2, 3 as received in the data packet DI, 2, 3, a wide range of velocity vector magnitude vl,2,3 (i.e., velocity vector magnitudes) can be transferred while saving bandwidth.

Definitions:

Throughout the application documents, the term “aircraft” relates to all VFR-operated or VFR-operatable manned and teleoperated or automated unmanned flying or flyable objects such as gliders, towplanes, helicopters, parachutes, dropplanes, hanggliders, paragliders, single-engine piston planes, multi-engine piston planes, jet planes, (hot air) balloons, airships (such as, e.g. blimps), and UAVs (unmanned aerial vehicles such as drones).

The term “pilot” refers to either the human on board the aircraft or on the ground or on board another aircraft and supervising or piloting the aircraft from a distance. Additionally, in the case of fully automated systems, the term “pilot” may refer to, e.g. a flight control system.

The term “broadcast” relates a method of transferring a message (or here, the data packet) from a single transmitter to all recipients within radio range simultaneously, e.g. non-connection based. This is in contrast a point-to-point (e.g. connection or link-based) method in which a single sender communicates with a single receiver. Whenever the term “transmitter” or “sender” is used, it shall relate to “broadcaster”.

Aircraft Motion Prediction (AMP) Protocol Description

1. Introduction

This section describes a possible implementation of the Aircraft Motion Prediction (AMP) Protocol, i.e. the structure and generation of a data packet used for the invention. All information in this section is to be treated in a non-limiting manner but as examples/ advantageous embodiments only. The AMP Protocol enables the following applications:

• Situation awareness

• Traffic monitoring

• Collision avoidance

• Tracking

• Identification

The following description omits the description of the physical and data link layers, as they are implemented by standard electronic components which are known to the skilled person (e.g. nRF905 from Nordic Semiconductors). 1.1. Coordinates Datum

The WGS-84 standard is used throughout. Elevation is referenced to the WGS-84 ellipsoid surface (i.e. not the geoid, not MSL). Longitude and latitude are encoded in degrees, scaled IE-7. South and West are negative.

1.2. Timing and Synchronization

To support a large number of broadcast devices, radio access is organized in time frames. A single time frame has a duration of, e.g. 1 s. Global time is available to any broadcast device via the positioning device. The number of data packets a broadcast device is allowed to send per time frame (i.e. the duty cycle) is regulated by law (e.g. 1% over one hour). All data packets per time frame have the same information contents (such as position, speed etc.) but can differ in timestamp, protocol version used, etc. (see section 2.4 below).

Transmission are organized in a plurality of transmit-windows per single time frame such that a single data packet is transmitted per transmit-window. As an example, with 1 sec time frames and 2 transmit-windows per time frame, the broadcast device nominally transmits a data packet once in each of these two transmit- windows. The nominal transmit/update rate is thus 2 data packets per second in this example.

Send timing is random within the transmit-window. If a packet collision is detected, a broadcast device retries after a random time delay. If the transmit-window ends before a successful transmission is made, the data packet is lost.

2. Algorithms and Methods

2.1. Extended Range Encoding

Extended Range Encoding (ERC) is a nonlinear encoding technique to encode a value with a large input range efficiently, using less bandwidth (bits) compared to a simple linear encoding. Due to the nonlinearity of the approach, it sacrifices (absolute) resolution at higher values (i.e. it utilizes a larger quantization interval), but achieves a much larger value range. The relative resolution (i.e. the ratio between the quantization interval and the encoded value) can be tuned to suit the intended application. The method is comparable to floating point representations. The difference is that ERC uses integers and is flexible to adjust to the individual values and fields in the AMP protocol. ERC is parametrizable:

• Total number of bits N to use for code points (i.e. ERC representation of a numerical value v)

• Number of bits N _m to use for mantissa m, and, derived from N and N _m , the number of bits N _e to use for the exponent e.

• Whether negative values v are allowed, or only non-negative. Signed values are encoded by using the most significant bit of the exponent as a minus sign.

• A linear scaling factor A for every field representing the physical unit/ resolution for the encoded numerical value

Assume the pair c = (e, m) is a code point indicative of a to-be-transmitted value v, the code point comprising an exponent e and a mantissa m. Both e and m are nonnegative integers, while v is a non-negative real number with a physical unit determined by the scaling factor A. The mantissa m has a bit width of N _m . The total bit with of c is N, thus the bit width of the exponent e is N _e = (N-N _m ).

For better readability, v* is defined as v* = R0UND(v / A) as the result of the value divided by the scaling factor, rounded to the nearest integer. Conversely, let v = A * v*. Then, the numerical input value v encoded by code point c is given by: v = A * v*

The reverse operation for computing c = (e, m) from v is defined by the following algorithm:

1. Let v* = R0UND(v / A)

2. Compute the exponent: Find the largest e from the set of integers 0. . . - 1) that satisfies v* >= 2Nm ₍₂ e _ i) 3. Compute the mantissa:

The code point c = (e, m) can be represented as the binary concatenation of the exponent e and the mantissa m, yielding the binary representation of c: where “|” denotes the “bitwise OR” operation and “«” the “shift left” operator.

An example for N _e = 2, N = 9 yields:

• The encodable value range increases from 0. . .511 (for a 9 bit integer and for a linear mapping with 2^- 1 =511) to 0... 1912 (for a 9-bit ERC mapping with N _e = 2, N _m = 7, and with v _max = 2^ (27 + 127) - 2^ = 1912 with e = 2^-1 and m = 2 -1), see Fig. la.

• The absolute resolution decreases (or the quantization interval increases) at v = 128, v =128+256, v = 128+256+512 = 896 and so forth, see Fig. lb.

• The relative quantization error (i.e. the quantization step divided by the encoded value) decreases with higher ERC encoded values v, see Fig. 1c.

The parametrization in N, N _e therefore defines the range of encodable values v and how quickly the resolution degrades with larger values. In addition to these parameters, the AMP protocol also applies a linear scaling A to every field which defines the physical unit/ resolution for the encoded numerical value v (e.g. 0.1 m/s for ground speed).

2.2. Adaptive Coordinate Truncation

Adaptive Coordinate Truncation (ACT) is a system to reduce the required bandwidth for transmitting the 2D position, exploiting that sender and receiver are necessarily local due to radio range limitations. 2.2.1. Broadcasting ACT

To transmit positions Pl, the WGS-84 geodesic system is used. The base units are longitude and latitude, scaled to 1E-7 ^O (pre truncation). Signed integers are used, north and east are positive, respectively. Altitude is relative to the ellipsoid (i.e. not the geoid) and not truncated, i.e. not part of this algorithm.

In ACT, the WGS-84 coordinate space is divided into grid cells, where the grid dimension is chosen to be well larger than the maximum expected radio range. A sender transmits its position relative to the local grid cell origin. A receiver can determine the grid cell that results in the lowest distance to the sender. By the principle of locality, this must then be the true solution, since other solutions are not physically possible due to the radio range.

The situation is depicted in Fig. 2: A receiver (“x”) determines the correct position (black solid dot) by proximity. Due to properties of the grid, any of the open dot positions are also correct and can only be discarded due to the principle of locality.

ACT uses a grid that is not uniform: If it were, the effective grid size would contract towards the poles for the longitude dimension. As a consequence, the longitudinal extent would drop below the radio range at some point; a receiver could then no longer unambiguously determine the sender’s position. Conversely, to maintain a sufficient grid size, longitude would require more bits in the transmission.

ACT addresses this by dynamically adapting the longitude grid size with higher/ lower latitudes.

The grid is created by binary truncation, which renders the calculations computationally efficient: Starting with a base 32-bit signed integer, a number of bits on the left and right are removed, and only the “center” part is transmitted:

In the example in Fig. 3, six leading bits labelled 26. . .31 and six trailing bits labelled 0. . .5 (white on black) are removed from a 32-bit input value, and the center 20 bits labelled 6. . .25 of the input value are transmitted (black on white). The number of deleted trailing bits determines the amount of lost resolution, while the number of deleted leading bits determine the grid size. The deletion of the trailing bits is performed after rounding to the closest admissible integer, i.e. a rounding up is performed if the most significant truncated bit is set to one. This is done by adding the value of the most significant to-be-truncated bit to the input value prior to trailing bit truncation. In the specific example of Fig. 3 with six truncated trailing bits, 2 is added to the input value prior to trailing bit truncation which results in a mapping of all original input values to 0, 64, 128, 192, ... after truncation. Negative input values map correctly due to the properties of 2's complement.

ACT uses a minimum grid size of 600 km (well above the radio range of approximately 100 km) and 20 bits (after truncation) for both longitude and latitude. Latitude deletes six leading and six trailing bits, resulting in a latitudinal grid size of approximately 750 km. For longitude, the white block with black text in Fig. 3 (the transmitted bits) is gradually shifted to the left as the sender approaches the poles. The shifts are optimized in a way to ensure a minimum longitudinal grid size (i.e. longitudinal extent of the grid cell) of 600 km.

The following table shows the rule for constructing the longitude grid:

Latitude range # leading # trailing Longitude bits truncated bits truncated Resolution

|lat| < 36° 6 6 0.0000064°

36° <=|lat| < 66° 5 7 0.0000128°

66° <=|lat| < 78° 4 8 0.0000256°

78° <=|lat| < 84° 3 9 0.0000512°

84° <=|lat| 2 10 0.0001024°

The resulting resolution and grid sizes are shown in Fig. 4.

2.2.2. ReceivingACT

A receiver can perform the following steps when receiving ACT coordinates:

1. Disambiguate latitude from the received truncated latitude data; this can be done due to the fact that the latitude grid (i.e. the latitudinal extent of the local grid cells) is uniform and that the own untruncated latitude data is known to the receiving broadcast device.

2. Determine longitude grid parameters from the table above and from the latitude data as computed in step 1. 3. Disambiguate longitude from the received truncated longitude data, the grid parameters from step 2, and the own untruncated longitude data as known to the receiving broadcast device.

2.3. Enhanced-Privacy Random ID

To improve the ability to conceal an identity of a broadcast device while maintaining consistency for collision avoidance, the AMP protocol features Enhanced-Privacy Random ID (EPRID).

The method provides a chain of identifiers (IDs) that are broadcasted in a data packet so that signals can be correlated over a short time (continuous reception), but not over a long time (with missed data packets). A broadcast device’s ID (i.e. the current identifier ID^ for the time T| thereby changes randomly over time: A randomly obtained number RON is generated, e.g. by randomly selecting it from a finite set of numbers or randomly generating it, e.g. by means of a true random number generator or some sufficiently seeded pseudo random number generator (PRNG). This RON is then mixed together with the previous identifier ID^_ | by means of a cryptographic hash function to generate the current identifier ID^ which is therefore not equal to the previous identifier ID^.j. The subsequent randomly obtained number (i.e. RON is transmitted as part of the data packet, such that a receiver can - upon receipt of the next data packet - correlate a then received ID^+ 1 to the previously received ID^ without effort. The RON is advantageously chosen from the range 0. . .2^ ^e - 1, where Ne is the number of random bits used for generating the RON.

If a receiver continuously receives at least one data packet per distinct RON/ID pair, then it can readily derive the next ID from this.

However, if a receiver loses one or more data packets, it must start the observation from new since with unknown RON, the new ID cannot be related to the old one. Alternatively, the receiver can try to “guess” the RON. Guessing is rather fast for one or a few missed data packets (only a few bits of randomness were added), but the complexity increases exponentially with the number of bits that need to be guessed. The capability of a receiver to successfully calculate the correct sequence of RONs is effectively limited by two effects:

1. Computational feasibility: Especially when tracking hundreds of targets, e.g. in a wide-area receiver network, or when limited processing power is available, as is often the case in embedded (on-board) systems.

2. Ambiguity: When the number of random bits added (i.e. the amount of randomness introduced by each RON times the number of missed ID-updates) approaches the number of total bits of the ID, the ambiguity increases to the point where a unique reconstruction is no longer possible: There is always a sequence of RONs that generate any given ID from any other.

Example: The random ID has a bit width of 32 bits. An 8 bit value is used for the randomly obtained number RON. Note: The amount of randomness in the RON can be varied, 2 bits are advantageously chosen.

2.3.1. Prerequisites

Advantageously, a cryptographic hash function comprising bitwise XOR-/and bitshifting-operations is used for mixing the randomly obtained number RON^. i with the previous identifier ID^_ | for generating the current identifier ID^. Such a cryptographic hash function HASH() thus is of the form ID^ = HASH/RON^. i , ID^_ | ). HASH() is deterministic, fast to compute, small changes of the input lead to large changes of the output, and it is computationally infeasible to find the reverse operation.

2.3.2. Sender

On the sender, EPRID comprises the following steps:

1. Set k=l, initialize ID | by choosing a random unsigned 32-bit integer and/or using a fixed value which is e.g. stored in a non volatile memory.

2. At each time T| , randomly choose a RON| from the set 0. . .2^ ^e - 1.

3. Use the pair (RON^, ID in AMP data packet broadcasts. 4. Advantageously after between 2 and 10 seconds, at time T _k + _k increment k by 1 and compute the new ID _k from the previous ID _k _ | and the RON^. i :

ID _k = HASH(RON _{k. k} ID]^);

5. Go to step 2.

The duration between the ID updates (step 4) and the number of bits to use for the randomness (Ne) can be chosen by the sender to trade off privacy vs. trackability. The default value is 10 s, 1 s is the minimum value. The default value for Ne is 2, the minimum 1, the maximum 8. The broadcast device may adapt the interval in flight. The ID update may happen at any time, but only after the RF time frame is completed.

2.3.3. Receiver

The receiver of a packet with a (RON, ID) pair performs the following steps:

1. Initialize an internal memory store for storing a list of (ID, ID’) pairs, referenced as ID[i], ID’ [i], wherein i is a natural number and refers to the i-th entry in the memory store.

2. When receiving a (RON, ID) pair, check the memory store if an entry i exists with ID = ID[i] . If a match is found, assume the new data packet originates from a known sender with a known current identifier ID. Break.

3. Else, check the memory store if an entry i exists with ID = ID’ [i]. If a match is found, assume the new data packet originates from a known sender with an updated current identifier ID and update the memory store entry to (ID, HASH(RON, ID)). Break.

4. Else, assume the new data packet originates from an unknown sender. Add the pair (ID, HASH(RON, ID)) to the memory store.

5. Continue with step 2.

If no matching ID is found in step 3, a receiver may optionally employ a deeper search, i.e. over multiple ID updates, assuming that it has received EPRID-enabled data packets before. This requires a brute-force search. This is feasible mostly for ground-based receivers. Airborne devices for collision avoidance purposes will probably not do this, e.g. due to computational limitations of embedded systems.

2.4. Dynamic Message Versioning

Dynamic Message Versioning (DMV) is a method for simplifying data packet protocol updates (e.g., changing the precision, layout, size, or semantics of the con- tents/values, or modifying other aspects of the broadcasted data packets such as modulation, error correction, encryption, preambel etc.) while eliminating the putative need for a hard firmware expiration mechanism that may be present in prior art broadcast devices: The fundamental nature of such a distributed system as the broadcast devices according to the invention is that all participating nodes/broadcast devices need to understand the updated data packet protocol to retain compatibility. With the mentioned firmware expirations, prior art broadcast devices that did not receive a recent firmware or protocol upgrade stopped operating at a predefined date. Thus, the active firmware and thus data packet protocol versions at any given date could be controlled, allowing a concerted, global protocol update, e.g. once every year. However, this is only possible at the cost of manual user intervention for all devices, which is sometimes cumbersome and expensive, particularly in complex aircraft avionics systems.

DMV-enabled broadcast devices do not require such a firmware expiration mechanism while still allowing the protocol to change and improve, e.g. subject to the capabilities of involved broadcast devices. A DMV-enabled broadcast device can therefore be made backward-compatible indefinitely, i.e. it is then capable of receiving and sending AMP data packets of any (lower) version. This makes an older broadcast device visible to newer ones automatically. For vice-versa visibility, DMV can dynamically balance the use of different versions of the data packet protocol based on the capabilities of other receiving broadcast devices in the vicinity of the transmitting broadcast device. The maximum protocol version a DMV-enabled broadcast device is capable of receiving, processing, and transmitting is published in the “ver max” field in the AMP data packet header and is thus transmitted with every AMP data packet (see below). A transmitting broadcast device can then fallback to a lower protocol version if a receiving broadcast device only understands this. 2.4.1. DMV Operating Principle

• Every transmitting DMV-enabled broadcast device (“sender”) updates and maintains a list of nearby receiving DMV-enabled broadcast devices (“receivers”) and their published maximum supported protocol version ( “ver max”- field in the header section of an AMP data packet, see section 3.1.1 below). Note that this maximum supported protocol version may deviate from the actual version used in a specific data packet (“ver”-field in the header section of an AMP data packet, see section 3.1.1 below).

• For every AMP data packet that is transmitted, the sender chooses the protocol version (“ver”) based on this list. Heuristics are applied to determine this protocol version used for transmissions, thus maintaining a backward-compatible minimum connectivity with older clients, albeit at a lower update frequency. The parameters used thereby may be dynamically adapted over time: For instance, a lower data packet protocol version may get a higher priority and thus be transmitted more frequently, e.g., at least once every 2 seconds, immediately after a new AMP-protocol release. This allows as many clients as possible to catch up with the respective update. After a transition period, for example after 4 to 8 weeks, the use of lower- versioned AMP data packets may gradually be reduced, e.g., to at least once every 6 seconds.

• The most compatible protocol version is 0, compatible with all DMV-enabled and possibly even prior art broadcast devices that are not DMV-enabled. A base rate (e.g. once every 15 seconds) of protocol 0 data packets can be used to remain compatible indefinitely.

2.4.2. Implementation

In this section, the DMV-enabled broadcast device under consideration is denoted as “host” and nearby DMV-enabled broadcast devices are referred to as “clients”. This section explains how the host selects the protocol version for broadcasting data packets based on data packets received by the host from the clients. Note that both roles (host and client) are usually present in any DMV-enabled broadcast device, such that this rule applies symmetrically for every client as well. Nonsenders (e.g. ground-based receiver stations) have no means to publish their DMV- capabilities. It is expected that these are updated frequently and/or support the latest protocol version at any time or at least with a short delay after a protocol/firmware update becomes available. Non-receivers (e.g. paraglider beacons) can transmit a predefined value for “ver max”, thus indicating that they cannot receive data packets. Thus, they can then be excluded from DMV.

Let i be an index for the list of received AMP clients, as stored in the host’s memory, with i being a non-zero natural number and i = 1 . . .Nc with Nc being the total number of clients from which data packets are received. Clients from which no data packets are currently received are removed from this list. Let then “ver max ’ be a client’s maximum supported AMP protocol version, as last received in the header section of a data packet sent by the client i. Hereby, it is assumed that “ver maxf’ does not change over time of operation of client i, i.e. during broadcasting of data packets.

This is because firmware and thus protocol updates are usually not performed during operation of the broadcast device.

Let m[i] then be the count of missed (i.e. unreceivable) data packets for each client i, i.e. data packets that the respective client i cannot have received (e.g. due to the data packet not being sent) or data packets that the respective client i cannot have parsed (e.g. due to the data packet having a “ver” > “ver max ’). This number-count m[i] is derived at the nominal AMP transmit or update rate taking transmit-windows into account: In other words, if the host deliberately does not send a data packet at all (e.g. due to RF collision or bandwidth management), this unsent data packet counts as a miss for all clients i and m[i] += 1 for all i = 1 . . .Nc.

Because a plurality of transmit-windows is used per time frame (see section 1.2 above), the protocol version “ver” of the to-be-broadcasted data packet is determined before the start of each transmit-window. Whatever happens during the transmit-window’s duration does not influence the transmission.

The array m[i] of missed data packets for each client i is then updated as follows: 1. At the beginning of any transmit-window, increment m[i] for every client i by

1.

2. If a data packet is sent successfully during the transmit- window: For every client i, set m[i] to zero for client i if the transmitted version “ver” as sent by the host is smaller than or equal to client’s maximum supported version “ver_maxj”. Thus, if a client i can receive and process the data packet, m[i] = 0 for this respective client i.

Before the beginning of a transmit-window, the protocol version “ver” to be used for the broadcasted data packet in this transmit-window is determined as follows:

1. Let the desired client update interval t _c p [i] for each client i be: t _c li[i]= f(D[i], ver maxp Mj) where D[i] is a norm function indicative of the distance between the host and the client i, ver maxj is the maximum supported protocol version of the client i, Mj is indicative of metadata available for the client i, e.g. aircraft type or firmware version, and f() is a dynamic client update function. For a definition of the dynamic client update function f(), see section 2.4.3 below.

2. If the client i is in conflict with the host (i.e. if the client i is in danger of a collision with the host), set t _c p[i] to 1.

3. Let the send gap g[i] for client i be the discrepancy between the number of missed data packets m[i] by the client i and the desired client update interval t _c li[i]. For any client i, the send gap g[i] is then given by g[i] = m[i] - t _c ij[i]

Note that the send gap g[i] starts negative and continuously increases if no client-supported transmission has been made. A gap of 0 or higher indicates that the desired client update interval t _c p[i] is not fulfilled and therefore a need for transmitting a supported data packet to the client i arises.

4. If no g[i] is 0 or higher for all clients i, select the maximum supported protocol version of the host. Break. 5. If some or all g[i] are non-negative, i.e. 0 or higher (i.e. if the desired client update interval t _c p[i] is missed for some or all clients i), select the protocol version “ver” as the minimum of “ver max ’ of all clients with non-negative g[i].

2.4.3. Dynamic Updates

The desired client update interval t _c p[i] of supported data packets for each client i is not fixed but may be adapted to the current situation in the population of broadcast devices, e.g. based on active firmware/ protocol versions and/ or based on situational parameters. This is reflected in the dynamic client update function f(D[i], ver maxp Mj) used for deriving t _c p[i] as discussed above. The following basic rules apply:

• A base desired client update interval is based on the vehicle type Mj of the client i. Examples are: o Hang glider, paraglider: 4: If 4 data packets have been missed by client i (i.e. if no supported data packet is received by client i for 4 transmitwindows), then transmit a supported fallback data packet (i.e. with a compatible protocol “ver”) to the client i. o UAV: 2: If 2 data packets have been missed by client i (i.e. if no supported data packet is received by client i for 2 transmit-windows), then transmit a supported fallback data packet (i.e. with a compatible protocol “ver”) to the client i. o All others: 1 : If 1 data packet has been missed by client i (i.e. if no supported data packet is received by client i for 1 transmit-window), then transmit a supported fallback data packet (i.e. with a compatible protocol “ver”) to the client i.

• If a client i signals no RX capabilities (i.e. if the published ver maxj is a predefined value), set t _c p[i] = 20: If 20 data packets have been missed by client i (i.e. if no supported data packet is received by client i for 20 transmit-windows), then transmit a supported fallback data packet (i.e. with a compatible protocol “ver”) to the client i. This leads to an effective disregard of client i. • If the horizontal distance as given by the norm function D[i] is larger than, e.g. 3 km, multiply t _c p[i] by 2: If the client i is “far away” horizontally, then increase the number of allowed missed data packets before transmitting a supported fallback data packet (i.e. with a compatible protocol “ver”) to the client i by a factor of 2.

• If the vertical separation as given by the norm function D[i] is larger than, e.g. 500 m, multiply t _c p[i] by 2: If the client i is “far away” vertically, then increase the number of allowed missed data packets before transmitting a supported fallback data packet (i.e. with a compatible protocol “ver”) to the client i by a factor of 2.

• If the approach time (distance divided by the relative speed vector projected on the relative position vector) of the client i is less than 30 seconds, set t _c p[i] to 1 : If the client i is on a collision course with the host with an expected approach in less than 30 sec, then transmit a supported fallback data packet (i.e. with a compatible protocol “ver”) to the client i.

• If a client i’s ver maxj is far behind the latest AMP protocol (e.g. if a firmware update for the client i is available for more than 2 years), multiple t _c p[i] by 2: If the client i’s firmware is “old”, then increase the number of allowed missed data packets before transmitting a supported fallback data packet (i.e. with a compatible protocol “ver”) to the client i by a factor of 2.

A placeholder client for protocol version 0 (most compatible protocol version) and a client update interval of 10 may be added to retain a base, worst-case compatibility.

3. AMP protocol

3.1. Marshalling and Semantics

A data packet comprises a header section and a payload data section. The size of the header section is 8 bytes, of the payload section is 16 bytes. The header is transmitted in clear (non-encrypted), the payload is encrypted. The data packet can be constructed as follows: 3.1.1. Header

The header section of the data packet comprises:

Field Description id Indicative of the identifier of the sender, can be either a current identifier ID^ for EPRID (see above) or fixed. ron Indicative of a subsequent randomly obtained number RON| for generating ID^+ i urgency Message urgency, can be between NORMAL and MAYDAY. Can have an effect on, e.g. hop max. hop max Maximum number of retransmissions for creating a mesh-network for message relaying between a plurality of broadcast devices. hop count Current count of retransmissions, incremented with each message relay. ver AMP protocol version used by sender in this data packet. Can be evaluated by the receiver to determine which fields to process. ver max Maximum AMP protocol version supported by the sender for receive or transmit.

3.1.2. Payload

Field Unit/Scaling Description lat trunc Latitude, truncated with ACT, see above. lon trunc Longitude, truncated with ACT, see above. alt m Altitude. acft type enum Aircraft type: Undefined, Glider, Towplane, Helicopter, Parachute, Dropplane, Hangglider, Paraglider, Single-engine piston, Jet, Multi-engine, Balloon, Airship, Blimp, UAV, Static. track 1° Ground track. speed .1 m/s Ground speed, unsigned ERC, see above. climb .1 m/s Climb rate, signed ERC, see above. turnrate .1 7s Turn rate, signed ERC. mov mode enum Discrete movement mode: On ground, Flying, Circling. stealth flag Indicating intent of sender to reduce visibility notrack flag Indicating intent of sender not to track his signal, e.g. with ground station receivers. timestamp .25 s Unix epoch timestamp, in quarter seconds, UTC from GNSS. 3.2. Encryption and Decryption

The payload is encrypted to ensure message integrity, system safety and provide protection for the relevant content against eavesdropping.

The AES algorithm with a key size of 128 bits is used. The key is fixed and shared by all participants of the system. Only the payload block (see Section 3.1.2) is encrypted, the header is transmitted in clear.

Prior to encryption, a 128-bit cryptographic nonce is mixed with the payload. The nonce is created deterministically from the header of the data packet, a time stamp of the data packet, and a secret constant. Because the cryptographic nonce contains the time stamp, replay attacks are not feasible.

The broadcasted, encrypted payload is generated as payloadp = AES(nonce ^A payload, key) where “ ^A ” denotes the bitwise-XOR operator.

Further aspects of the invention are described in the following clauses:

Clause 1. A broadcast device (10) for wirelessly broadcasting information pertaining to a first aircraft (1), the broadcast device (10) comprising:

- a positioning device (11) configured to determine position data (PD1) indicative of a position (Pl) of the broadcast device (10),

- a control unit (12) configured to:

* receive the position data (PD1) as determined by the positioning device (11),

* determine truncated position data (PDF) using at least a part of the received position data (PD1), wherein a bit width (ST) of the truncated position data (PDF) is smaller than a bit width (SI) of the position data (PD1), and

* generate a data packet (DI) comprising the truncated position data (PDF) and an identifier (ID1) of the broadcast device (10) or values indicative thereof, and wherein the broadcast device (10) further comprises

- a radio transmitter (13) configured to receive the generated data packet (DI) and wirelessly broadcast the received data packet (DI), wherein the broadcasted data packet (DI) is indicative of the to-be- broadcasted information.

Clause 2. The broadcast device (10) of clause 1 configured to determine the position (Pl) such that the position data (PD1) comprises

- latitude data (PD1 LAT) indicative of a latitude of the broadcast device (10) and

- longitude data (PD1 LON) indicative of a longitude of the broadcast device (10), and in particular wherein the position data (PD1) further comprises

- altitude data (PD1 ALT) indicative of an altitude of the broadcast device (10).

Clause 3. The broadcast device (10) of clause 2 configured to determine the truncated position data (PDF) using

- the latitude data (PD1 LAT) such that the truncated position data (PDF) comprises truncated latitude data (PDF LAT) and - the longitude data (PD1 LON) such that the truncated position data (PDF) comprises truncated longitude data (PDl’ LON), and in particular wherein the broadcast device (10) is configured to determine the truncated position data (PDF) such that the truncated position data (PDF) comprises the altitude data (PD1 ALT).

Clause 4. The broadcast device (10) of any of the preceding clauses configured to encode the position data (PD1) and/or the truncated position data (PDF) as an integral data type.

Clause 5. The broadcast device (10) of any of the preceding clauses configured to, for determining the truncated position data (PDF), reduce a resolution of the latitude data (PD1 LAT) and the longitude data (PD1 LON).

Clause 6. The broadcast device (10) of clause 5 configured to, for reducing the resolution, truncate the latitude data (PD1 LAT) and the longitude data (PD1 LON) in a binary representation by a first number of trailing bits, and in particular wherein said first number is between 6 and 10.

Clause 7. The broadcast device (10) of any of the clauses 5 to 6 configured to, prior to or together with reducing the resolution, round the latitude data (PD1 LAT) and the longitude data (PD1 LON).

Clause 8. The broadcast device (10) of any of the preceding clauses configured to, for determining the truncated position data (PDF), reduce an encodable value range for the latitude data (PD1 LAT) to obtain the truncated latitude data (PDl’ LAT) and for the longitude data (PD1 LON) to obtain the truncated longitude data (PDl’ LON).

Clause 9. The broadcast device (10) of clause 8 configured to, for reducing the encodable value range, truncate the latitude data (PD1 LAT) and the longitude data (PD1 LON) in a binary representation by a second number of leading bits, and in particular wherein said second number is between 2 and 6. Clause 10. The broadcast device (10) of any of the clauses 8 to 9 configured to set the encodable value range such that an encodable longitudinal separation and an encodable latitudinal separation are both larger than a radio range of the broadcast device (10), in particular by a factor of 2 or more.

Clause 11. The broadcast device (10) of any of the clauses 8 to 10 configured to set the encodable longitudinal separation depending on the latitude data (PD1 LAT).

Clause 12. The broadcast device (10) of any of the preceding clauses further comprising a radio receiver (14) configured to receive a foreign data packet (D2) as broadcasted from a foreign broadcast device (20), the foreign data packet (D2) being indicative of information pertaining to a second aircraft (2), wherein the foreign data packet (D2) comprises foreign truncated position data (PD2’), in particular foreign truncated latitude data (PD2’_LAT) and foreign truncated longitude data (PD2’_LON), and in particular wherein the broadcast device (10) is configured to calculate a collision probability and/or visualize information indicative of a situational awareness between the first aircraft (1) and the second aircraft (2) based on the information pertaining to the first aircraft (1) and based on the received information pertaining to the second aircraft (2).

Clause 13. The broadcast device (10) of clause 12 configured to

- disambiguate the foreign truncated position data (PD2’), in particular the foreign truncated latitude data (PD2’_LAT) and the foreign truncated longitude data (PD2’_LON) using the position data (PD1), in particular using the latitude data (PD1 LAT) and the longitude data (PD1 LON).

Clause 14. The broadcast device (10) of any of the preceding clauses configured to generate the data packet (DI) in such a way that the data packet (DI) comprises a header section and a payload section, and in particular wherein the header section is non-encrypted and/or wherein the payload section is encrypted.

Clause 15. The broadcast device (10) of clause 14 configured to encrypt the payload section of the data packet (DI) by means of a symmetric cryptographic algorithm, and in particular wherein the broadcast device (10) is configured to use a cryptographic nonce based on the header section of the data packet (DI), based on a time stamp, and based on a secret constant for encryption.

Clause 16. The broadcast device (10) of any of the preceding clauses configured to generate the data packet (DI) in such a way that the data packet (DI) comprises at least one of

- a timestamp, in particular in the pay load section of the data packet,

- a packet protocol version, in particular in the header section of the data packet, and

- a maximum supported packet protocol version, in particular in the header section of the data packet.

Clause 17. The broadcast device (10) of any of the preceding clauses configured to

- repeatedly determine updated position data (PD1) indicative of an updated position (Pl) of the broadcast device (10),

- repeatedly determine updated truncated position data (PDF) using at least a part of the updated position data (PD1), and

- repeatedly generate and broadcast an updated data packet (DI) based on the updated truncated position data (PDF) and based on the identifier (ID1) of the broadcast device (10), and in particular wherein any time interval between two of such consecutive updates is between 0. 1 s and 5 s, in particular is between 0.5 s and 1 s, and in particular is 1 s.

Clause 18. The broadcast device (10) of any of the preceding clauses configured to generate the data packet (DI) in such a way that the data packet (DI) comprises a pair c = (e, m) with an exponent e being a natural number and with a mantissa m being a natural number, wherein the pair c is indicative of a value v, wherein the mantissa m has a bit width of N _m and wherein the exponent e has a bit width of N _e , wherein and wherein the bit widths N _m and N _e are selected such that a total bit width N=N _e +N _m of the pair c is smaller than a total bit width of the value v. Clause 19. A method for, by means of a broadcast device (10), in particular of any one of the preceding clauses, wirelessly broadcasting information pertaining to a first aircraft (1), the method comprising steps of

- providing the broadcast device (10) comprising a positioning device (11), a control unit (12), and a radio transmitter (13),

- by means of the positioning device (11) determining position data (PD1) indicative of a position (Pl) of the broadcast device (10), wherein the position data (PD1) comprises latitude data (PD1 LAT) indicative of a latitude of the broadcast device (10) and longitude data (PD1 LON) indicative of a longitude of the broadcast device (10),

- by means of the control unit (12):

* receiving the position data (PD1) as determined by the positioning device (11) and storing the received position data (PD1) in the memory,

* determining truncated position data (PDF) using the latitude data (PD1 LAT) and the longitude data (PD1 LON), wherein a bit width (ST) of the truncated position data (PDF) is smaller than a bit width (SI) of the position data (PD1), and

* generating a data packet (DI) comprising the truncated position data (PDF) and an identifier (ID1) of the broadcast device (10) or values indicative thereof, and wherein the method comprises a further step of

- by means of the radio transmitter (13) receiving the generated data packet (DI) and wirelessly broadcasting the received data packet (DI), wherein the broadcasted data packet (DI) is indicative of the to-be- broadcasted information.

Clause 20. The method of clause 19 wherein the position data (PD1) comprising a further step of

- for determining the truncated position data (PDF), reducing a resolution of the latitude data (PD1 LAT) and of the longitude data (PD1 LON), in particular by truncating the latitude data (PD1 LAT) and the longitude data (PD1 LON) in a binary representation by a first number of trailing bits and/or

- for determining the truncated position data (PDF), reducing an encodable value range for the latitude data (PD1 LAT) to obtain the truncated latitude data (PDF LAT) and for the longitude data (PD1 LON) to obtain the truncated longitude data (PDF LON), in particular by truncating the latitude data (PD1 LAT) and the longitude data (PD1 LON) in a binary representation by a second number of leading bits.

Clause 21. The method of clause 20 comprising a further step of

- setting the encodable value range such that an encodable longitudinal separation and an encodable latitudinal separation are both larger than a radio range of the broadcast device (10), in particular by a factor of 2 or more and in particular wherein the method comprises a further step of

- setting the encodable longitudinal separation depending on the latitude data (PD1 LAT).

Clause 22. The method of any of the clauses 19 to 21 wherein the broadcast device (10) further comprises a radio receiver (14) and wherein the method comprises a further step of

- by means of the radio receiver (14) receiving a foreign data packet (D2) as broadcasted from a foreign broadcast device (20), the foreign data packet (D2) being indicative of information pertaining to a second aircraft (2), wherein the foreign data packet (D2) comprises foreign truncated position data (PD2’) with foreign truncated latitude data (PD2’_LAT) and foreign truncated longitude data (PD2’_LON), and wherein the method comprises a further step of

- disambiguating the foreign truncated latitude data (PD2’_LAT) and the foreign truncated longitude data (PD2’_LON) using the latitude data (PD1 LAT) and the longitude data (PD1 LON), and in particular wherein the method comprises a further step of

- calculating a collision probability and/or visualizing information indicative of a situational awareness between the first aircraft (1) and the second aircraft (2) based on the information pertaining to the first aircraft (1) and based on the received information pertaining to the second aircraft (2).

Clause 23. A computer program product comprising instructions to cause a device of any of the clauses 1 to 18 to execute the steps of a method of any of the clauses 19 to 22.

Clause 24. A computer-readable medium having stored thereon the computer program product of clause 23. Clause 25. A use of a broadcast device (10) of any one of the clauses 1 to 18 at a first aircraft (1) for wirelessly broadcasting information pertaining to the first aircraft (1), in particular for collision avoidance and/or situational awareness.

Clause 26. A system for aircraft collision avoidance comprising

- a first broadcast device (10) of any of the clauses 1 to 18 at a first aircraft (1) for wirelessly broadcasting information pertaining to the first aircraft (1), and

- a second broadcast device (20) of any of the clauses 1 to 18 at a second aircraft (2) for wirelessly broadcasting information pertaining to the second aircraft (2).

Note:

Any embodiments described with respect to the device shall similarly pertain to the method, the computer program product, the use, and the system. Synergetic effects may arise from different combinations of the embodiments although they might not be described in detail.

While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.