Method for managing secondary radar operating in mode s for avoiding BDS swap problem

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 generally relates to radar systems, and more particularly to a method and apparatus for managing secondary radars operating in Mode S.

Background Art

[0002] Secondary radars are used in air traffic control (ATC). Unlike primary radars that utilize the reflection of electromagnetic waves from aircraft, secondary radars communicate by calling at least the transponder mounted on the aircraft or the IFF responder mounted on military aircraft to obtain the identification (code 3 / A) and altitude (code C) of the aircraft, and then determining its position.

[0003] In the first-generation secondary radars (Mode AC), the transponders responded uniformly to calls from all secondary radars, resulting in a lot of RF contamination or interference. Furthermore, the content of the response was restricted by the number of data bits (at most 12 bits).

[0004] Mode S (where S means selective) is currently widely implemented and can call and track multiple aircraft within the radar coverage area while reducing RF contamination.

[0005] Similar to the first-generation secondary radars, Mode S secondary radars receive the altitude and address of an aircraft when a response is detected, and then determine the position of the aircraft. The radar can also receive information regarding the type, speed, destination, etc. of the aircraft. These additional data are identified by numbers and presented in the form of registers (BDS in the case of the Comm-B data selector) that are available upon request (one BDS per call).

[0006] In Mode S, the call can be either a general call or a roll call. In a general call, the radar requests all transponders located within its coverage area to provide their Mode S addresses, i.e., their identification numbers, in 24 bits.

[0007] When the radar detects an aircraft, i.e., when the aircraft's position is predictable, in each new antenna rotation, to address prediction-related uncertainties, it first calls out to the aircraft within a large spatial window in roll call mode; this phase is hereafter referred to as "search mode." By thus obtaining the aircraft's initial precise position, the radar can reduce the size of the call window used for subsequent roll call calls, allocating other space / time to calling out to other aircraft within its monitoring area. This second phase is hereafter referred to as "tracking mode."

[0008] Figure 1 shows the reduction in the size of the “tracking mode” window for a given antenna rotation, which is centered on the aircraft detected in “search mode”.

[0009] If an aircraft is detected, the radar will lock out the aircraft's transponder for 18 seconds. The aircraft's locked-out transponder will no longer respond to any calls from calling radars having the same radar base code as the radar that locked out the aircraft. This lockout is renewed with each roll call that includes a lockout command (the lockout time is reset, i.e., a new lockout lasting 18 seconds is initiated).

[0010] If two radars send calls to a given transponder, and these calls reach the transponder almost simultaneously, the transponder will send only one response to the two calls.

[0011] A radar (Radar 1) whose call is recognized by its transponder receives a response (including a BDS) that matches the call directed to it. In contrast, a secondary radar (Radar 2) whose call is ignored receives the same (and therefore incorrect) response corresponding to the call from Radar 1, rather than the call itself. Furthermore, Radar 2 never receives a response to the call it sent.

[0012] This type of problem is called BDS swapping. BDS swapping is observed in areas with a high density of Mode S secondary radars. This problem affects the effectiveness of surveillance systems and therefore requires correction. Known automated systems for managing air traffic can handle such malfunctions, but only after the antenna has rotated multiple times (i.e., multiple rotations of the movable part of the antenna).

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

[0014] Figure 2 shows the result of a BDS swap when the radar makes an error regarding the actual distance to the aircraft due to inaccurate data. The radar, having detected an incorrect BDS response, derives an incorrect distance from it. Therefore, the size of the monitoring window in "tracking mode" is reduced, but only to the extent of this incorrect distance. With the same antenna rotation, the radar continues to call on the transponders, but ignores their responses because they are not within the correct monitoring window.

[0015] During subsequent antenna rotations, the radar continues to call upon the transponder within a predicted window calculated based on its previous position, but this position is incorrect. As shown in Figure 3, because this predicted position is far from the true path, the radar has no chance of reacquiring the transponder (for example, it appears to the radar that the target has suddenly turned 90°).

[0016] The transponder is locked out and therefore not responding to any calls, and consequently the radar cannot reposition it. We need to wait for the transponder lockout to end and for the radar to reacquire the aircraft.

[0017] Conventional technologies offer solutions to the problems of BDS swaps.

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

[0019] If the response does not address the request, the response will be rejected.

[0020] For example, for a call UF=4 (monitoring, high-level request), only a response DF=4 (monitoring, high-level response) or a response DF=20 (Comm-B, high-level response) is expected. For a call 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 cannot avoid the BDS swap problem when two calling devices request the same information. Unfortunately, it is not uncommon for the same information to be requested in air traffic control.

[0022] The second solution involves verifying the integrity of the first byte of register BDS1,0, register BDS2,0, or further 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 call targeted these BDS.

[0023] However, this solution cannot be applied 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 BDSs returned from the transponder. Therefore, when the calling device receives the response, the parity check can verify 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 introduction of this solution is restricted 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 problem of BDS swapping.

[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 the tracking of aircraft by two radars but does not seek to solve the problem of BDS swapping.

Prior Art Documents

Non-Patent Documents

[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 Problems 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 Problems

[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”, said “search mode” being carried out until an aircraft is detected by the secondary radar, said “search mode” comprising, in a first monitoring window, a plurality of all - call secondary radar call periods and a plurality of selective - call periods, detection b) Detection in “Tracking Mode,” which is performed when a valid response to a roll call is detected in “Search Mode,” and which includes multiple full call periods and multiple roll call periods in a second monitoring window, the second monitoring window being around the aircraft position predicted by secondary radar in response to the full call. A method including a1) which is performed between detection in “search mode” and detection in “tracking mode,” wherein the intermediate step is To detect 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, at least one roll call should be made using the first monitoring window. This method is characterized by including [a certain element].

[0031] This method advantageously includes the following embodiments.

[0032] The secondary radar includes an antenna, and the first monitoring window is calculated for a given aircraft based on the rotational speed of the secondary radar antenna and the predicted aircraft position with respect to subsequent antenna rotations.

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

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

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

[0036] The predicted aircraft position is calculated based on a set of calculated discrepancies, where the discrepancy corresponds to the difference between the estimated 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 with respect to time using a recursive filter.

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

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

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

[0041] Other features, details, and advantages of the present invention will become apparent by reading the given description with reference to the accompanying drawings, which are given as examples. [Brief explanation of the drawing]

[0042] [Figure 1] As already described, this is a reduction in the call window between "discovery mode" and "tracking mode" in nominal cases (where BDS swapping is not present). [Figure 2] As already described, one failure case (following a BDS swap) involves a narrowing of the call window between "discovery mode" and "tracking mode." [Figure 3] As already described, this involves the BDS swap and the appearance of the aircraft in the wrong position. [Figure 4] This is the method according to the present invention. [Figure 5] These are various window and response and associated plot representations used in the method according to the present invention. [Figure 6] This is an example of a new appeal being made. [Modes for carrying out the invention]

[0043] The method according to the present invention is shown in Figure 4.

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

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

[0046] The first monitoring window can be calculated for a given aircraft based on the rotation speed of the secondary radar antenna and the predicted aircraft position with respect to subsequent antenna rotations.

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

[0048] The prediction window is constructed around the predicted position of the aircraft and takes into account 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 the estimation of measurement errors that occurred in the preceding call and the estimated inaccuracies of the predictive model used to predict the aircraft's position.

[0050] The estimation of measurement error is based on data provided by the manufacturer.

[0051] The inaccuracies of the prediction model are estimated by radar. These represent the difference between the aircraft's ideal path and the aircraft's position predicted by radar. The aircraft's ideal path is calculated assuming straight flight at a constant speed or turns with constant lateral and longitudinal acceleration.

[0052] The inaccuracy of the aircraft's predicted position is estimated based on the discrepancy (azimuth angle, distance from radar) between the measured position and the predicted position in preceding and current antenna rotations (since the object was first detected), integrated over time using a recursive filter (e.g., an extended Kalman filter), and also based on the integration over time of radar detection errors (manufacturer-provided data) and uncertainty in the path model (operator-defined radar parameters).

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

[0054] Intermediate step a1) includes two substeps.

[0055] The first partial step i) includes detecting whether the aircraft's response to the roll call is identified within the noise window of the secondary radar.

[0056] There are two possible reasons why the aircraft's response is not identified within the noise window.

[0057] The first reason concerns aircraft control, specifically the initiation or termination of lateral acceleration of up to 5g or longitudinal acceleration of up to 1g.

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

[0059] In this case, the secondary radar makes a new roll call using the first monitoring window (second partial step ii). The probability of a new BDS swap occurring is extremely small, and therefore the target's response should be identified within the noise window of the first monitoring window of the new call.

[0060] In the case of civilian transport, although extremely rare, this might involve using the first monitoring window to issue a new roll call, assuming that no new response from the aircraft following a new roll call is identified within the noise window.

[0061] The algorithm used for scheduling calls and monitoring windows must account for new calls made to avoid BDS swapping issues and be able to manage this slight extra processing load. Consequently, monitoring time constraints will be temporarily increased.

[0062] Despite the above, this processing mode immediately resolves the BDS swapping problem, and the device can avoid track loss.

[0063] If intermediate step a1) is performed, detection can be performed in "tracking mode". "Tracking mode" allows the aircraft to be tracked in a smaller monitoring window.

[0064] To do this, the radar makes a series of roll call calls in a second monitoring window that is smaller in size than the first monitoring window in “search mode.” The second monitoring window is the minimum size required to capture responses from aircraft whose distance to the radar is precisely known.

[0065] In a given antenna rotation, calls are made in "tracking mode" until all transactions (calls / responses) are completed or until the radar antenna no longer points in the direction of the aircraft.

[0066] Furthermore, if no valid response is received to any of the roll call calls in intermediate step a1) and the scanning has been completed, this method terminates.

[0067] The number of roll call transmissions in either "search mode" or "tracking mode" is dynamically determined based on the expected location of the aircraft being called. The algorithm used to make the calls must, in particular, ensure that the expected response is not received simultaneously as another response to a second call.

[0068] The method according to the present invention does not require modification or further upgrading of transponders currently installed on aircraft. Furthermore, the upgrading of secondary radars can be carried out in stages. Not all secondary radars in a given area need to implement the method according to the present invention.

[0069] The present invention also relates to a secondary radar capable of carrying out the method described above. The secondary radar according to the present invention is structurally no different from a secondary radar known to those skilled in the art, except that a processing module is configured to insert an additional call window when a BDS swap occurs.

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", the "track mode" being implemented when a valid response to a roll call interrogation is detected in the "search mode", the "track mode" including 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 an aircraft position predicted by the secondary radar in response to a full call interrogation. In a method comprising the steps of: i) detecting the presence or absence of said aircraft response within a noise window of said secondary radar; ii) making at least one roll call interrogation using the first monitoring window if the response of the aircraft is not identified within the noise window. The method according to claim 1, further 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, the all possible aircraft positions being 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 an aircraft position predicted by the secondary radar.

5. The method of claim 4 , wherein the ideal path for 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 the claims 1 to 9.

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