Technical Field

The present disclosure relates to positioning, navigation, and timing (PNT) services, and in particular to a system comprising terrestrial transmitters and a receiver of digital wireless broadcast signals, corresponding methods, and said broadcast signal.

Background Art

Global navigation satellite systems (GNSS) such as GPS or GALILEO transmit unencrypted signals of very low power. These signals may easily be jammed, for example using a 5W jammer, or even spoofed using low-cost software-defined radio (SDR) communication systems.

Accordingly, there is a need for a GNSS backup system to provide timing and position to critical infrastructure, such as power grids, ports, airports, road tolls, cash machines, stock exchanges, and the like.

Summary

The object of the present disclosure is to provide a robust GNSS backup system.

The claimed subject-matter is defined by the appended independent claims. Preferred embodiments are set forth in the dependent claims and in the following description and drawings.

A first aspect of the present disclosure relates to a method of geographic positioning of a receiver based on communicated positioning information. The method comprises: transmitting, by three or more time-synchronized terrestrial transmitters, a respective digital wireless broadcast signal comprising transmitter information directly or indirectly indicative of a geographic position of the transmitter of the broadcast signal, and timing information indicative of a timing of the broadcast signal; receiving, from the three or more transmitters, the respective broadcast signal at a respective time of arrival (TOA); calculating, in dependence of the respective timing information and the respective TOA, a plurality of time differences of arrival (TDOAs) of each of the transmitters but a reference transmitter of the transmitters; determining, in dependence of the respective transmitter information, a geographic position of each of the transmitters; and calculating a geographic position of the receiver in dependence of the geographic positions of the transmitters and the plurality of TDOAs.

The method may further comprise receiving a correction information indicative of a temporal correction of the timing information of the respective broadcast signal.

The correction information may depend on a ground conductivity detected at a geographic position in between the transmitters.

The broadcast signal may comprise a Further Evolved Multicast Broadcast Mobile Service (FeMBMS) signal including the transmitter information and the timing information.

The broadcast signal may comprise DVB-T2 extension frames (FEF) including the FeMBMS signal.

The broadcast signal may comprise an NB-IoT signal including the transmitter information and the timing information; and the broadcast signal may comprise a guard band of another broadcasting service, the guard band including the NarrowBand Internet of Things (NB-IoT) signal.

The transmitter information may comprise at least one of: a transmitter identifier, and a geographic transmitter position.

The timing information may comprise at least one of: a timestamp of the transmission by the transmitter of the broadcast signal, and a positioning reference signal (PRS) mapped into the broadcast signal. A mapping of the PRS to time-frequency resources of the broadcast signal may depend on a cryptographic key.

The time-synchronizing of the transmitters may comprise providing a common primary reference clock (PRC) source for the transmitters.

The time-synchronizing may comprise using Precision Time Protocol (PTP).

The time-synchronizing may comprise using Synchronous Ethernet (SyncE) protocol.

A second aspect of the present disclosure relates to a method of transmitting positioning information for geographic positioning of a receiver. The method comprises: transmitting a digital wireless broadcast signal, comprising transmitter information directly or indirectly indicative of a geographic position of the transmitter of the broadcast signal, and timing information indicative of a timing of the broadcast signal.

The broadcast signal may comprise an FeMBMS signal including the transmitter information and the timing information.

The broadcast signal may comprise DVB-T2 FEF including the FeMBMS signal.

The broadcast signal may comprise an NB-IoT signal including the transmitter information and the timing information; and the broadcast signal may comprise a guard band of another broadcasting service, the guard band including the NB-IoT signal.

The transmitter information may comprise at least one of: a transmitter identifier, and a geographic transmitter position.

The timing information may comprise at least one of: a timestamp of the transmission by the transmitter of the broadcast signal, and a PRS mapped into the broadcast signal. A mapping of the PRS to time-frequency resources of the broadcast signal may depend on a cryptographic key.

The time- synchronizing of the transmitter may comprise providing a common PRC source for the transmitter.

The time-synchronizing of the transmitter may comprise using PTP.

The time-synchronizing of the transmitter may comprise using SyncE.

A third aspect of the present disclosure relates to a terrestrial transmitter of a digital wireless broadcast signal. The transmitter comprises: a processing unit configured to transmit the digital wireless broadcast signal comprising transmitter information directly or indirectly indicative of a geographic position of the transmitter of the broadcast signal, and timing information indicative of a timing of the broadcast signal.

The broadcast signal may comprise an FeMBMS signal including the transmitter information and the timing information.

The broadcast signal may comprise DVB-T2 FEF including the FeMBMS signal.

The broadcast signal may comprise an NB-IoT signal including the transmitter information and the timing information; and the broadcast signal may comprise a guard band of another broadcasting service, the guard band including the NB-IoT signal.

The transmitter information may comprise at least one of: a transmitter identifier, and a geographic transmitter position.

The timing information may comprise at least one of: a timestamp of the transmission by the transmitter of the broadcast signal, and a PRS mapped into the broadcast signal. A mapping of the PRS to time-frequency resources of the broadcast signal may depend on a cryptographic key.

A fourth aspect of the present disclosure relates to a method of geographic positioning of a receiver based on received positioning information. The method comprises: receiving, from three or more time-synchronized terrestrial transmitters, a respective digital wireless broadcast signal at a respective TOA, the respective broadcast signal comprising transmitter information directly or indirectly indicative of a geographic position of the transmitter of the broadcast signal, and timing information indicative of a timing of the broadcast signal; calculating, in dependence of the respective timing information and the respective TOA, a plurality of TDOAs of each of the transmitters but a reference transmitter of the transmitters; determining, in dependence of the respective transmitter information, a geographic position of each of the transmitters; and calculating a geographic position of the receiver in dependence of the geographic positions of the transmitters and the plurality of TDOAs.

The broadcast signal may comprise an FeMBMS signal including the transmitter information and the timing information.

The broadcast signal may comprise DVB-T2 FEF including the FeMBMS signal.

The broadcast signal may comprise an NB-IoT signal including the transmitter information and the timing information; and the broadcast signal may comprise a guard band of another broadcasting service, the guard band including the NB-IoT signal.

The transmitter information may comprise at least one of: a transmitter identifier, and a geographic transmitter position.

The timing information may comprise at least one of: a timestamp of the transmission by the transmitter of the broadcast signal, and a PRS mapped into the broadcast signal. A mapping of the PRS to time-frequency resources of the broadcast signal may depend on a cryptographic key.

The method may further comprise receiving a correction information indicative of a temporal correction of the timing information of the respective broadcast signal.

The correction information may depend on a ground conductivity detected at a geographic position in between the transmitters.

The determining the geographic position of each of the transmitters may comprise retrieving the respective geographic position of the respective transmitter in dependence of the transmitter identifier, for example from a database hosted or cached by the receiver 5, or directly from the geographic transmitter position of the respective transmitter information.

The calculating the geographic position of the receiver may comprise performing multilateration in dependence of the geographic positions of the transmitters and a plurality of pseudo-range differences corresponding to the plurality of TDOAs .

A fifth aspect of the present disclosure relates to a receiver of digital wireless broadcast signals. The receiver comprises: a processing unit configured to receive, from three or more time-synchronized terrestrial transmitters, a respective digital wireless broadcast signal at a respective TOA, the respective broadcast signal comprising transmitter information directly or indirectly indicative of a geographic position of the transmitter of the broadcast signal, and timing information indicative of a timing of the broadcast signal; calculate, in dependence of the respective timing information and the respective TOA, a plurality of TDOAs of each of the transmitters but a reference transmitter of the transmitters; determining, in dependence of the respective transmitter information, a geographic position of each of the transmitters; and calculating the geographic position of the receiver in dependence of the geographic positions of the transmitters and the plurality of TDOAs.

The broadcast signal may comprise an FeMBMS signal including the transmitter information and the timing information. The broadcast signal may comprise DVB-T2 FEF including the FeMBMS signal.

The broadcast signal may comprise an NB-IoT signal including the transmitter information and the timing information; and the broadcast signal may comprise a guard band of another broadcasting service, the guard band including the NB-IoT signal.

The transmitter information may comprise at least one of: a transmitter identifier, and a geographic transmitter position.

The timing information may comprise at least one of: a timestamp of the transmission by the transmitter of the broadcast signal, and a PRS mapped into the broadcast signal.

A mapping of the PRS to time-frequency resources of the broadcast signal may depend on a cryptographic key.

A sixth aspect of the present disclosure relates to a system of communicating positioning information for geographic positioning of a receiver comprises three or more time-synchronized terrestrial transmitters of respective digital wireless broadcast signals according to the third aspect or any of its implementations; and a receiver of the digital wireless broadcast signals according to the fifth aspect or any of its implementations.

A seventh aspect of the present disclosure relates to a digital wireless broadcast signal. The broadcast signal comprises: transmitter information directly or indirectly indicative of a geographic position of a terrestrial transmitter of the broadcast signal; and timing information indicative of a timing of the broadcast signal.

The broadcast signal may comprise an FeMBMS signal including the transmitter information and the timing information.

The broadcast signal may comprise DVB-T2 FEF including the FeMBMS signal. The broadcast signal may comprise an NB-IoT signal including the transmitter information and the timing information; and the broadcast signal may comprise a guard band of another broadcasting service, the guard band including the NB-IoT signal.

The transmitter information may comprise at least one of: a transmitter identifier, and a geographic transmitter position.

The timing information may comprise at least one of: a timestamp of the transmission by the transmitter of the broadcast signal, and a PRS mapped into the broadcast signal.

A mapping of the PRS to time-frequency resources of the broadcast signal may depend on a cryptographic key.

Advantageous Effects

The present disclosure provides a terrestrial GNSS backup system that is largely invulnerable to jamming and spoofing. The system is based on broadcasting of digital television (TV) signals including 3GPP signals, such as FeMBMS or NB-IoT, with embedded positioning information. The resulting positioning approach requires no extra / dedicated spectrum, is compatible with Observed Time Difference Of Arrival (OTDOA) positioning in 3GPP 4G, can be carried out using existing 3GPP 4G modem chipsets (with OTDOA support, commercially available in US) with mainly SW modifications, can be used worldwide, is energy efficient due to integration in TV transmitters, is more economical, as existing TV transmitters and antennas are reused and transmitters do not have to be significantly modified, and is attack-proof.

Brief Description of Drawings The above-described aspects and implementations will now be explained with reference to the accompanying drawings, in which the same or similar reference numerals designate the same or similar elements.

The features of these aspects and implementations may be combined with each other unless specifically stated otherwise.

The drawings are to be regarded as being schematic representations, and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to those skilled in the art.

FIG. 1 illustrates a system comprising terrestrial transmitters and a receiver in accordance with the present disclosure, and corresponding methods in accordance with the present disclosure;

FIGs. 2 - 5 schematically illustrate digital wireless broadcast signals in accordance with the present disclosure;

FIG. 6 schematically illustrates a terrestrial transmitter in accordance with the present disclosure; and

FIG. 7 schematically illustrates a receiver in accordance with the present disclosure.

Detailed Descriptions of Drawings

FIG. 1 illustrates a system 4, 5 comprising terrestrial transmitters 4 and a receiver 5 in accordance with the present disclosure, and corresponding methods 1, 2, 3 in accordance with the present disclosure; The system 4, 5 is for communicating positioning information from the three (or more) time- synchronized terrestrial transmitters 4 for geographic positioning of the receiver 5 of the digital wireless broadcast signals 6.

Each of the terrestrial transmitters 4 shown to the left of FIG. 1 performs a method 2 of transmitting positioning information for geographic positioning of the receiver 5.

The method 2 comprises a step of time-synchronizing 201 the transmitters 4. The time-synchronizing 201 of the transmitters 4 may comprise providing the transmitters 4 with a common primary reference clock (PRC) source in accordance with a master-slave synchronization approach. The time synchronizing 201 of the transmitters 4 may comprise using the Precision Time Protocol (PTP, IEEE 1588) for clock synchronization and/or the Synchronous Ethernet (SyncE) protocol for clock syntonization (i.e., frequency synchronization). For example, it may be chosen to use a “High Accuracy Request-Response Default PTP Profile” (Annex 1.5) of IEEE 1588-2019 to interconnect switches and nodes, which is also commonly referred to as CERN’s “White Rabbit” time synchronization protocol. The transmitters 4 need to be synchronized to 3GPP frame boundaries, and the PRS occasions for all transmitters 4 on a same frequency layer need to be aligned in time.

The method 2 further comprises a step of transmitting 202 a digital wireless broadcast signal 6. The digital wireless broadcast signal 6 comprises transmitter information 601 and timing information 602.

The transmitter information 601 is directly or indirectly indicative of a geographic position of a terrestrial transmitter 4 of the broadcast signal 6. The transmitter information 601 may comprise at least one of: a transmitter identifier, and a geographic transmitter position.

The timing information 602 is indicative of a timing of the broadcast signal 6. The timing information 602 may comprise at least one of: a timestamp of the transmission by the transmitter 4 of the broadcast signal 6, and a positioning reference signal (PRS) 607 mapped into the broadcast signal 6. A PRS as used herein may refer to a pseudo-random QPSK sequence that is mapped into time- frequency resources of a 4G/5G signal in diagonal patterns with shifts in frequency and time. PRS facilitate ranging measurements of a terminal (receiver) from eNodeB (transmitter) signals to improve positioning performance. PRS are transmitted in pre-defined “positioning occasions”. Positioning occasions occur with a periodicity TPRS (subframes, or milli-seconds) and then extend across a number NPRS of consecutive subframes. A starting subframe of PRS transmission, relative to SFN=0, is given by the cell specific PRS subframe offset APRS. A PRS configuration index IPRS defines the PRS transmission schedule (i.e., APRS and TPRS). A mapping of the PRS 607 to time- frequency resources of the broadcast signal 6 may depend on a cryptographic key. The mapping of the PRS 607 to time-frequency resources of the broadcast signal 6 may be made time-variant such that an ability to continuously track the PRS 607 requires knowledge and use of the cryptographic key. By deriving the PRS 607 time-frequency pattern from a pseudo-random sequence, usage may be restricted to subscribed users, and the navigation signal may be authenticated to harden the system against spoofing.

The broadcast signal 6 may comprise a 3 GPP multicast / broadcast signal. In particular, the broadcast signal 6 may comprise an FeMBMS signal 603 including the transmitter information 601 and the timing information 602, wherein the broadcast signal 6 may comprise DVB-T2 extension frames (FEF) 604 including the FeMBMS signal 603. Alternatively, the broadcast signal 6 may comprise an NB-IoT signal 605 including the transmitter information 601 and the timing information 602; and the broadcast signal 6 may comprise a guard band 606 of another broadcasting service (e.g., DVB-T2), the guard band 606 including the NB-IoT signal 605. Both concepts achieve a similar purpose, the NB-IoT approach being less spectrally efficient, however.

FeMBMS as used herein may refer to a 3GPP point-to-multipoint service for efficient cellular multicast/broadcast. The service involves deploying IP multicast, forming dynamic single- frequency networks (SFNs) across cells, and the like measures.

NB-IoT as used herein may refer to a 3GPP Low Power Wide Area Network (LPWAN) service for battery-efficient machine-to-machine (M2M) communication. NB-IoT derives its name from its support of bandwidth-limited receivers (200kHz). DVB-T2 as used herein may refer to an ETSI standard for broadcast transmission of digital terrestrial television and suited for carrying HDTV signals.

The receiver 5 on its part performs a method 3 of geographic positioning of the receiver 5 based on received positioning information.

The method 3 comprises a step of receiving 302 a respective digital wireless broadcast signal 6 at a respective time of arrival (TOA) from three or more time-synchronized terrestrial transmitters 4. The respective broadcast signal 6 comprises transmitter information 601 and timing information 602.

Time of arrival (TOA) as used herein may refer to an absolute time instant when a radio signal emanating from a transmitter reaches a remote receiver. TOA corresponds to a sum of a time of transmission (TOT) and a time of flight from the respective transmitter i (TOFi):

TOA, = TOT + TOF

Time difference of arrival (TDOA) as used herein may refer to a difference between TO As, which effectively cancels the TOT:

TDOA,, = TOA, TOA _j = TOT + TOF, (TOT + TOF _j ) = TOF, TOF _j .

Each TDOAi _j locates the receiver 5 on a hyperbola focused on the involved transmitters 4, and the geographic position of the receiver 5 is at an intersection of multiple hyperbolas.

The transmitter information 601 is directly or indirectly indicative of a geographic position of a terrestrial transmitter 4 of the broadcast signal 6. The transmitter information 601 may comprise at least one of: a transmitter identifier, and a geographic transmitter position. The timing information 602 is indicative of a timing of the broadcast signal 6. The timing information 602 may comprise at least one of: a timestamp of the transmission by the transmitter 4 of the broadcast signal 6, and a positioning reference signal (PRS) 607 mapped into the broadcast signal 6. A mapping of the PRS 607 to time-frequency resources of the broadcast signal 6 may depend on a cryptographic key.

The broadcast signal 6 may comprise a 3 GPP multicast / broadcast signal. In particular, the broadcast signal 6 may comprise an FeMBMS signal 603 including the transmitter information 601 and the timing information 602, wherein the broadcast signal 6 may comprise DVB-T2 extension frames (FEF) 604 including the FeMBMS signal 603. Alternatively, the broadcast signal 6 may comprise an NB-IoT signal 605 including the transmitter information 601 and the timing information 602; and the broadcast signal 6 may comprise a guard band 606 of another broadcasting service (e.g., DVB-T2), the guard band 606 including the NB-IoT signal 605.

The method 3 further comprises a step of calculating 303 a plurality of time differences of arrival (TDOAs) of each of the transmitters 4 but a reference transmitter 4 of the transmitters 4 in dependence of the respective timing information 602 and the respective TOA.

The method 3 may further comprise a step of receiving 304 a correction information indicative of a temporal correction of the timing information 602 of a respective broadcast signal 6. The correction information may depend on a ground conductivity, since its variations may influence a propagation of the broadcast signal 6. Ground conductivity may be detected by a network of terrestrial monitoring stations located at geographic positions in between the transmitters 4 and broadcasting or otherwise communicating a difference between their fixed geographic positions and the geographic positions indicated by the transmitters 4. Receivers 5 may correct their pseudo-range differences by the same amount.

The method 3 further comprises a step of determining 305 a geographic position of each of the transmitters 4 in dependence of the respective transmitter information 601. The determining 305 the geographic position of each of the transmitters 4 may comprise retrieving the respective geographic position of the respective transmitter 4 in dependence of the transmitter identifier, for example from a database hosted or cached by the receiver 5, or directly from the geographic transmitter position of the respective transmitter information 601.

The method 3 further comprises a step of calculating 306 a geographic position of the receiver 5 in dependence of the geographic positions of the transmitters 4 and the plurality of TDOAs. The calculating 306 the geographic position of the receiver 5 may comprise performing multilateration in dependence of the geographic positions of the transmitters 4 and a plurality of pseudo-range differences corresponding to the plurality of TDOAs (pseudo-range differences are obtained when multiplying TDOAs by the group velocity, i.e. the speed of propagation of an information carried by the broadcast signal 6).

Multilateration as used herein may refer to a technique for geographic positioning based on measurement of TOAs of energy waves having a known group velocity when propagating from multiple transmitters. These transmitters are at known geographic locations and have synchronized clocks. A minimum of d+1 TOAs are required to determine the receiver’s geographic position in d dimensions.

Collectively, the transmitters 4 and the receiver 5 forming the system 4, 5 perform a method 1 of geographic positioning of the receiver 5 based on communicated positioning information. The method 1 substantially comprises the steps already explained in connection with the methods 2, 3 above.

FIG. 2 - 5 schematically illustrate digital wireless broadcast signals 6 in accordance with the present disclosure; and

A digital wireless broadcast signal 6 comprises transmitter information 601 and timing information 602.

The transmitter information 601 is directly or indirectly indicative of a geographic position of a terrestrial transmitter 4 of the broadcast signal 6. The transmitter information 601 may comprise at least one of: a transmitter identifier, and a geographic transmitter position. The timing information 602 is indicative of a timing of the broadcast signal 6. The timing information 602 may comprise at least one of: a timestamp of the transmission by the transmitter 4 of the broadcast signal 6, and a positioning reference signal (PRS) 607 mapped into the broadcast signal 6. A mapping of the PRS 607 to time-frequency resources of the broadcast signal 6 may depend on a cryptographic key.

The broadcast signal 6 may comprise a 3GPP multicast / broadcast signal. In particular, the broadcast signal 6 may comprise an FeMBMS signal 603 including the transmitter information 601 and the timing information 602, wherein the broadcast signal 6 may comprise DVB-T2 extension frames (FEF) 604 including the FeMBMS signal 603. Alternatively, the broadcast signal 6 may comprise an NB-IoT signal 605 including the transmitter information 601 and the timing information 602; and the broadcast signal 6 may comprise a guard band 606 of another broadcasting service (e.g., DVB- T2), the guard band 606 including the NB-IoT signal 605.

FIG. 6 schematically illustrates a terrestrial transmitter 4 in accordance with the present disclosure.

A terrestrial transmitter 4 of a digital wireless broadcast signal 6 comprises a processing unit 401.

The processing unit 401 is configured to transmit 202 the digital wireless broadcast signal 6 comprising transmitter information 601 and timing information 602.

The transmitter information 601 is directly or indirectly indicative of a geographic position of a terrestrial transmitter 4 of the broadcast signal 6. The transmitter information 601 may comprise at least one of: a transmitter identifier, and a geographic transmitter position.

The timing information 602 is indicative of a timing of the broadcast signal 6. The timing information 602 may comprise at least one of: a timestamp of the transmission by the transmitter 4 of the broadcast signal 6, and a positioning reference signal (PRS) 607 mapped into the broadcast signal 6. A mapping of the PRS 607 to time-frequency resources of the broadcast signal 6 may depend on a cryptographic key.

The broadcast signal 6 may comprise a 3GPP multicast / broadcast signal. In particular, the broadcast signal 6 may comprise an FeMBMS signal 603 including the transmitter information 601 and the timing information 602, wherein the broadcast signal 6 may comprise DVB-T2 extension frames (FEF) 604 including the FeMBMS signal 603. Alternatively, the broadcast signal 6 may comprise an NB-IoT signal 605 including the transmitter information 601 and the timing information 602; and the broadcast signal 6 may comprise a guard band 606 of another broadcasting service (e.g., DVB- T2), the guard band 606 including the NB-IoT signal 605.

FIG. 7 schematically illustrates a receiver 5 in accordance with the present disclosure.

A receiver 5 of digital wireless broadcast signals 6 comprises a processing unit 501.

The processing unit 501 is configured to receive 302, from three or more time-synchronized terrestrial transmitters 4, a respective digital wireless broadcast signal 6 at a respective time of arrival (TO A). The respective broadcast signal 6 comprises transmitter information 601 and timing information 602.

The transmitter information 601 is directly or indirectly indicative of a geographic position of a terrestrial transmitter 4 of the broadcast signal 6. The transmitter information 601 may comprise at least one of: a transmitter identifier, and a geographic transmitter position.

The timing information 602 is indicative of a timing of the broadcast signal 6. The timing information 602 may comprise at least one of: a timestamp of the transmission by the transmitter 4 of the broadcast signal 6, and a positioning reference signal (PRS) 607 mapped into the broadcast signal 6. A mapping of the PRS 607 to time-frequency resources of the broadcast signal 6 may depend on a cryptographic key. The broadcast signal 6 may comprise a 3GPP multicast / broadcast signal. In particular, the broadcast signal 6 may comprise an FeMBMS signal 603 including the transmitter information 601 and the timing information 602, wherein the broadcast signal 6 may comprise DVB-T2 extension frames (FEF) 604 including the FeMBMS signal 603. Alternatively, the broadcast signal 6 may comprise an NB-IoT signal 605 including the transmitter information 601 and the timing information 602; and the broadcast signal 6 may comprise a guard band 606 of another broadcasting service (e.g., DVB- T2), the guard band 606 including the NB-IoT signal 605.

The processing unit 501 is further configured to calculate 303, in dependence of the respective timing information 602 and the respective TOA, a plurality of time differences of arrival (TDOAs) of each of the transmitters 4 but a reference transmitter 4 of the transmitters 4.

The processing unit 501 may further be configured to receive 304 a correction information indicative of a temporal correction of the timing information 602 of a respective broadcast signal 6. The correction information may depend on a ground conductivity detected at a geographic position in between the transmitters.

The processing unit 501 is further configured to determine 305, in dependence of the respective transmitter information 601, a geographic position of each of the transmitters 4. The determining 305 the geographic position of each of the transmitters 4 may comprise retrieving the respective geographic position of the respective transmitter 4 in dependence of the transmitter identifier, for example from a database hosted or cached by the receiver 5, or directly from the geographic transmitter position of the respective transmitter information 601.

The processing unit 501 is further configured to calculating 306 the geographic position of the receiver 5 in dependence of the geographic positions of the transmitters 4 and the plurality of TDOAs. The calculating 306 the geographic position of the receiver 5 may comprise performing multi lateration in dependence of the geographic positions of the transmitters 4 and a plurality of pseudo-range differences corresponding to the plurality of TDOAs.