GB2477103A

GB2477103A - Passive monitoring of Mode S radar ground stations

Info

Publication number: GB2477103A
Application number: GB1000946A
Authority: GB
Inventors: Michael Francis Sharples
Original assignee: Qinetiq Ltd
Current assignee: Qinetiq Ltd
Priority date: 2010-01-21
Filing date: 2010-01-21
Publication date: 2011-07-27
Also published as: GB201000946D0

Abstract

To monitor a mode S radar ground station, a passive receiver 3 receives messages of two types, namely responses by aircraft to a ground station interrogation, and broadcasts by the aircraft of its position and identity (ADS-B). Common information in the first and second messages is used to associate the two provided they are received within a preset time interval. This links information derivable from the first message, such as the ground station radar identity, with the spatial location from the second message. This allows detection of problems associated with misallocation of the same radar ground station identity (interrogator code IC) to adjacent interrogators A and B which causes inappropriate lock-out (denial of service), and also allows the ground station radar locations to be determined geometrically using the received information.

Description

Method and Apparatus for Passive Monitoring of Radar Ground Stations The present invention relates to a method and apparatus for the passive monitoring of radar ground stations.

Mode S Secondary Radar is rapidly becoming the prime means of surveillance for civil air traffic control purposes in Europe and the US. The techniques for measuring the position of an aircraft or other target using Mode S radar are well established and widely known. The protocols, data formats and transmission signals involved in Mode S are also well established and widely known.

Mode S radar surveillance is dependent upon co-operative targets that are fitted with transponder equipment operating to an appropriate standard. The standards and recommended practices (SARPS) and minimum operating performance standards (MOPS) for Mode S transponder equipment on aircraft or other targets are published in the public domain.

The technique of ADS-B (Automatic Dependent Surveillance -Broadcast) refers to any platform that periodically transmits information about its current position and/or vector in a manner capable of reception by any number of suitable receiving devices within range.

The public domain standards for Mode S transponder equipment include the provision for supporting ADS-B using Mode S signal formats. Any suitable receiving station can decode Mode S ADS-B transmissions and pass the information to a surveillance or tracking system. This capability is well established and widely known.

Figure 1 of the accompanying drawings shows two Mode S radars, A and B. Each radar has an assigned Interrogator code, IC, which can be considered to be an address of the radar. There are only a limited number of addresses, so there is inevitably a degree of address re-use. Limits of defined coverage of radar A is shown in Figure 1 by CA and defined coverage of radar B is shown by OB.

Under normal operation, each radar has a different IC. An aircraft I outside of the defined coverage, but within radio range, will respond to "All Call" interrogations from each radar. The response is a reply format known as DF=1 1 or "All Call Reply".

Once an aircraft flies into the defined coverage range, say aircraft 1 moving to position la in Figure 1, then radar A will instruct the aircraft telling it not to respond to "All Call" interrogations from it, as identified by its IC (a protocol known as lockout). This is to minimise radio congestion. Aircraft la will, however, respond to any other "All Call" interrogations it receives from radars having different Cs. Aircraft la now only responds to specifically addressed interrogations from radar A (these eliciting reply formats DF=4, 5, 20 or 21, the details of which are not important -only the fact that these responses can be observed by an independent monitoring system 3 to be described below in relation to an embodiment of the present invention). The aircraft does not have to respond to "All call" interrogations with DF=1 1 messages because the radar A already knows of its whereabouts.

Similarly, an aircraft 2 that is in the defined coverage range of both radars A and B, will respond to "All Call" interrogations from both, provided that neither radar has locked it out (which in practice is likely to have happened). Assume aircraft 2 has flown from radar A, and is heading towards radar B. It will have been locked out to "All Call" interrogations having address ICA, as it was solely in the coverage region of radar A. As it meets the radio range of radar B, it will start to respond to "All Call" interrogations having address ICB. Thus radar B is able to detect the presence of the aircraft in good time. When aircraft 2 enters the defined coverage of Radar B then Radar B will instruct aircraft 2 to lockout, to minimise congestion. When the aircraft 2 goes outside of the defined coverage range CA of radar A it will then revert to respond to "All Call" interrogations from the address ICA, either after a defined timeout period (the failsafe mode) or under direct instruction from radar A before coverage is lost.

A potential problem with the scenario illustrated in Figure 1 has been identified by the present applicant. As there are only a limited number of ICs (for historical reasons), there is considerable need for re-use among the radar population. When a new radar goes online some re-organisation of these codes may be required, which can result in a radar being assigned the wrong IC.

Consider a situation in the above scenario where radar B mistakenly sets its IC to the same one as radar A (i.e. both are ICA). Aircraft 2, located solely in the region of radar A (at position 2a) will be locked out from ICA "All Call" interrogations. When aircraft 2 reaches the defined coverage area CB it will be locked out and not respond with DF=1 1 messages to "All call" interrogations from radar B, as it shares the same address ICA As the aircraft continues on to radar B, and meets the border of defined coverage with ICA, it will revert to respond to "All Call" interrogations with ICA. Only at that point will radar B know of its presence, because it will suddenly start responding with All Call DF=1 1 messages. Thus there has been a period of time (and distance d), as the aircraft moved from one border CB to the other CA, where radar B was unaware of the presence of the aircraft. This inadvertent denial of service is clearly a safety issue that is recognised by the regulatory authorities who identify a need for careful management (albeit that other systems such as primary radars may provide some mitigation).

It is desirable to provide a solution to the above-identified problem.

According to a first aspect of the present invention there is provided a method for use in the passive monitoring of a radar ground station, comprising the steps of: receiving a plurality of first messages, each first message being sent by an aircraft in reply to a message received previously from the radar ground station, and each first message comprising an identifier of the aircraft from which the message was sent; receiving a plurality of second messages, each second message being broadcast by an aircraft to convey its spatial location, and each second message comprising an identifier and spatial location of the aircraft from which the message was sent; and, for each received first message, associating the first message with any of the received second messages that identifies the same aircraft as the first message and is closely associated in time with the first message according to a predetermined measure, thereby associating any information derivable from the first message with the spatial location from the second message.

The derivable information may comprise information sent to the aircraft by the radar ground station.

The derivable information may comprise information being used by the radar ground station to identify itself.

The first messages may comprise Mode S messages.

The method may comprise determining the perimeter of a lockout coverage region for the radar ground station based on where aircraft start or stop sending Mode S messages of type DF=1 1 in response to a Mode S interrogation from the radar ground station.

The perimeter may be determined in its horizontal extent.

The perimeter may be determined in its vertical extent.

The method may comprise comparing the determined perimeter to that expected for the radar ground station, and indicating possible misallocation of IC codes if a difference is found.

The information being used by the radar ground station to identify itself may comprise the IC code of the radar ground station.

The method may comprise monitoring the derivable information and its associated spatial locations.

The method may comprise causing the monitoring of the derivable information and its associated spatial locations.

The method may comprise enabling the monitoring of the derivable information and its associated spatial locations.

The method may comprise monitoring the derivable information and its associated spatial locations to identify anything that would indicate anomalous behaviour and/or incorrect implementation of changes in radar ground station deployment and/or correct implementation of managed changes.

The derivable information may comprise information relating to the type of first message. For example, where the first message is a Mode S message, the first message could be of type DF=4, DF=5, DF1 1, DF20, DF=21, and so on.

The derivable information may comprise timing information, for example the time that the first message was sent by the aircraft or the time of receipt of the first message.

The method may comprise determining a first time difference between two first messages received from a first aircraft in two different respective radar rotations, and determining the period of radar rotation using the first time difference. This could be done multiple times, possibly using a number different aircraft, to obtain a more accurate estimate of the period of radar rotation (for example, an average of multiple differences could be used).

The method may comprise determining a second time difference between a first pair of first messages received respectively from second and third different aircraft during the same radar rotation, determining a third time difference between a second pair of first messages received respectively from fourth and fifth different aircraft during the same radar rotation, determining a first angle subtended at the radar ground station by the second and third aircraft using the second time difference and the period of radar rotation, determining a second angle subtended at the radar ground station by the fourth and fifth aircraft using the third time difference and the period of radar rotation, using the first angle and the spatial locations associated with the first pair of first messages to determine a first locus on which the radar ground station lies, using the second angle and the spatial locations associated with the second pair of first messages to determine a second locus on which the radar ground station lies, and determining the location of the radar ground station from the intersection of the first and second loci.

The first aircraft may be the same as any one of the second to fifth aircraft.

The third aircraft may be the same as the fourth aircraft.

It may be that two messages are determined to be closely associated in time according to the predetermined measure if a difference between their respective times of receipt is less than a predetermined threshold.

The second messages may comprise Mode S messages.

The second messages may comprise Automatic Dependent Surveillance -Broadcast (ADS-B) messages.

According to a second aspect of the present invention there is provided an apparatus for use in the passive monitoring of a radar ground station, comprising: means for receiving a plurality of first messages, each first message being sent by an aircraft in reply to a message received previously from the radar ground station, and each first message comprising an identifier of the aircraft from which the message was sent; means for receiving a plurality of second messages, each second message being broadcast by an aircraft to convey its spatial location, and each second message comprising an identifier and spatial location of the aircraft from which the message was sent; and means for, for each received first message, associating the first message with any of the received second messages that identifies the same aircraft as the first message and is closely associated in time with the first message according to a predetermined measure, thereby associating any information derivable from the first message with the spatial location from the second message.

According to a third aspect of the present invention there is provided a program for controlling an apparatus to perform a method according to the first aspect of the present invention or which, when loaded into an apparatus, causes the apparatus to become an apparatus according to the second aspect of the present invention. The program may be carried on a carrier medium. The carrier medium may be a storage medium. The carrier medium may be a transmission medium.

According to a fourth aspect of the present invention there is provided an apparatus programmed by a program according to the third aspect of the present invention.

According to a fifth aspect of the present invention there is provided a storage medium containing a program according to the third aspect of the present invention.

An embodiment of the present invention proposes a novel concept of associating Mode S radar replies with ADS-B reports to provide an effective means of monitoring IC and lockout activity that is independent of the radar systems that are being monitored.

An embodiment of the present invention facilitates the identification of changes in IC activity that may confirm correct implementation of planned changes in IC and lockout allocation. This can provide means to assist and validate any IC management process.

An embodiment of the present invention facilitates identifying changes in IC activity that may signify incorrect radar deployment and performance. This can form the basis of an alerting and anomaly reporting system.

An embodiment of the present invention assists in locating any radar that is believed to be the source of anomalous IC or lockout activity.

Referring again to the example illustrated in Figure 1, this time in relation to an embodiment of the present invention, the aircraft illustrated in Figure 1 will periodically be transmitting their positions using ADS-B. A receiver 3, positioned on the ground, is adapted to listen to positional information from ADS-B, and correlate this with the Mode S DF=1 1 information (as both have the address of the aircraft). The Mode S DF=1 1 information also has the IC code of the interrogating radar. Thus a receiver embodying the invention, having knowledge of the aircraft position (from ADS B) and the IC code it is responding to (from the Mode S All Call transmissions) would be able to see, in the above problem scenario, that aircraft 2 only started responding to All Call interrogations when it crossed border CA, when (from prior knowledge) it should have started earlier.

From the non All Call DF=11 transmissions (such as DF=4, 5, 20, 21 etc) the position of the aircraft can be tracked (using the ADS-B transmissions also), and when the DF=1 1 transmissions start, the IC identified.

The information may be used in other ways as described below, for example for the location of radars.

An embodiment of the present invention proposes using a passive 1090MHz monitoring system that is capable of performing two distinct functions: Firstly, to identify aircraft position by decoding transmissions carrying Automatic Dependent Surveillance information.

Secondly, to identify aircraft replies to Mode S radar specifically of a type known as All Call" and inferring the IC code of the radar that initiated the transmission from the transmission cyclic redundancy check (CRC), and then associating this information with the position information obtained from function 1.

Such a method allows the user to establish geographic and altitude bounded patterns of IC activity from which characteristics of radar deployment and performance can be determined by analysis.

More particularly, the method facilitates identifying changes in IC activity that may signify incorrect radar deployment and performance. This can form the basis of an alerting and anomaly reporting system.

Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1, discussed hereinbefore, is a schematic illustration of a typical scenario addressed by an embodiment of the present invention, in which aircraft are travelling between two different areas of radar coverage; Figure 2 is a schematic illustration of a passive monitoring system according to an embodiment of the present invention; Figure 3 shows the structure of a Mode S message of type DF=1 1; Figure 4 shows the structure of IC information, being 7 bits of information comprising a 3-bit Code Label (CL) field and a 4 bit Interrogator Identifier (II) field and which is contained within the P1 field of a Mode S message of type DF=1 1 as shown in Figure 3; Figure 5 illustrates one possible visualisation of the process of monitoring IC activity on a geographical basis; and Figure 6 is for use in explaining a method determining radar position according to one embodiment of the present invention.

As described previously, the efficient operation of Mode S requires that each interrogator operates with a designated identifier known as an Interrogator Code (IC), and the efficiency is achieved by a mechanism known as "IC lockout". A Mode S radar will normally be given a defined lockout region. Any other Mode S radar operating with the same IC will not normally be able to acquire any target that lies within that lockout region.

The purpose of IC and lockout is to optimise the efficiency of Mode S operation and thus maximise target detection performance. However, as described previously, misallocation of ICs and lockout coverage maps can lead to operational conflicts between interrogators. Consequently there is a potential for regionalised denial of service. To minimise the risk of this, allocation of IC is centrally managed within Europe.

Whilst management processes are intended to avoid IC conflicts, the possibility for human error or system malfunction creates a real risk of an interrogator operating with an incorrect IC or lockout coverage map.

An embodiment of the present invention provides a means of monitoring IC activity in a manner that is independent of the Mode S interrogators. Thus it will be possible to monitor, alert and manage IC code operation in a highly effective manner.

An embodiment of the present invention exploits the fact that a large and increasing proportion of aircraft are transmitting ADS-B information, thus allowing the location of the aircraft to be determined by an appropriate passive receiving system. As will be seen in more detail below, an embodiment of the present invention associates ADS-B information with other aircraft transmissions generated in response to Mode S radar interrogation. Subsequently, an embodiment of the present invention is able to establish geographic and altitude bounded patterns of IC activity from which aspects of radar deployment and performance can be calculated.

An embodiment of the present invention proposes using information from a passive Mode S receiving system. A Mode S receiving system will comprise one or more receiving stations able to monitor the Mode S transponder transmission frequency of 1090MHz. Such a system is capable of performing two distinct functions: (a) decoding ADS-B transmissions from aircraft; and (b) decoding aircraft replies to Mode S interrogations.

The passive Mode S receiving system is not of itself the novel aspect of this invention.

However, an embodiment of the present invention does place extra requirements on the performance of the receiving system: Firstly, information decoded from transponder replies to Mode S interrogation must be available at the output of the system (some systems may block or discard this information and thus would not be appropriate).

Secondly, every decoded transmission should be annotated with a time of reception (time-stamped) with sufficient accuracy and stability to support subsequent calculations.

An embodiment of the present invention provides a means of monitoring IC activity by associating replies to Mode S interrogators with ADS B transmissions from aircraft.

The association is achieved using a common element in both message types, namely the Aircraft Address. The Aircraft Address is a 24-bit identifier that is designed to be unique to every aircraft.

Transmissions that are directly associated by Aircraft Address and closely associated in time can be assumed as transmitted from the same location to a high degree of certainty. Information within the ADS-B transmission identifies the location of the aircraft. Information within the Mode S replies provides information relating to radar activity, and specifically information on IC activity. Hence aspects of radar performance at a specific location can be determined.

The monitoring process as described can be applied to all and any aircraft that is operating with an ADS-B capability and is also undergoing interrogation from one or more Mode S radar. The movement of multiple aircraft across a geographic region of interest will allow patterns of IC activity to be determined, and anomalous behaviour to be identified.

In more detail now with reference to Figure 2, a Mode S monitoring system according to one embodiment of the present invention incorporates one or more receiving stations in order to monitor the Mode S transponder transmitting frequency of 1090MHz. The number and arrangement of receiving stations will provide a geographic region of coverage appropriate for the IC monitoring capability that is required.

The Mode S monitoring system provides as an output all successfully received messages of recognised Mode S format. These will include messages generated automatically and periodically by aircraft, and also messages generated by aircraft in response to Mode S radar interrogation. The type of each Mode S message is identified by a 5 bit prefix known as the DF field.

Each successfully received message that is output is annotated (time-stamped) by the Mode S monitoring system with the time of reception.

The Mode S transponder on the aircraft transmits information on a frequency of 1090MHz as follows: (a) it shall respond to Mode S interrogations from Mode S radar; and (b) it may periodically generate ADS-B transmissions.

The Mode S Radar measures position of targets using timing between Mode S interrogations and associated Mode S replies.

The Mode S monitor at the Mode S Monitor System decodes any 1090MHz transmission of recognised Mode S type within its detection range.

The instantaneous position of an aircraft is determined from the information contained within ADS-B messages transmitted periodically using Mode S message formats known as Extended Squitter and identified by a DF field having the value DF17, DF=18 or DF=19. The information includes position, altitude, vector and Aircraft Address.

IC activity is determined from information contained within Mode S messages identified by DF=11. DF=11 refers to a dual purpose message that may be generated automatically and periodically by the aircraft whereby it is known as a Short Squitter, or in response to a Mode S interrogation whereby it is known as an All Call Reply.

The structure of a DF=1 1 message is shown in Figure 3. The structure of the message is the same regardless of whether it is a Short Squitter or an All Call Reply. However, any DF11 message that is in response to a Mode S interrogation shall contain information of the interrogator IC within the P1 field. The structure of the IC information

within the P1 field is shown in Figure 4.

The mechanism of obtaining the IC value is as follows: Firstly, a standard Mode S Cyclic Redundancy Check (CRC) calculation is carried out on the complete message.

Secondly, any non-zero result of the CRC calculation that, as a binary representation, consists of more than 7 bits, or is not an acceptable value as defined in Figure 4 is deemed to be due to transmission error and the message is discarded from further consideration.

Thirdly, any non-zero result of the CRC calculation that, as a binary representation, consists of 7 bits or fewer, and is an acceptable value as defined in Figure 4, is deemed to indicate the IC value of an interrogator that initiated the transmission.

It may be observed that the mechanism described may incorrectly identify IC codes where in fact a Mode S message contains a transmission error. This is a natural consequence of the way that the CRC is being used to obtain the IC value. However, the proportion of errors can be expected to be low. Furthermore, as an embodiment of the present invention involves identifying patterns of activity over time, the probability will be negligible of errors correlating in a manner that could skew performance.

Acceptable IC values are associated with ADS-B position reports on the basis of: (a) the same Aircraft Address; and (b) close correlation of time of reception.

The correlation of time of reception may typically associate an IC value with the ADS-B position report that is closest in time, provided that the time difference does not exceed an appropriate maximum value. However, other rules of correlation may be equally effective. An appropriate maximum value is derived from the distance an aircraft may travel within the period and the consequent effect on the accuracy of the method, and typically may be two seconds.

The correlation of IC and ADS-B information provides a continuous monitor of IC activity on a geographic basis. This in turn allows information relating to IC activity to be calculated, either in real-time or through the analysis of recorded data. Figure 5 provides, for the purposes of example, a simple visualisation of this process.

Additional radar information can be determined by monitoring Mode S message types of DF=4, DF=5, DF20 and DF=21. These messages do not provide IC information but are indicative of radar activity and should be evident in any region where an aircraft is expected to be in Mode S radar coverage. These Mode S message types will be associated with ADS-B position reports on the same basis as IC association. The associated radars can be inferred by any method known in the art.

Various examples will now be described of Mode S radar performance that can be monitored by a method embodying the present invention. These are examples only, and the skilled person would appreciate that other applications and examples are possible.

According to a first example, the perimeter of a lockout coverage region in its horizontal extent can be identified. Aircraft moving into the region will stop generating Mode S messages of type DF=1 1 for that IC value when the perimeter is crossed. Aircraft moving out of the region will commence generating Mode S messages of type DF1 1 when the perimeter is crossed, or shortly thereafter.

According to a second example, the perimeter of a lockout coverage region in its vertical extent can be identified. Aircraft climbing or descending into the region will stop generating Mode S messages of type DF=1 1 for that IC value when the perimeter is crossed. Aircraft climbing or descending out of the region will commence generating Mode S messages of type DF=1 1 when the perimeter is crossed, or shortly thereafter.

According to a third example, the location of a radar operating with a specific IC code can be identified. The periodicity of Mode S messages of type DF=4, DF=5, DF=11, DF=20, DF=21 generation from any individual aircraft will indicate the rotation rate of the radar. Consequently, the interval between Mode S message generation of any two aircraft will indicate the angle subtended by that aircraft pair at the radar and hence identify a locus (circle) upon which the radar must lie. The equivalent measurements with one or more additional aircraft will create additional loci that will intersect with the first locus to identify the location of the radar.

This is illustrated in more detail in Figure 6, showing three aircraft 1, 2 and 3, as well as a radar R rotating in a clockwise direction and having an IC code of X'. Firstly, the period icity of DF1 1 messages from aircraft 1 and for IC=x is measured as T', which can be considered as the period of rotation of the radar R. Secondly, the time difference of DF1 1 and IC=x messages between Aircraft 1 and Aircraft 2 is measured as t12. Thirdly, the time difference of DF1 1 lCx messages between Aircraft 2 and Aircraft 3 is measured as t23. This sets 01 = 2ir x (t12IT), where 01 is the angle subtended by aircraft pair 1, 2 at the radar R, and furthermore sets 02 = 2rr x (t23IT), where 02 is the angle subtended by aircraft pair 2, 3 at the radar R. Because of the location information for aircraft 1, 2 and 3 correlated from the ADS-B information, the distance between aircraft 1 and 2 is known, as is distance between aircraft 2 and 3. It is well known that, for any triangle with angles ABC and sides abc, if A' and a' are fixed then apex A' describes a circle as the other angles are varied.

These circles are illustrated in Figure 6 for the two triangles formed by radar R and aircraft 1, 2 and 3, with the position of the radar R being pinpointed at the intersection of the two circles. Note that the diagram assumes that radar always rotate in the same direction (which they do); if they did not there would be a further solution on the opposite side of the baseline.

With the third example, it will be appreciated that Mode S messages of type DF=4, DF=5, DF=20 and DF=21 do not contain IC code information to identify a radar explicitly, unlike a DF1 1 message. All DF types contain the aircraft identity so there is no ambiguity there; however, there is no IC information in DF=4, DF=5, DF=20 and DF=21 messages so any conclusions as to radar have to be made by inference. In one possible use, the assumption might be that anomalous behaviour has been identified through DF=1 1 activity and the method is then attempting to locate candidates for the rogue radar from both DF=1 1 and non-DF=1 1 transmissions. Non-DF=1 1 replies may provide more than one "candidate" for the rogue, but other indicators including background knowledge of which radar should be legitimately locking out that target should narrow this down.

If only a single radar is interrogating the target for non-DF=1 1 types then the periodicity should be plainly evident. If more than one radar is interrogating, the interpretation becomes more difficult but will still be possible in principle so long as the rotation rates are not very similar. As more radars are interrogating then matching patterns becomes increasingly more complex.

It could be considered that the above method of the third example assumes that only one message is sent per rotation. However, for DF11 there may often be more than one. For selective DFs (e.g. DF=4 etc) there will usually only be one but may be more if the radar needs to re-interrogate for some reason. However, multiple interrogations in a single beam dwell will be very closely spaced in time. A target is typically only within the radar beam for tens of milliseconds. This is so short in relation to the rotation period (typically between 4s and lOs) that clusters of replies can be considered as a single event. However, the width of this interrogation window will create a slight blurring error in estimating radar range using two or more aircraft if those aircraft are closely spaced, and this can be taken into account. In all of the above calculations (e.g. that for determining the period T'), adjustments could also be made for the fact that the aircraft will have travelled a certain distance in between rotations of the radar R. It should be noted that the method of the third example may be successful even where the radar location is well outside the coverage region of the Mode S monitoring system -hence offering an "over the horizon" capability.

According to a fourth example, changes in normal IC activity that indicate anomalous IC activity can be identified. The movement of large numbers of aircraft across the region of interest will allow patterns of normal or typical behaviour to be built up.

Unexpected changes it these patterns of behaviour can be a used to alert a user of potential anomalous behaviour.

According to a fifth example, changes in normal IC activity confirming correct implementation of managed changes can be identified. Whenever planned changes in the IC and lockout allocations are carried out, the timing and correctness of those changes can be confirmed from observed changes in patterns of IC activity.

Although embodiments of the present invention have centred around ADS-B and Mode S communication standards, it will be appreciated that the present invention will be equally applicable to other similar communication standards.

It will be appreciated that operation of one or more of the above-described functions can be controlled by a program operating on a device or apparatus. Such an operating program can be stored on a computer-readable medium, or could, for example, be embodied in a signal such as a downloadable data signal provided from an Internet website. The appended claims are to be interpreted as covering an operating program by itself, or as a record on a carrier, or as a signal, or in any other form.

Claims

CLAIMS: 1. A method for use in the passive monitoring of a radar ground station, comprising the steps of: receiving a plurality of first messages, each first message being sent by an aircraft in reply to a message received previously from the radar ground station, and each first message comprising an identifier of the aircraft from which the message was sent; receiving a plurality of second messages, each second message being broadcast by an aircraft to convey its spatial location, and each second message comprising an identifier and spatial location of the aircraft from which the message was sent; and, for each received first message, associating the first message with any of the received second messages that identifies the same aircraft as the first message and is closely associated in time with the first message according to a predetermined measure, thereby associating any information derivable from the first message with the spatial location from the second message.
2. A method as claimed in claim 1, wherein the derivable information comprises information sent to the aircraft by the radar ground station.
3. A method as claimed in claim 2, wherein the derivable information comprises information being used by the radar ground station to identify itself.
4. A method as claimed in any preceding claim, wherein the first messages comprise Mode S messages.
5. A method as claimed in claim 4, comprising determining the perimeter of a lockout coverage region for the radar ground station based on where aircraft start or stop sending Mode S messages of type DF1 1 in response to a Mode S interrogation from the radar ground station.
6. A method as claimed in claim 5, wherein the perimeter is determined in its horizontal extent.
7. A method as claimed in claim 5 or 6, wherein the perimeter is determined in its vertical extent.
8. A method as claimed in claim 5, 6 or 7, comprising comparing the determined perimeter to that expected for the radar ground station, and indicating possible misallocation of IC codes if a difference is found.
9. A method as claimed in any one of claims 4 to 8, when dependent on claim 3, wherein the information being used by the radar ground station to identify itself comprises the IC code of the radar ground station.
10. A method as claimed in any preceding claim, comprising monitoring or causing or enabling the monitoring of the derivable information and its associated spatial locations.
11. A method as claimed in claim 10, comprising monitoring the derivable information and its associated spatial locations to identify anything that would indicate anomalous behaviour and/or incorrect implementation of changes in radar ground station deployment and/or correct implementation of managed changes.
12. A method as claimed in any preceding claim, wherein the derivable information comprises information relating to the type of first message.
13. A method as claimed in any preceding claim, wherein the derivable information comprises timing information, for example the time that the first message was sent by the aircraft or the time of receipt of the first message.
14. A method as claimed in claim 13, comprising determining a first time difference between two first messages received from a first aircraft in two different respective radar rotations, and determining the period of radar rotation using the first time difference.
15. A method as claimed in claim 14, comprising determining a second time difference between a first pair of first messages received respectively from second and third different aircraft during the same radar rotation, determining a third time difference between a second pair of first messages received respectively from fourth and fifth different aircraft during the same radar rotation, determining a first angle subtended at the radar ground station by the second and third aircraft using the second time difference and the period of radar rotation, determining a second angle subtended at the radar ground station by the fourth and fifth aircraft using the third time difference and the period of radar rotation, using the first angle and the spatial locations associated with the first pair of first messages to determine a first locus on which the radar ground station lies, using the second angle and the spatial locations associated with the second pair of first messages to determine a second locus on which the radar ground station lies, and determining the location of the radar ground station from the intersection of the first and second loci.
16. A method as claimed in claim 14, wherein the first aircraft is the same as any one of the second to fifth aircraft, and/or the third aircraft is the same as the fourth aircraft.
17. A method as claimed in any preceding claim, wherein two messages are determined to be closely associated in time according to the predetermined measure if a difference between their respective times of receipt is less than a predetermined threshold.
18. A method as claimed in any preceding claim, wherein the second messages comprise Mode S messages.
19. A method as claimed in any preceding claim, wherein the second messages comprise Automatic Dependent Surveillance -Broadcast, ADS-B, messages.
20. A method substantially as hereinbefore described with reference to the accompanying drawings.
21. An apparatus for use in the passive monitoring of a radar ground station, comprising: means for receiving a plurality of first messages, each first message being sent by an aircraft in reply to a message received previously from the radar ground station, and each first message comprising an identifier of the aircraft from which the message was sent; means for receiving a plurality of second messages, each second message being broadcast by an aircraft to convey its spatial location, and each second message comprising an identifier and spatial location of the aircraft from which the message was sent; and means for, for each received first message, associating the first message with any of the received second messages that identifies the same aircraft as the first message and is closely associated in time with the first message according to a predetermined measure, thereby associating any information derivable from the first message with the spatial location from the second message.
22. An apparatus substantially as hereinbefore described with reference to the accompanying drawings.
23. A program for controlling an apparatus to perform a method as claimed in any one of claims 1 to 20.
24. A program as claimed in claim 23, carried on a carrier medium.
25. A program as claimed in claim 24, wherein the carrier medium is a storage medi urn.
26. A program as claimed in claim 24, wherein the carrier medium is a transmission medium.
27. A storage medium containing a program as claimed in any one of claims 23 to 26.