How to manage secondary radars operating in Mode S to avoid BDS swap issues

Abstract

To provide a method for managing a secondary radar operating in mode S for avoiding a BDS swap problem.SOLUTION: A method for managing a secondary radar operating in mode S, the method including the detection comprises: a) detection in a "search mode", the "search mode" to be executed until an aircraft is detected by a secondary radar; b) detection in a "tracking mode", the "tracking mode" to be executed when a right response to a roll call is detected in the "searching mode". The method comprises an intermediate step a1) to be executed between the detection in the "search mode" and the detection in the "tracking mode", the intermediate step including: detecting presence or absence of the aircraft's response in a noise window of the secondary radar; and when the aircraft's response is not specified in the noise window, executing at least one roll call using a first monitoring window.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates generally to radar systems, and more particularly to a method and apparatus for managing a secondary radar operating in Mode S. [Background technology]

[0002] Secondary radars are used in air traffic control (ATC). In contrast to primary radars, which use electromagnetic wave reflections from aircraft, secondary radars interact with transponders onboard aircraft or IFF responders on military aircraft by interrogating them to obtain at least the aircraft's identification (Code 3 / A) and altitude (Code C) and to determine its position.

[0003] In first generation secondary radars (Mode AC), the transponder responded uniformly to all secondary radar interrogations, resulting in a lot of RF pollution or interference. Furthermore, the content of the response was limited by the number of data bits (at most 12 bits).

[0004] Mode S (S stands for selective) is now widely implemented and allows for interrogation and tracking of multiple aircraft within a radar coverage area while reducing RF pollution.

[0005] Like first-generation secondary radars, Mode S secondary radars receive aircraft altitude and address when they detect a response, and then determine the aircraft's location. The radar can also receive information about aircraft type, speed, destination, etc. This additional data is presented in the form of registers (or BDSs in the case of the Comm-B Data Selector), identified by number and available on request (one BDS per interrogation).

[0006] In Mode S, interrogations are either general interrogations or selective interrogations. In a general interrogation, the radar asks all transponders located within its coverage area to provide their 24-bit Mode S addresses, i.e., their identification numbers.

[0007] If the radar acquires an aircraft, i.e., if its position is predictable, then with each new antenna rotation, it first interrogates the aircraft within a large spatial window in roll-call mode to accommodate the uncertainty associated with the prediction; this phase is hereafter referred to as "search mode." By obtaining this first precise location of the aircraft, the radar can reduce the size of the interrogation window used for subsequent roll-call interrogations, freeing up more space / time for interrogating other aircraft within its surveillance area. This second phase is hereafter referred to as "track mode."

[0008] FIG. 1 shows the reduction in the size of the window in "track mode" for a given antenna rotation, this window being centered on the aircraft detected in "search mode."

[0009] If acquisition of an aircraft is confirmed, the radar will lock out the target aircraft's transponder for 18 seconds. The locked-out transponder of the aircraft will no longer respond to all call interrogations from interrogating radars with the same radar station code as the radar that locked out the aircraft. This lockout is refreshed (the lockout time is reset, i.e., a new lockout lasting 18 seconds is initiated) with each roll call interrogation that includes a lockout command.

[0010] If two radars transmit interrogations to a given transponder and the interrogations arrive at the transponder at approximately the same time, the transponder will only emit one response to the two interrogations.

[0011] The radar whose interrogation is recognized by the transponder (Radar 1) receives a response (including a BDS) that matches its own interrogation. In contrast, the secondary radar whose interrogation is ignored (Radar 2) receives an identical (and therefore incorrect) response that corresponds to Radar 1's interrogation, not its own. Radar 2 also never receives a response to the interrogation it sent.

[0012] Such a problem is called a BDS swap. The BDS swapping problem is observed in areas with a high density of Mode S secondary radars. This problem impacts the effectiveness of the surveillance system and must be corrected. Known automated systems for managing air traffic can handle such a malfunction, but only after multiple rotations of the antenna (i.e., multiple rotations of the moving part of the antenna).

[0013] The above mentioned problems and consequences have been detailed by the European Aviation Safety Agency in its document EASA.2016.FC19 SC.001, in particular if the radar uses inaccurate data it may lead to erroneous acceleration or changes in flight path.

[0014] Figure 2 shows the consequences of a BDS swap when inaccurate data causes the radar to err on the side of the actual distance to the aircraft. The radar detects the erroneous BDS response and derives an incorrect distance from it. The size of the monitoring window in "tracking mode" therefore shrinks, but only to the extent of this erroneous distance. In the same antenna rotation, the radar continues to interrogate the transponder, but ignores its replies because they do not fall within the correct monitoring window.

[0015] On subsequent antenna rotations, the radar continues to interrogate the transponder within a prediction window calculated based on the previous position, but at an incorrect location. As shown in Figure 3, this predicted location is so far from the true path that the radar has no chance of reacquiring the transponder (e.g., it appears to the radar that the target has suddenly made a 90° turn).

[0016] The transponder will not respond to all interrogations because it is locked out, so the radar cannot relocate it. It must wait for the transponder lockout to end and for the radar to acquire the aircraft again.

[0017] The prior art provides solutions to the problem of BDS swaps.

[0018] The first solution involves verifying the integrity of the UF / DF messages (UF / DF stands for uplink format / downlink format): the interrogation (UF message) sent from the radar to the transponder, and the response (DF message) sent from the transponder to the radar.

[0019] If the response does not correspond to the request, the response is rejected.

[0020] For example, for an interrogation UF=4 (monitoring, altitude request), only a response DF=4 (monitoring, altitude response) or a response DF=20 (Comm-B, altitude response) is expected. For an interrogation UF=5 (monitoring, request identification), only a response DF=5 (monitoring, response identification) or a response DF=21 (Comm-B, response identification) is expected.

[0021] However, this solution does not avoid the BDS swap problem when two interrogating devices request the same information, which unfortunately is not uncommon in air traffic control.

[0022] The second solution involves verifying the integrity of the first byte of register BDS1,0, register BDS2,0 or even register BDS3,0. In the case of these registers only, if the BDS number can be extracted from the response, the processing pipeline verifies whether the challenge was targeted at these BDSs.

[0023] However, this solution is not applicable to all BDS registers.

[0024] A third known solution involves including the number of BDSs in the parity code of the DF response containing the BDS returned by the transponder. Thus, when the interrogator receives the response, it can verify by a parity check whether the response corresponds to the expected BDS register. This solution corresponds to a technique called BDS overlay, introduced in Amendment 89 of ICAO Annex 10 Volume IV.

[0025] However, the implementation of this solution is limited by the need to update the transponder software.

[0026] Document D1, “Data Integrity Augmentation by ADS-B SSR Hybrid Techniques” (Mariano et al., 2018 Integrated Communications, Navigation and Surveillance Conference), presents an interactive surveillance solution for air traffic management systems. However, document D1 does not seek to solve the BDS swapping problem.

[0027] Document D2 “Autonomous Continuous Target Tracking Technology for Safety in Air Traffic Radar Systems Network” (Koga et al., 2011 IEEE 6th International Symposium on Service Oriented System Engineering) describes aircraft tracking using two radars, but does not seek to solve the BDS swapping problem. [Prior art documents] [Non-patent literature]

[0028] [Non-Patent Document 1] EASA.2016.FC19 SC.001 [Non-patent document 2] Amendment 89 of ICAO Annex 10 Volume IV [Non-patent document 3] “Data Integrity Augmentation by ADS-B SSR Hybrid Techniques”(Mariano et al.,2018 Integrated Communications, Navigation and Surveillance Conference) [Non-patent document 4] “Autonomous Continuous Target Tracking Technology for Safety in Air Traffic Radar Systems Network”(Koga et al.,2011 IEEE 6th International Symposium on Service Oriented System Engineering) Summary of the Invention [Problem to be solved by the invention]

[0029] Therefore, there is a need for an improved method and apparatus for managing secondary radar. [Means for solving the problem]

[0030] The subject of the present invention is therefore a method for managing a secondary radar operating in Mode S, comprising: a) detection in a "search mode," the "search mode" being performed until an aircraft is detected by a secondary radar, the "search mode" including a plurality of full-call secondary radar interrogation periods and a plurality of roll-call interrogation periods in a first monitoring window; b) detection in a "track mode," which is implemented when a valid response to the roll call interrogation is detected in the "search mode," and which includes a plurality of full call interrogation periods and a plurality of roll call interrogation periods in a second monitoring window, the second monitoring window being around the aircraft position predicted by the secondary radar in response to the full call interrogation; In a method comprising the steps of: Detecting the presence or absence of an aircraft response within the noise window of the secondary radar; if no aircraft response is identified within the noise window, issuing at least one roll call interrogation using a first monitoring window. The method is characterized by comprising:

[0031] The method advantageously includes the following embodiments:

[0032] The secondary radar includes an antenna, and the first surveillance window is calculated for a given aircraft based on the rotation rate of the antenna of the secondary radar and the predicted aircraft position for a subsequent antenna rotation.

[0033] A predicted aircraft position is calculated based on all possible aircraft positions, which are calculated based on the known path of the aircraft and rate-related aircraft parameters received by the secondary radar.

[0034] The noise window is calculated based on the difference between the ideal path of the aircraft and the aircraft position predicted by the secondary radar.

[0035] The ideal path of the aircraft is calculated based on a set of assumptions that the aircraft flies along a straight line path at a constant aircraft speed.

[0036] A predicted aircraft position is calculated based on the set of calculated discrepancies, where the discrepancies correspond to the difference between the estimate of the aircraft position and the aircraft position measured in one of the preceding rotations, starting from when the aircraft was first detected.

[0037] The discrepancy is integrated over time using a recursive filter.

[0038] The recursive filter is an extended Kalman filter.

[0039] The predicted aircraft position is calculated based on the radar measurement error.

[0040] The invention also relates to a secondary radar configured to implement the method described above.

[0041] Other characteristics, details and advantages of the invention will become apparent on reading the description given with reference to the accompanying drawings, given as an example, in which: [Brief explanation of the drawings]

[0042] [Figure 1] Already described is the reduction of the interrogation window between "search mode" and "track mode" in the nominal case (no BDS swap). [Figure 2] Already described is the reduction of the interrogation window between "search mode" and "track mode" in one failure case (following a BDS swap). [Figure 3] Already mentioned are the occurrence of BDS swaps and incorrect aircraft positions. [Figure 4] This is the method according to the present invention. [Figure 5] 1 is a representation of various windows used in the method according to the invention and the responses and associated plots. [Figure 6] This is an example of a new call being made. DETAILED DESCRIPTION OF THE INVENTION

[0043] The method according to the present invention is illustrated in FIG.

[0044] In the first step a), a "search mode" is implemented until an aircraft is detected by the secondary radar.

[0045] As shown above, the first monitoring window used in "search mode" is quite large to accommodate both the rate-related constraints specified in the European standard for Mode S and the estimated uncertainty in the actual position of the aircraft.

[0046] The first monitoring window may be calculated for a given aircraft based on the rotation rate of the secondary radar's antenna and the predicted aircraft position for subsequent antenna rotations.

[0047] The monitoring window shown in FIG. 5 includes several independent elements: a prediction window and a noise window.

[0048] A prediction window is constructed around the predicted aircraft position and takes into account the rate-related change criteria required by the European standard for Mode S, such as lateral acceleration up to 5g and longitudinal acceleration up to 1g.

[0049] The noise window is centered on the aircraft's position and takes into account estimates of measurement errors made in previous interrogations and estimated inaccuracies in the prediction models used to predict the aircraft's position.

[0050] Measurement error estimates are data provided by the manufacturer.

[0051] The inaccuracies in the prediction models are estimated by the radar. They represent the difference between the ideal path of the aircraft and the aircraft position predicted by the radar. The ideal path of the aircraft is calculated assuming straight flight at constant speed or turning with constant lateral and longitudinal acceleration.

[0052] The uncertainty of the aircraft's predicted position is estimated based on the discrepancy (azimuth, distance to radar) between the measured position and the predicted position in the previous and current antenna rotation (since the object was first detected), integrated over time using a recursive filter (e.g., an extended Kalman filter), and based on the radar detection error (manufacturer-provided data) and the path model uncertainty integrated over time (radar parameters specified by the operator).

[0053] If, for a given antenna rotation, an aircraft is detected within the antenna lobe of the secondary radar and before the detection switches to "track mode", the method includes an intermediate step a1).

[0054] The intermediate step a1) comprises two substeps.

[0055] The first substep i) involves detecting whether an aircraft response to the roll call interrogation is identified within the noise window of the secondary radar.

[0056] There are two possible reasons why the aircraft response may not be located within the noise window.

[0057] The first reason concerns the maneuvering of the aircraft, i.e. the initiation or termination of lateral accelerations up to 5g or longitudinal accelerations up to 1g.

[0058] The second reason is that the response received was not intended for that radar, which is typical in the case of a BDS swap. The Mode S protocol does not identify the aircraft's BDS register in the response.

[0059] In this case, the secondary radar issues a new roll call interrogation using the first monitoring window (second substep ii). The probability of a new BDS swap occurring is extremely small, so the target response should be identified within the noise window of the first monitoring window of the new interrogation.

[0060] In the case of civil traffic, which is extremely rare, the first monitoring window may be used to make a new roll call, assuming that no new responses from aircraft following the new roll call are identified within the noise window.

[0061] The algorithms used to place the interrogations and monitor windows must be able to take into account new interrogations made to avoid BDS swapping issues and manage this small amount of extra processing load. Therefore, the monitor time constraints are temporarily increased.

[0062] Notwithstanding the above, this mode of operation immediately solves the problem of BDS swapping and allows the device to avoid track loss.

[0063] If intermediate step a1) has been carried out, detection can be carried out in "track mode", which allows tracking of aircraft in a smaller surveillance window.

[0064] To do this, the radar sends a series of roll call interrogations in a second observation window that is smaller than the first observation window in "search mode" and is the minimum size necessary to capture a response from an aircraft whose distance to the radar is precisely known.

[0065] For a given antenna rotation, interrogations are made in "track mode" until all transactions (interrogation / response) have been made or the radar antenna is no longer pointed towards the aircraft.

[0066] Furthermore, if in intermediate step a1) no valid response is received to any of the call interrogations and the scan is finished, the method ends.

[0067] The number of times a roll call interrogation is transmitted in either "search mode" or "track mode" is dynamically determined depending on the expected location of the interrogated aircraft. The algorithms used to conduct the interrogations must, among other things, conduct the interrogations in such a way that an expected response cannot be received simultaneously as another response to a second interrogation.

[0068] The method according to the invention does not require any modification or further updating of transponders currently installed on aircraft. Furthermore, the updating of secondary radars can be done in stages. It is not necessary for all secondary radars in a given area to implement the method according to the invention.

[0069] The invention also relates to a secondary radar capable of implementing the above-mentioned method, which does not differ structurally from secondary radars known to those skilled in the art, except that the processing module is configured to insert an additional interrogation window in the event of a BDS swap.

Claims

1. 1. A method for managing a secondary radar operating in Mode S, comprising: a) detection in a "search mode," the "search mode" being performed until an aircraft is detected by the secondary radar, the "search mode" including a plurality of full-call secondary radar interrogation periods and a plurality of roll-call interrogation periods in a first monitoring window; b) detection in a "track mode," which is implemented when a valid response to a roll call interrogation is detected in the "search mode," and which includes a plurality of full call interrogation periods and a plurality of roll call interrogation periods in a second monitoring window, the second monitoring window being around the aircraft position predicted by the secondary radar in response to the full call interrogation. and an intermediate step a1) performed between said detection in "search mode" and said detection in "track mode", said intermediate step comprising: i) detecting the presence or absence of said aircraft response within the noise window of said secondary radar; ii) if the response of the aircraft is not identified within the noise window, making at least one roll call interrogation using the first monitoring window. A method comprising:

2. 2. The method of claim 1, wherein the secondary radar includes an antenna, and the first monitoring window is calculated for a given aircraft based on a rotational rate of the antenna of the secondary radar and a predicted aircraft position for a subsequent antenna rotation.

3. 3. The method of claim 2, wherein the predicted aircraft position is calculated based on all possible aircraft positions, and wherein all possible aircraft positions are calculated based on known paths of the aircraft and rate-related aircraft parameters received by the secondary radar.

4. The method of claim 1 , wherein the noise window is calculated based on a difference between an ideal path of the aircraft and a position of the aircraft predicted by the secondary radar.

5. The method of claim 4 , wherein the ideal path of the aircraft is calculated based on a set of assumptions that the aircraft flies along a straight line path at a constant aircraft speed.

6. 3. The method of claim 2, wherein the predicted aircraft position is calculated based on a set of calculated discrepancies, a discrepancy corresponding to a difference between an estimate of the aircraft's position and a measured aircraft position in one of the preceding rotations, starting from when the aircraft was first detected.

7. The method of claim 6 , wherein the discrepancy is integrated over time using a recursive filter.

8. The method of claim 7 , wherein the recursive filter is an extended Kalman filter.

9. The method of claim 6 , wherein the predicted aircraft position is calculated based on measurement errors of the radar.

10. A secondary radar, characterized in that it is adapted to carry out the method according to any one of claims 1 to 9.

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