Method and ADS-B base station for validating position information contained in a mode S extended squitter message (ADS-B) from an aircraft

Abstract

The invention refers to a method and a base station for validating information regarding the position of a target-aircraft, the information contained in an ADS-B signal periodically broadcast by the target-aircraft, with the method being executed in the ADS-B base station.

Claims

1. Method for validating information regarding the position of a target-aircraft, the information contained in an ADS-B signal periodically broadcast by a target-aircraft, the method being executed in an ADS-B base station and comprising the steps of: receiving the ADS-B signal from the target-aircraft at the base station, extracting the position information contained in the ADS-B signal, detecting, receiving and decoding an interrogation signal from a secondary surveillance source directed to the target-aircraft and detecting and receiving a reply signal transmitted by the target-aircraft in response to the interrogation signal, determining a time of arrival (TOA) of the received interrogation signal and of the received reply signal at the base station, based on the time of arrival (TOA) of the interrogation signal and on the position information, determining at least one expectation time window, in which the reply signal from the target-aircraft is expected to be received by the base station, determining whether the reply signal from the target-aircraft is received during one of the at least one expectation time window, if the reply signal from the target-aircraft is received by the base station during one of the at least one expectation time window, enhancing the confidence level of the position information contained in the ADS-B signal.

2. Method according to claim 1, wherein the received interrogation signal has been transmitted by another interrogator-aircraft.

3. Method according to claim 1, wherein the received interrogation signal has been transmitted by another base station.

4. Method according to claim 1, wherein separate expectation time windows are determined for at least one of each other base station and interrogator-aircraft within a region of interest for the base station and which could potentially have transmitted the received interrogation signal.

5. Method according to claim 1, wherein the interrogation signals and reply signals are transmitted as part of a Traffic Collision Avoidance System (TCAS) or an Aircraft Collision Avoidance System (ACAS), a Multilateration (MLAT) system or a Wide Area Multilateration (WAM) system.

6. Method according to claim 1, wherein the interrogation signal is transmitted at 1,030 MHz and the monitored reply signal is transmitted at 1,090 MHz.

7. Method according to claim 1, wherein the received interrogation signal has been transmitted by another interrogation-aircraft and wherein the at least one expectation time window, in which the reply signal from the target-aircraft is expected to be received by the base station, is determined further based on previously verified position information of the other transmitting interrogator-aircraft having an enhanced confidence level.

8. Method according to claim 1, wherein the expectation time window has a minimum length corresponding to an assumed response time of a transponder in the target-aircraft, which transmits the reply signal in response to the interrogation signal.

9. Method according to claim 8, wherein a position in time and a duration of the expectation time window are determined based on a response delay and a time sampling accuracy of the transponder.

10. Method according to claim 1, wherein the confidence levels of a plurality of aircraft within a region of interest for the base station are stored in a credibility matrix, to which the base station has access.

11. Method according to claim 10, wherein at least one other base station also executing the method according to one of the preceding claims has access to the credibility matrix, in order to update the content of the credibility matrix and to make use of the content of the credibility matrix when verifying information regarding the position of a target-aircraft (1, 2, 2), the information contained in an ADS-B signal periodically broadcast by the target aircraft.

12. Method according to claim 1, wherein information regarding the velocity of the target-aircraft is used for validating and if necessary correcting the position information extracted from the ADS-B signal or is used for estimating the position information if extraction of the position information from the ADS-B signal is not possible or has failed.

13. ADS-B base station adapted for receiving an ADS-B signal periodically broadcast by a target-aircraft and containing information regarding the position of the target-aircraft, wherein in order to validate the position information contained in the ADS-B signal, the base station comprising: receiving means for receiving the ADS-B signal from the target-aircraft, a first processing means for extracting the position information contained in the ADS-B signal, monitoring means for detecting, receiving and decoding an interrogation signal from a secondary surveillance source directed to the target-aircraft and for detecting and receiving a reply signal transmitted by the target-aircraft in response to the interrogation signal, a second processing means adapted for determining a time of arrival (TOA) of the received interrogation signal and of the received reply signal at the base station, the second processing means further adapted for determining at least one expectation time window, in which the reply signal from the target-aircraft is expected to be received by the base station, wherein the determination of the expectation time window is based on the time of arrival (TOA) of the received interrogation signal and on the position information contained in the ADS-B signal, the second processing means further adapted for determining whether the reply signal from the target-aircraft is received during one of the at least one expectation time window, the second processing means further adapted for enhancing the confidence level of the position information contained in the ADS-B signal, if the reply signal from the target-aircraft is received by the base station during one of the at least one expectation time window.

14. ADS-B base station according to claim 13, wherein the first and second processing means are adapted to execute the method for validating information regarding the position of a target-aircraft, the information contained in an ADS-B signal periodically broadcast by a target-aircraft, the method being executed in an ADS-B base station and comprising the steps of: receiving the ADS-B signal from the target-aircraft at the base station, extracting the position information contained in the ADS-B signal, detecting, receiving and decoding an interrogation signal from a secondary surveillance source directed to the target-aircraft and detecting and receiving a reply signal transmitted by the target-aircraft in response to the interrogation signal, determining a time of arrival (TOA) of the received interrogation signal and of the received reply signal at the base station, based on the time of arrival (TOA) of the interrogation signal and on the position information, determining at least one expectation time window, in which the reply signal from the target-aircraft is expected to be received by the base station, determining whether the reply signal from the target-aircraft is received during one of the at least one expectation time window, if the reply signal from the target-aircraft is received by the base station during one of the at least one expectation time window, enhancing the confidence level of the position information contained in the ADS-B signal wherein separate expectation time windows are determined for at least one of each other base station and interrogator-aircraft within a region of interest for the base station and which could potentially have transmitted the received interrogation signal.

15. ADS-B base station according to claim 13, wherein the base station is ground based or located on-board a satellite.

16. ADS-B base station according to claim 13, wherein the base station is in connection with at least one other base station in order to exchange credibility information regarding the confidence level of the position information (X.sub.1, Y.sub.1; X.sub.2, Y.sub.2; X.sub.3, Y.sub.3) transmitted by various aircraft in their respective ADS-B signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the present invention are described in more detail in the following description of a preferred embodiment of the invention making reference to the enclosed drawings. The figures show:

(2) FIG. 1 a first possible scenario for realizing a preferred embodiment of the present invention, wherein the SSR interrogator is a TCAS/ACAS enabled aircraft;

(3) FIG. 2 a flowchart of a method according to a preferred embodiment the present invention;

(4) FIG. 3 a second possible scenario for realizing another preferred embodiment of the present invention, wherein the SSR interrogator is a base station different from the ADS-B base station;

(5) FIG. 4 a third possible scenario for realizing another preferred embodiment of the present invention, wherein the SSR interrogator is the ADS-B base station; and

(6) FIG. 5 a base station according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION

(7) In FIG. 1 a possible scenario for realizing the present invention is shown in the two dimensional plane. Of course, it is understood that the present invention will be realized in the three dimensional space. However, for the purpose of easier explanation, the invention will be described hereinafter in the two dimensional plane. The scenario comprises an ADS-B base station 10 located on the ground. Of course, base station 10 could also be located in space on-board a satellite. The satellite could be a LEO, MEO, GSO or a GEO satellite. Base station 10 is operated by an Air Navigation Service Partner (ANSP), like the German ANSP, Deutsche Flugsicherung (DFS), the Airservices Australia and the US Federal Aviation Administration (FAA), for example. Furthermore, FIG. 1 shows an example of three aircraft 1, 2, 3 within the range of interest 23 of the base station 10. An ADS-B base station 10 has a range of interest 23 of 250 nm (463 km) or more.

(8) It is assumed that aircraft 1 is a target-aircraft and transmits an ADS-B signal 20, which among others is also received by the base station 10. The ADS-B signal 20 is preferably a 1,090 MHz Extended Squitter (ES), for example transmitted in a downlink format DF17. The ADS-B signal 20 comprises position information X.sub.1, Y.sub.1 relating to the aircraft 1. The position information X.sub.1, Y.sub.1 may be determined on board the aircraft 1 by many means, such as global navigation satellite system (GNSS), for example NAVSTAR GPS (USA), GLONASS (Russia), COMPASS (China) or GALILEO (Europe). Of course, any GNSS yet to come could also be used to determine the position information of the aircraft 1. The position information is then inserted into the ADS-B signals 20 and broadcast by means of a transponder on board the target-aircraft 1.

(9) The method according to the present invention shown in FIG. 2 starts at step 100. In step 102 the base station 10 receives the ADS-B signals 20 from aircraft 1 and in step 104 extracts the position information (X.sub.1, Y.sub.1) contained therein. The ADS-B signal 20 is preferably a 1,090 MHz Extended Squitter (1090 ES) periodically transmitted, preferably in a downlink format DF17, by an appropriate transponder on-board the aircraft 1. The base station 10 uses the position information for tracking the aircraft 1 and for realizing air traffic management (ATM) within the base station's region of interest 23. However, the base station 10 has no information whatsoever regarding the correctness and trustworthiness of position information received from aircraft 1 via the ADS-B signals 20. The present invention proposes a method and a base station 10 with an enhanced functionality allowing a verification or validation of the ADS-B position information of aircraft 1.

(10) The general idea of the present invention is to receive any kind of interrogation and reply signals from any type of secondary surveillance source, for example making part of an SSR system, for validating the position information (X.sub.1, Y.sub.1) contained in the ADS-B signals 20 received by the base station 10. The SSR system may be, for example, a Traffic Collision Avoidance System (TCAS) or an Aircraft Collision Avoidance System (ACAS), a Multilateration (MLAT) system or a Wide Area Multilateration (WAM) system. All these SSR systems transmit and receive interrogation and reply signals among the participating SSR devices. The interrogators may be any kind of appropriate SSR interrogator, such as an interrogator-aircraft 2, 3 (FIG. 1) and/or a base station 11 different from the ADS-B base station 10 (FIG. 3). The interrogation signals 21 and reply signals 22 are preferably transmitted omni-directional by the respective interrogators of the secondary surveillance source. According to a preferred embodiment of the invention shown in FIG. 1, it is suggested that the interrogation and reply signals make part of a TCAS/ACAS system.

(11) In the case of a TCAS/ACAS system the monitored interrogation signal 21 is transmitted at 1,030 MHz and the monitored reply signal 22 is transmitted at 1,090 MHz. This is typically the frequency range in which TCAS/ACAS signals defined in ICAO DOC 9863 are transmitted. The TCAS/ACAS interrogation signals 21 typically use uplink formats UFO (for tracking) and/or UF16 (for conflict resolution). The respective TCAS/ACAS reply signals 22 typically use the corresponding downlink formats DF0 or DF16. TCAS/ACAS interrogation signals 21 are transmitted by interrogator-aircraft to selected other target-aircraft which have found to be in a nominal range of interest, e.g. 14 nm (=25,928 km). The interrogation signals 21 are directed to the specific target-aircraft (in the example to aircraft 1) and contain its address. The interrogation signals 21 provoke the target-aircraft 1, to which the signal is directed, to respond with a reply signal 22 directed to the interrogator-aircraft 2; 3 which transmitted the interrogation signal 21. However, the interrogation and reply signals 21, 22 can be received by any appropriate receiver within reach, too. The interrogation signals 21 contain no identification information of the interrogator. The reply signal 22 contains information regarding the flight level of the target-aircraft 1 transmitting the response signal 22. For the sake of the present invention, not the content of the interrogation and/or reply signals 21, 22 is important but rather the time of arrival (TOA) of the signals 21, 22 at the receiving base station 10. In the embodiment of FIG. 1 the interrogation signal 21 is part of a TCAS/ACAS system and is transmitted by interrogator-aircraft 3 and addressed to target-aircraft 1.

(12) In step 106 of the method according to the present invention the interrogation signal 21 is received by base station 10. To that end, the base station 10 is equipped with appropriate receiving means adapted for receiving the 1,030 MHz signal in the uplink format UFO or UF16, respectively. In step 108 processing means of the base station 10 determine the time of arrival (TOA) of the interrogation signal 21. The base station 10 has no information regarding the origin of the interrogation signal 21, that is which interrogator transmitted the signal 21. Therefore, expectation time windows (or expected response windows, ERW) are determined in step 110 for each of the possible interrogators, that is aircraft 2 and aircraft 3, which may have transmitted the interrogation signal 21. The expectation time windows represent estimated time windows, during which a reply signal 22 form target-aircraft 1 in response to the interrogation signal 21 is expected to be received by the base station 10, based on the assumption that the position information X.sub.1, Y.sub.1 previously received by the base station 10 via the ADS-B signal 20 from target-aircraft 1 is correct. For calculating the expectation time windows various parameters may be considered, comprising: the exact position X.sub.S, Y.sub.S (in respect to the earth's surface) of base station 10 receiving the ADS-B signal 20 from target-aircraft 1 and the interrogation signal 21 from the interrogator of the secondary surveillance source (interrogator-aircraft 3), the position information X.sub.1, Y.sub.1 (in respect to the earth's surface) of target-aircraft 1 contained in the ADS-B signal 20, the position X.sub.2, Y.sub.2; X.sub.3, Y.sub.3 (in respect to the earth's surface) of the possible interrogators of the secondary surveillance source(s) (interrogator-aircraft 2 and/or interrogator-aircraft 3) possibly having transmitted the interrogation signal 21 received by the base station 10, the time of arrival (TOA) of the interrogation signal 21 at the base station 10, and the address of an aircraft (target-aircraft 1), to which the interrogation signal 21 received by the base station 10 is addressed, the address contained in the interrogation signal 21.

(13) From the determined TOA of the interrogation signal 21 at the base station 10 it is possible to work back to an assumed time of interrogation (TOI) of the interrogation signal 21 for the one or more possible interrogators (aircraft 2 or aircraft 3) of the secondary surveillance source within the region of interest of target-aircraft 1, which previously transmitted the ADS-B signal 20. The one or more possible interrogators (interrogator-aircraft 2 or interrogator-aircraft 3), for which the assumed TOI is determined, may possibly have transmitted the interrogation signal 21, received by the base station 10. If the received interrogation signal 21 is addressed to the target-aircraft 1, which previously transmitted the ADS-B signal 20, the target-aircraft 1 is expected to send a reply signal 22 in response to the interrogation signal 21 soon. In particular, based on the assumption that all position information X.sub.S, Y.sub.S; X.sub.2, Y.sub.2; X.sub.3, Y.sub.3 available at the base station 10 is correct, the reply signal 22 from the target-aircraft 1, which previously transmitted the ADS-B signal 20, can be expected to be received at the base station 10 at a certain point in time or within a certain expectation time window (ERW), depending on the position of the target-aircraft 1, the position of the interrogator (interrogator-aircraft 3) of the secondary surveillance source, which transmitted the interrogation signal 21, and the TOA of the interrogation signal 21 at the base station 10.

(14) In step 112 the base station 10 determines whether the reply signal 22 from target-aircraft 1 is received within the previously determined expectation time window (ERW). If the reply signal 22 is indeed received by the base station 10 within the previously determined expectation time window (yes), one can go on the assumption that the position information X.sub.1, Y.sub.1 of the target-aircraft 1, which transmitted the ADS-B signal 20, and possibly also the position information X.sub.3, Y.sub.3 of the interrogator-aircraft 3 of the secondary surveillance source, which transmitted the interrogation signal 21, are correct (step 114). As time goes on and as the method for validation according to the present invention has been performed more and more times for the same target-aircraft 1, the position information X.sub.1, Y.sub.1 contained in the ADS-B signal 20 received from that target-aircraft 1 is assigned an increasingly high level of confidence. As time goes on and as the method for validation according to the present invention has been performed for more and more different target-aircraft 1, 2, 3 the position information X.sub.1, Y.sub.1; X.sub.2, Y.sub.2, X.sub.3, Y.sub.3 contained in the ADS-B signals 20 from an increasing number of target-aircraft 1, 2, 3 within the region of interest 23 of the base station 10 is validated. In step 116 it is determined whether the method has reached the end. If so, in step 118 the method is terminated. If not, another iteration of the method, for the same or another target-aircraft i (i=1, . . . , n) within the range of interest 23 of base station 10 is executed.

(15) The following time delays or inaccuracies may be used/considered when determining the expectation time windows, which are ADS-B Mode S transponder ICAO defined delays: 128 s (128.Math.10.sup.6 s) transponder reply, 0.5 s (5.Math.10.sup.7 s) transponder uncertainty, 0.08 s (8.Math.10.sup.8 s) transponder reply delay jitter, 7 ns (7.Math.10.sup.9 s) time stamp accuracy,
wherein the speed of travel of the signal is 299,792,458 m/s. Of course, in practice for example the time stamp accuracy could vary from the indicated value. Furthermore, the above value for the speed of travel of the signal is indicated for vacuum. The speed of travel could vary from the indicated value, for example if there is no real vacuum between the sender and the receiver. The values for the possible transponder delays and consequently the calculation of the expectation time window(s) would have to be adapted to the actual circumstances in each single case.

(16) Based on the determined TOA of the interrogation signal 21 at the base station 10 and considering one or more of the above mentioned time delays and inaccuracies, together with the (verified or unverified) positions X.sub.1, Y.sub.1; X.sub.2, Y.sub.2, X.sub.3, Y.sub.3 of the interrogators and target (aircraft 1, 2, 3) of the secondary surveillance source, the following basic formulae can be used:
Ait=(Ts+Tsa),
wherein
Ait=Assumed Interrogation Time (from 0 seconds),
Ts=Travel time to sensor (at base station 10),
Tsa=Time stamping accuracy of the sensor.
ERW=Ait+Ts+Rd+Td+Tsa,
wherein
ERW=Expected response window(=expectation time window),
Ait=Assumed Interrogation Time (starting from 0 seconds),
Ts=Travel time to sensor,
Td=Travel to destination target (aircraft 1),
Tsa=Time stamping accuracy of the sensor.

(17) As mentioned above, the indicated lengths of the expectation time windows have been determined to be the minimum value corresponding to the jitter of the transponder on-board the target-aircraft 1. In a more practical approach, the lengths of the expectation time windows would be longer, approximately in the range of 1 s (1.Math.10.sup.6 s) to 5 s (5.Math.10.sup.6 s).

(18) Using this information, it can be verified whether the reply signal 22 from the target-aircraft 1 in response to the interrogation signal 21 transmitted by an interrogator-aircraft, aircraft 2 or aircraft 3 (the base station 10 does not yet know, which of the two possible interrogator-aircraft 2, 3 actually transmitted the interrogation signal 21 addressed to the target-aircraft 1), is received within one of the previously defined expectation time windows. Hence, if there are a plurality of target-aircraft within the range of interest 23 of base station 10 for which the respective position information transmitted by the respective target has to be validated, a corresponding number of tables would be generated, one for each target-aircraft.

(19) If the reply signal 22 from the target-aircraft 1 in response to the interrogation signal 21 transmitted by an interrogator-aircraft, aircraft 2 or aircraft 3, is received within one of the previously defined expectation time windows, base station 10 has information regarding which of the two possible interrogation-aircraft 2, 3 actually transmitted the interrogation signal 21 addressed to the target-aircraft 1 and further the ADS-B position information previously received from target-aircraft 1 via the ADS-B signal 20 can be regarded as being correct and can be trusted for future ATM calculations. Consequently, the level of confidence of the ADS-B position information transmitted by target-aircraft 1 via the ADS-B signal 20 can be increased. The target-aircraft's ADS-B position information has been successfully validated.

(20) The data regarding the validation of the various ADS-B position information received in the ADS-B signals 20 from the various aircraft 1, 2, 3 within the range of interest 23 of base station 10 can be entered into and updated in a credibility matrix, for example stored in a database 14 (see FIGS. 4 and 5). At the beginning of the method according to the present invention the credibility matrix can have the following content:

(21) TABLE-US-00001 Position information validated Aircraft 1 Aircraft 2 Aircraft 3 No X X Yes X

(22) After the described above and considering that the target-aircraft 1 is validated based on the interrogation reply 22 being received within the ERW if interrogator-aircraft 3 is the aircraft that sent the interrogation signal 21, that was addressed for the target-aircraft 1. The embodied invention has not validated aircraft 1 position received in the ADS-B signal 20 that it transmitted and was received at ADS-B base station 10. In addition, because the ADS-B base station 10 determined the ERW for the instance where interrogator-aircraft 3 is the interrogator that sent the interrogation signal 21 with the address of the target-aircraft 1 and the position of the interrogator-aircraft 3, that was transmitted via its ADS-B signal 20, then the interrogator-aircraft 3 is validated. The following table could be updated based on this signal iteration.

(23) TABLE-US-00002 Position information validated Aircraft (20) Aircraft (21) Aircraft (22) No X Yes X X

(24) If thereafter, during one of the following iterations of the method according to the present invention, the ADS-B position information received via the ADS-B signal 20 from aircraft 1 is successfully validated, the matrix could have the following content:

(25) TABLE-US-00003 Position information validated Aircraft 1 Aircraft 2 Aircraft 3 No Yes X X X

(26) The content of the credibility matrix is highly dynamic. It cannot only vary regarding the validation of the ADS-B position information received from the aircraft 1, 2, 3, but it can also vary regarding the considered aircraft. For example, with time one of the aircraft, for example aircraft 2, can leave the range of interest 23 of base station 10 and new aircraft, for example aircraft 4 and 5, can enter the range of interest 23. This would result in the following content of the credibility matrix:

(27) TABLE-US-00004 Position information validated Aircraft 1 Aircraft 3 Aircraft 4 Aircraft 5 No X X Yes X X

(28) Instead of only Yes or No the confidence level of the ADS-B position information can also comprise a plurality of different levels, for example 0 (binary 00), 1 (01), 2 (10) and 3 (11). It is assumed that at the beginning of the method according to the present invention the ADS-B position information from aircraft 1 has not yet been validated at all (0), the position information from aircraft 2 has been successfully validated once (1) and the position information from aircraft 3 has already been fully validated (3). In that case the credibility matrix would have the following content:

(29) TABLE-US-00005 Confidence level Aircraft 1 Aircraft 2 Aircraft 3 0 X 1 X 2 3 X

(30) Each time an iteration of the method according to the present invention has been successfully executed and ADS-B position information from an aircraft successfully validated, the confidence level of the position information for that aircraft is increased by 1. It is possible that each time an iteration of the method according to the present invention has been unsuccessfully executed and ADS-B position information from an aircraft could not be validated, the confidence level of the position information for that aircraft is decreased by 1. It is assumed that after various iterations of the method according to the present invention the position information from aircraft 1 and aircraft 2 has been fully validated (3), whereas the validation of position information received from aircraft 3 has failed once (3-1=2). In that case the credibility matrix would have the following content:

(31) TABLE-US-00006 Confidence level Aircraft 1 Aircraft 2 Aircraft 3 0 1 2 X 3 X X

(32) In the above description the present invention has been described in the context of aircraft 2, 3 as possible interrogators (interrogator-aircraft) of a secondary surveillance source transmitting the interrogation signals 21. Of course, other interrogators of different secondary surveillance sources could be used, too, for transmitting interrogation signals 21, which are received by base station 10 performing the verification and validation of the ADS-B position information received from target-aircraft 1. For example, the interrogators could be other (independent) ground based transmitter stations 11 as suggested in the embodiment of FIG. 3. In FIG. 3 the same reference signs are used for the same components of the invention as in FIG. 1. Of course, it would also be possible to use a combination of different secondary surveillance sources (e.g. TCAS/ACAS and MLAT/WAM ground stations), like another base station 11 and/or another aircraft 2 or 3, transmitting interrogation signals 21, in order to verify or validate the ADS-B position information contained in the broadcast signal 20 received from target-aircraft 1. Of course, the other independent transmitter stations (e.g. base station 11) transmitting the interrogation signals 21 could also be space based, for example mounted on one or more satellites.

(33) FIG. 4 shows an embodiment of a scenario for realizing the present invention, in which the aircraft 3 is the target-aircraft. To this end, in this scenario the position information X.sub.3, Y.sub.3 of the target-aircraft 3 is validated. The aircraft 1 acts as the SSR interrogator. In particular, it is suggested that interrogator-aircraft 1 is a TCAS/ACAS enabled aircraft which transmits interrogation signals 21 addressed to the target-aircraft 3. The interrogation signals 21 are also received by base station 10. In response to the interrogator signal 21 the target-aircraft 3 transmits one or more reply signals 22, which are received by the interrogator-aircraft 1 as well as by the base station 10. The base station 10 determines the expectation time window(s) and validates the position information X.sub.3, Y.sub.3 of the target-aircraft 3 contained in the ADS-B signal 20. Each of the aircraft 1, 2, 3 periodically broadcasts ADS-B signals 20, which are received by the base station 10 as well as by other base stations 11. Both base stations 10, 11 have access to a common database 14, where the credibility matrix is stored.

(34) FIG. 5 shows an embodiment of a base station 10 according to the present invention in more detail. The base station 10 comprises receiving means 12a comprising an antenna for receiving the ADS-B signal 20 from any capable ADS-B aircraft. Furthermore, the base station 10 comprises processing means 12b for pre-processing, for example decoding, the received ADS-B signal 20 and for extracting the position information X.sub.1, Y.sub.1 contained therein. Further, the base station 10 comprises monitoring means for detecting, receiving and decoding an interrogation signal 21 transmitted by a secondary surveillance source (ACAS/TCAS aircraft 2, 3 or other base station 11) and for detecting and receiving, possibly also decoding, a reply signal 22 transmitted by the target-aircraft 1 in response to the interrogation signal 21. The monitoring means for receiving the ADS-B Signal 20, the interrogation signal 21 and the response signal 22 can comprise a single antenna structure. However, for simplicity in the diagram separate antennae are shown. In particular, the base station 10 further comprise an antenna 12c and processing means 12d for pre-processing, for example decoding, the received interrogation signal 21. The interrogation signal 21 may be transmitted in the 1,030 MHz frequency range. Further, the monitoring means comprise an antenna and processing means for the reply signal 22, which may be identical to the antenna 12a and the processing means 12b for the ADS-B signal 20. This is possible if both signals 20, 22 are transmitted in the 1,090 MHz frequency range. Further, the base station 10 has processing means 13 for determining the TOA of the received interrogation signal 21 and the received reply signal 22. The processing means 13 are also adapted for determining the expectation time window and for verifying whether the reply signal 22 is actually received by the base station 10 within the expectation time window. Finally, depending on the outcome of this verification, the processing means 13 increase (or decrease) the confidence level of the position information X.sub.1, Y.sub.1 contained in the ADS-B signal 20 and update the content of the credibility matrix stored in database 14. It is noted that the processing means 12b, 12d, 13 could be integrated a single processing apparatus comprising the processing functionalities of all processing means 12b, 12d, 13.

(35) Summing up, there are several known types of systems for aircraft surveillance in the world. The known systems are cooperative or non-cooperative systems. A system is cooperative if the target needs any equipment so the surveillance system can work. So known primary radar is a non-cooperative technology, whereas secondary surveillance is a cooperative technology. In secondary surveillance (cooperative) the aircraft must have a transponder. Further, a system is dependent or independent. This indicates which system makes the position calculation. Primary Radar, Monopulse Secondary Service Radar (MSSR) as well as MLAT/WAM are independent technologies, i.e. the sensors/system makes the calculation. The dependent technology is thus a system where the position is determined by the target, which makes ADS-B a dependent technology.

(36) When an operator (ANSP: Air Navigation Service Provider) looks at using any information, whether it is a primary radar return or a secondary radar return, operators typically prefer multiple services. Very few operators use only a single source. But at least with Primary Radar, MSSR or MLAT/WAM operators consider these as trustworthy sources, mainly because the operators deploy, test and maintain these sources.

(37) One of the problems with ADS-B from an operator's point of view is its lack of trustworthiness because the operator does not deploy the equipment that makes the position calculation, nor does he maintain it etc. So as ADS-B is adopted, most operators will deploy other systems, like Primary Radar, MSSR or MLAT/WAM to be a second source of data to confirm the position of the ADS-B feed.

(38) Another issue with ADS-B is the potential to spoof targets; it is rather simple to build a home based transmitter, that would send out a target or a thousand fake targets into an environment. One can easily imagine what would happen around a civilian airport if a thousand fake targets would show up. The airspace would shut down because the controllers would need to verify if the targets are real or not.

(39) One of the concepts for ADS-B in the future is that an operator only needs ADS-B stations, no MSSRs, no WAMIMLAT even no PSRs. An ADS-B station is rather inexpensive compared to a full PSR, MSSR or even a MLAT/WAM (MLAT/WAM is comprised of several sensors typically the minimum is 5 sensors.) The present invention provides for a method to validate position information contained in the ADS-B signal with one and the same single ADS-B sensor. It represents a great cost saving to a customer and keeps the ideal concept of a single ADS-B station to cover one complete airspace. By using other sources of interrogation like described above, especially ACAS/TCAS and WAM/MLAT, the pieces of equipment can still be kept down to one box and at the same time still provide a validated or trustworthy ADS-B position information.