The present invention relates to an aeronautical obstacle with warning lights for warning aircraft in order to prevent collisions, a control system for warning lights for aeronautical obstacles and a method for activating warning lights on aeronautical obstacles .

It is well known that aeronautical obstacles such as towers, masts, chimneys, wind turbines, are fitted with warning lights in order to warn the crew of aircraft to their presence, in order to avoid collisions .

The use of warning lights, depend on national regulations, in accordance with and implementing international agreements under International Civil Aviations Organization (ICAO) . In the following reference is made to Danish regulations as laid down in "Lov om Luftfart" , and the invention will be ex- plained in that context. Regulations in other countries may and will deviate from the Danish regulations, but this is immaterial to the technical aspects of the invention.

Aeronautical obstacles exceeding 150 m above ground level (AGL) must be marked with white flashing light. Aeronautical obstacles exceeding 100 m AGL may be required to be marked with continuously lit red lights. In practice this optional requirement is a de facto requirement, always posed by the authorities in charge of the airspace. The additional costs for establishing and maintaining the marking of the aeronautical obstacles exceeding 100 m AGL are imposed on the proprietor of the obstacle. Aeronautical obsta-

cles not exceeding 100 m AGL can also be required to be marked with continuously lit red lights. In that case, however, the costs for establishing and maintaining the marking are at the expense of the Danish state.

In the past decades, the size of wind turbines erected in the landscape and off-shore have grown steadily. Currently wind turbines with rated powers of more than 3 MW are commercially available and wind turbines rated more than 4 MW are being tested. Such wind turbines have wing lengths of 50 m or more, and consequently their heights lie in the region between 100 m to 150 m AGL, where they would normally be required to be marked with permanently lit red lights, at the expense of the proprietor.

The erection of wind turbines, and other structures, however, involves other authorities than those in charge of airspace, inter alia the local authorities in charge of buildings in general and the au- thorities in charge of conservation of the nature.

It has been increasingly difficult to get permission to erect wind turbines. One problem in relation to the larger wind turbines in the range of 100 m to 150 m AGL, is the de facto requirement of mark- ings with permanently lit red lights. The local authorities will be reluctant to permit erection of wind turbines in the range of 100 m to 150 m AGL because of the light pollution created. Perhaps not surprisingly, considering that such red lights would, at least at night time, be visible 30 to 50 km or more away from the wind turbine.

It is the object of the present invention to reduce the light pollution from aeronautical obsta-

cles, such as towers, masts, chimneys, and in particular wind turbines.

According to a first aspect of the invention this object is achieved by an aeronautical obstacle with warning lights for warning aircraft in order to prevent collisions, characterised in that the aeronautical obstacle comprises an antenna array of at least two antennas for receiving a radio signal and means for activating said warning lights upon receipt of said radio signal from an aircraft.

According to a second aspect of the invention the object is achieved by a control system for warning lights for aeronautical obstacles, characterized in that said control system comprises an antenna ar- ray of at least two antennas for receiving a radio signal and means for activating said warning lights upon receipt of said radio signal from an aircraft.

According to a third aspect of the invention the object is achieved by a method for activating warning lights on aeronautical obstacles, charcter- ized in transmitting a radio signal within a predetermined frequency range by means of a radio transmitter on an aircraft, monitoring said predetermined frequency range with a receiving means in connection with the aeronautical obstacle, evaluating signals received by said receiving means in order to estimate the relative position of the aircraft with respect to the aeronautical obstacle, and activating said warning lights if the aircraft is within a predetermined distance and/or flying time of the aeronautical obstacle .

The invention thus makes use of the fact that most aircraft, be it civil, military, or state air-

craft, autonomously emit a number of well defined detectable radio signals. Autonomously in this context being understood as continuously or repeatedly by itself or in response to an interrogation or triggering signal, but without the intervention of the crew. The system according to the invention is thus a passive system in the sense that it does not itself emit any interrogation signals. Only activating the warning lights upon detection of one such signals, allow the warning lights to be turned off most of the time, thus reducing light pollution, and the costly operation of the lights, when no aircraft in present in the vicinity of the obstacle anyway.

Moreover, using an antenna array allows the system to have increased directionality, thereby increasing the sensitivity to signals close to the horizontal plane from the obstacle. This is a simple and efficient way to achieve suppression of signals from aircraft passing in safe altitude above the ob- stacle, so that they will not activate the warning lights .

According to a further preferred embodiment the array is a phased array.

Taking the phase difference between the antenna elements into consideration allows the azimuth angle to the aircraft to be determined. The combined knowledge about the received signal strength and the azimuth angle, allows the relative position or flying time of the aircraft to be determined, and the warn- ing lights to be switched on only when an aircraft is in the vicinity of the aeronautical obstacle.

According to another preferred embodiment the antenna array is adapted to receive radio signals

with frequencies of approximately 1 GHz, preferably between 900 MHz and 1300 MHz, and in particular between 960 MHz and 1285 MHz.

These frequencies are advantageous for several reasons. One reason being that several autonomously emitting systems for aircraft use such frequencies. Another reason is that the typical dimensions of antennas for these frequencies, allow them to easily find place and be fitted on the structures which need to be equipped with warning lights.

According to a preferred embodiment of the control system according to the invention, the system comprises means for adding and subtracting the signals from the individual antennas of the antenna ar- ray.

Using combinations of subtracted signals and added signals allow further increase the accuracy of the position determination for the aircraft.

According to a further preferred embodiment of the control system, a digital signal processor is used for evaluating the signals from the individual antennas of the antenna array.

The use of a digital signal processor allows the use of several different combinations of sub- tracted signals and added signals to further increase the accuracy of the position determination for the aircraft. Also, it allows the introduction of an azimuth off-set angle in order to e.g. suppress signals from ground based transmitters, or the introduction of vertical sweeps for even further precision in azimuth determination. Furthermore it allows the system to be easily reconfigured, if error sources such as

local ground based transmitters are established, or the frequency thereof changed.

According to a preferred embodiment of the method according the invention, the predetermined distance and/or flying time depends on the azimuth of the aircraft with respect to said aeronautical obstacle .

By taking both azimuth and distance into account, it can be avoided that the warning lights are switched on by aircraft passing in a safe altitude above the aeronautical obstacle.

According to a further preferred embodiment, a receiving means comprising an antenna array and means for evaluating the signals received by the individual antennas of the antenna array is" used to determine the azimuth.

This allows the position of the aircraft with respect to the aeronautical obstacle to be determined with a high degree of precision. According to another preferred embodiment of the method, a digital signal processor is used for evaluating the signals from the individual antennas of the antenna array.

Using a digital signal processor allows the method to utilize an azimuth off-set angle in order to e.g. suppress signals from ground based transmitters, or utilizing vertical sweeps for even further precision in azimuth determination. Furthermore, it allows the system to be easily reconfigured, if error sources such as local ground based transmitters are established, or the frequency thereof changed.

In particular, according to yet another embodiment of the method signals received from the individ-

ual antennas of the antenna array are stored, and several calculations using various combinations of signals are performed by the digital signal processor. According to a further preferred embodiment of the method according to the invention, the radio signal transmitted by the radio transmitter on the aircraft is selected among signals emitted by a system selected from the group consisting of SSR, TACAN, Link 16, DME.

These systems are advantageous in that they have rather well defined properties in respect of transmitting power and/or frequencies.

The invention will now be described in greater detail based on non-limiting exemplary embodiments in connection with the drawings on which, fig. 1 is a schematic view of an aeronautical obstacle equipped with warning lights and a control system according to the invention for the warning lights, fig. 2a is a free space elevation diagram of the gain of a two element array when combining the signal from each element, fig. 2b is a free space elevation diagram of the gain of a two element array when subtracting the signal from one element from the signal of the other element , fig. 2c illustrate both diagrams of fig. 2a and fig. 2b, fig. 3a is a free space elevation diagram of the gain of an eight element array when combining the signal from each element,

fig. 3b is a free space elevation diagram of the gain of an eight element array when subtracting the signal from one group of four elements from the signal of the other group of four elements, fig. 4 is schematic diagram of a signal processing system for an eight element array, and fig. 5 is a flow chart illustrating a method for switching on the warning lights of an aeronautical obstacle. Fig. 1 illustrates an aeronautical obstacle 1. The aeronautical obstacle 1 is equipped with warning lights 2. An aircraft 3 passes in an altitude at a distance from the aeronautical obstacle 1. The aircraft 3 autonomously transmit a radio signal from an antenna 4. Autonomously in this respect is to be understood as the signal being transmitted continuously or repeatedly without any specific activation, i.e. once the transmitter is switched on, it autonomously transmits without any intervention by the aircraft crew.

The aeronautical obstacle 1 is equipped with a receiving means adapted to receive a number of different signals commonly transmitted from aircraft. These signals may include SSR, TACAN, DME, Link 16, which are all transmitted at a frequency of approximately 1 GHz. The receiving means comprises an antenna array 5 and a receiver 6. The receiver 6 is connected to a means 7 for activating the warning lights 2. The system may be designed to respond to one or more of a number of different signals, such as SSR, TACAN, DME, and Link 16 mentioned above. The currently most preferred signals are from the Distance

Measuring Equipment, (DME) . All Instrument Flight Rules (IFR) equipped civil aircraft and almost all military aircraft have the DME system as a part of the VOR/DME and the Instrument Landing System (ILS) or as a part of the TACtical Air Navigation (TACAN) system. When the DME system is activated it will emit an identifiable pulse train on one of 100 frequencies in a 100 MHz range slightly above 1 GHz. In search mode the transmitter will transmit between 120 and 150 pulse pairs per second. If a ground station is within reach it will respond with a corresponding set of pulses on a different frequency, having a fixed off-set with respect to the first frequency. When contact is established between the DME transmitter on the aircraft and the transponder on the ground, the pulse pair rate will be reduced to between 24 and 30 per second. The DME transmitters installed in aircraft have comparable transmitting power, though the military ones are typically more powerful. The trans- mitting power, however, is in either case sufficiently well defined to allow a range estimate based on the received signal strength at the aeronautical obstacle 1.

SSR signals may also be used, but for reasons explained below they are preferably only to be used as a supplement to the above DME signal.

Classical SSR is based on ground based radar which interrogates the transponders in the air space around the radar. The airborne transponder responds with a coded pulse train at 1090 MHz when it is interrogated by the ground based equipment at a frequency of 1030 MHz. Both these signals lie in the same overall frequency range as the DME signals and

may thus be received with the same antennas array 5. Though it is usually the case in Denmark, where low altitude instrument flight paths exist, low altitude coverage of the radar cannot always be guaranteed. Thus, low-flying aircraft may be below the radar coverage, and their transponder would not transmit any signal . It should be noted, that the SSR transponder signal from the aircraft 3 for the purposes of this invention is considered to be an autonomously emitted signal, because it responds autonomously to a third party signal, which is transmitted independently of the aeronautical obstacle 1. Apart from the simple detection of azimuth and signal strength, the system can benefit from the fact that the SSR transponder can transmit the pressure altitude of the aircraft as part of the coded pulse train mentioned above. The pressure altitude is the altitude based on the reference ground level pressure of 1013.25 HPa. Ground level pressure, however, is not always 1013.25 HPa but depends on both ground level elevation above mean sea level (MSL), and local atmospheric conditions. The received pressure altitude thus differs from the altitude AGL, which is evidently more relevant. Thus, the aeronautical obstacle preferably includes means for measuring the actual ground level pressure, or means for receiving meteorological data to be combined with information about the ground level elevation.

SSR mode S includes navigation data and would therefore also be able to supply additional information supplementing the signal detection.

Link 16 is a tactical spread spectrum TDMA mobile communications network allowing communications

between up to 32 subscribers, such as various tactical players such as aircraft, tanks, infantry units, or the like. Link 16 is also known as Joint Tactical Information Distribution System (JTIDS) . In the Link 16 network data and voice information is exchanged continuously between the parties in respectively assigned time slots in the TDMA system. The operating frequency band is from 960 MHz to 1215 MHz.

The signals transmitted from the Link 16 trans- mitter on an aircraft thus lie in the same overall frequency range as the DME signals and may thus be received with the same antenna array 5, irrespective of the fact that the signals are not transmitted on a fixed frequency in that frequency range. In this re- spect it should be noted that the receiving means does not necessarily scan the individual channels of the frequency range, but may in a simple embodiment be tuned to detect any signal in the entire frequency range . Thus, the Link 16 signals transmitted from aircraft forming part of a Link 16 network, can be used to determine the presence in the vicinity of the aeronautical obstacle 1, and activate the warning lights 2. In principle, it is sufficient for the functioning of the invention to switch on the warning lights 2 upon receipt a signal of a predetermined signal strength of any of the signals described above, leaving the warning lights 2 on for a prede- termined period of time after receipt of the signal, or preferably a predetermined period after the signal is no longer received, and then switching them off again, thereby reducing the light pollution. This

predetermined signal strength, then corresponds to a predetermined minimum distance set by the authorities. Alternatively, the distance may be expressed as a minimum flying time at a given maximum speed for low altitude flight, e.g. 400 knots (approximately 720 km/h) .

However, since the purpose of the invention is to reduce light pollution, it is desirable to make the system more selective, so as to reduce the number of times when the warning lights 2 are switched on.

It is currently assumed that for the system to be an acceptable alternative to continuously lit warning lights 2, aircraft 3 have to be detected within a flying time of approximately 45 seconds at an altitude of 1000 ft (approximately 305 m) from the obstacle, in order to have sufficient response time for fast flying low altitude aircraft 3, such as fighters and other combat aircraft 3. Realistic speeds for combat aircraft at that altitude would be 400 knots (approximately 720 km/h) , corresponding to a range of approximately 5 nautical miles (approximately 9 km) from the aeronautical obstacle. The actual value of the predetermined distance is not per se a technical feature of the invention but depends on what the authorities in a given country may find appropriate, or may even depend on the actual location of the aeronautical obstacle.

This horizontal range, however, is far more than necessary in the vertical direction, and would lead to activation of the warning lights 2 also by aircraft 3 passing in safe altitude above the aeronautical obstacle 1.

According to the invention the receiving means of the aeronautical obstacle 1 comprises an antenna array 5, in particular an array of vertically aligned vertical antenna elements 5a. This increases the gain in the horizontal direction and reduces the gain in the vertical directions, thus increasing the directionality of antenna array 5. Thus, if the aircraft 3 has a high altitude it will have to be far closer to the aeronautical obstacle 1 before the received sig- nal is sufficient to activate the warning lights 2. Increasing the number of vertical antennas 5a will increase this vertical directionality of the antenna array 5, and allow aircraft 3 to pass above the obstacle without activating the warning lights 2 at all . This can be seen by comparing the gain diagram of fig. 2a for a two antenna array 5 with the gain diagram of fig. 3a for an eight element array 5.

To further suppress false detections known error sources could be filtered out. Such an error source could be an SSR interrogating radar in the vicinity of the aeronautical obstacle 1.

To further improve the ability of the system to prevent unnecessary activation of the warning lights 2 the receiving means comprise in addition to the an- tenna array 5 means for evaluating the signals received by the individual antenna elements 5a.

In a simple form such a means for evaluating the signals may comprise simple means such as a magic T for the analogue addition and subtraction of the signals from the signals of two antenna elements 5a of a two-antenna array 5. Knowledge about both the added signal and the subtracted signal provides the possibility of a far more precise determination of

the azimuth to the aircraft 3, than does the signal from a single antenna or does the added signal from two antenna elements 5a.

It is however presently preferred to use a digital solution, rather than a simple analogue one. Thus according to the presently preferred embodiment, illustrated in fig. 6, the evaluation means comprises a digital signal processor (DSP) 8. In the system illustrated in fig. 6 an antenna array 5 comprising eight identical antenna elements 5a. The signals received by the antenna elements 5a are delivered to the DSP via eight identical paths. Each path illustrated comprises an antenna element 5a, a band pass filter (BPF) 9, a down converter (DC) 10, an ana- logue/digital-converter (ADC) 11, a data buffer (DB) 12 and a common data bus 13.

In each path, the signal from the transmitter on board the aircraft 3 is received by the antenna 5. The antennas are dimensioned for the frequency range around approximately 1 GHz, in order to receive signals in the frequency band from 963 MHz to 1283 MHz. The signals received are then band pass filtered in a band pass filter 9 in order to suppress unwanted signals from outside the relevant frequency band. The signals are then converted down to a frequency range from 0-320 MHz. The down-converted signals are digitized in an analogue/digital -converter 11. Optionally there is a data buffer 12 storing the digitized signal before it is placed on a common data bus 13, and transmitted to the digital signal processor 8. The digital signal processor processes the received signals and transmits resulting signals to a microprocessor (μP) 14 for evaluation.

The digital signal processor 8 processes the signals received from each path. Using a digital signal processor allows different signal combinations to be formed from the individual signals of the eight antenna elements 5a. Using different combinations of signals offers a variety of ways of improving the determination of the azimuth to the aircraft 3.

One way is to divide the antenna elements 5a into groups. If the eight antenna elements 5a repre- sented in fig. 4 represent eight vertically arranged and vertically aligned antenna elements 5a, the antenna elements 5a could be split into two groups, one containing the upper four antenna elements and another containing the lower four antenna elements . The combined signals from the lower four antenna elements could be subtracted from the combined signal of the upper four antenna elements, forming a resulting subtraction signal. Then based on the same signals, the combined signal from all antenna elements 5a could be formed, thus providing a resulting combination signal. Based on both resulting signals, the microprocessor 14 could perform the evaluation and determine whether the warning lights 2 should be switched on.

The digital signal processor, however, may per- form further signal processing than simply adding and subtracting signals. Also, the individual signals could be delayed, thus introducing a predetermined phase shift, allowing the vertical directionality of the antenna array 5 to be controlled. An antenna ar- ray 5 operated in this way by control of the signal phase to each individual antenna element 5a is commonly known as a phased array. By using a phased array the system could compensate to some extent for

the fact that the antenna array 5 will normally not be placed at ground level, but have an elevated position. Thus, the antennas will often be placed high above the ground, e.g. on the nacelle if the aeronau- tical obstacle 1 is a wind turbine.

Also, the directionality of the antenna array 5 could be controlled dynamically, so as to make sweeps up and down searching for aircraft 3 within the predetermined range around the aeronautical obstacle 1. Using a digital signal processor 8 and a microprocessor 14 furthermore allows the system to selectively suppress known error sources in the vicinity of the aeronautical obstacle, e.g. by simply comparing the phases or the signals from two or more an- tenna elements 5a of the antenna array 5. Thus, not only fixed frequency error sources, such as the SSR interrogating radar mentioned above, can be filtered out, but also error signals from ground based DME transponders. The frequency of the ground based DME transponders always lie approximately 50 MHz from the frequency of the aircraft transmitter. The necessary information about inter alia the frequencies of local SSR radars and other data necessary for the digital signal processor is stored in an appropriate data storage means such as RAM and/or ROM 17.

Preferably the system also includes means for logging and communicating instances when the warning lights 2 are switched on. Such means comprises memory means 18 such as RAM. To keep time track of the re- cordings the system comprises a GPS unit 19, receiving timing signals form the satellites of the Global Positioning System. Alternatively, signals from the GLONASS or Galileo systems could be used. For trans-

mitting the data the system comprises communication means, preferably a GSM telephone unit 20, but land telephone or data lines could also be used.

From the microprocessor 14 controlling the sys- tern, the warning lights 2 are controlled via an input/output (I/O) unit 15. The input/output unit 15 preferably also allows the input of commands to the microprocessor 14 for forcing the warning lights 2 on. The input/output unit may also include means for issuing an error signal, e.g. indicating a malfunction. Though not shown, the force and error signals communicated to and from the microprocessor 14 could be received via the GSM telephone unit 20.

The microprocessor 14 is used to evaluate the signals from the digital signal processor 8, and decide whether the warning lights 2 should be switched on or not .

The microprocessor applies several criteria to signals in order to make the decision on whether to switch the warning lights 3 on.

One criterion is the signal frequency. If frequencies, which correspond to known error sources in the vicinity of the aeronautical obstacle 1 are not filtered out by the digital signal processor 8 or by filters in the signal path, the microprocessor 14 and/or the digital signal processor can be programmed to suppress such signals as known errors.

Another criterion is the signal type. Because the signals derive from different signals sources, they are transmitted with different powers. The strength of the received signal, on which the range is estimated, will thus differ between the various

types of signals. Consequently, the microprocessor 14 looks at the signal in order to determine its type.

A DME signal search signal transmits 120-150 pulse pairs per second in the search mode and 24-30 per second once contact to a ground transponder is established.

A Link 16 signal is has a frame length of 7.8125 ms, but is not transmitted on a fixed frequency. The frame is sent as pulses of a duration of 6.4 μs , where the frequency changes between each pulse according to a pseudorandom code.

The SSR transponder signal is a coded pulse train transmitted at a frequency of 1090 MHz.

When the microprocessor 14 identifies a signal as one of the above, it evaluates the signal taking into account the transmission power, in order to determine the distance to the aircraft. If the distance is less than the safety range, the microprocessor 14 switches on the warning lights 2. The microprocessor will control the warning lights 2 to make them stay on as long as the signal is detected, and preferably for a predetermined time thereafter, also controlled by the microprocessor 14.

Another criterion is motion. If an object ap- pears not to be moving, there will normally not be any risk of collision, because the object is unlikely to be an aircraft. Thus, if the received signal strength and the azimuth remain stable over a period of time, the microprocessor 14 may suppress the switching on of the warning lights 2, or switch them off sooner than if a moving object was detected.

An embodiment of the method according to the invention will now be described. Fig. 5 is a block

diagram illustrating the basic process in the method according to the invention.

Once started in step 101, the system receives radio signals in the relevant frequency bands in step 102. Preferably any received signal will be processed in a processing step 103. The processing is preferably carried out in a digital signal processor 8 as described above, but could also include the direct filtering out of known error sources, before the sig- nals reach the digital signal processor 8.

Based on the processed signals, a decision is made in step 104, as to whether the signal represents an aircraft 3, which is too close to the aeronautical obstacle 1 and below a predetermined altitude. The decision making step 104 is preferably performed by a microprocessor 14. If it is decided in step 104 that a aircraft 3 it too close to the aeronautical obstacle 1 the warning lights 2 are switched on in step 105, and the process returns to step 102 to receive further signals, e.g. from the same aircraft 3 if it is still too close to the aeronautical obstacle 1.

If it is decided in step 104 that no aircraft 3 is too close to the aeronautical obstacle 1, the process returns directly to step 102 without execut- ing step 105.

The invention has been described above based on a non-limiting embodiment. The skilled person will understand that modifications and variations are possible without deviating from the inventive concept. In particular it is not excluded that other radio signals in other frequency bands could be used instead of, but preferably in further addition to the signals mentioned above. Though, the inventive con-

cept relates to a passive system, it is evident that it does not exclude the possible use of a dedicated interrogation transmitter and responder, in addition to the passive detection.