# Patent

## Title
Multilateration system and method

## Abstract

A multilateration system and method includes a plurality of receiver stations for receiving signals from an aircraft, and a controller that derives the position of the aircraft by applying a multilateration process to outputs from the receiver stations. For this purpose, the controller determines the altitude of the aircraft and selects a multilateration process that is to be used for position determination, based on the determined altitude.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of Application No. Ser. No. 12/429,564, filed Apr. 24, 2009, now U.S. Pat. No. 8,138,967, which claims priority to United Kingdom Application GB 0807586.3, filed Apr. 26, 2008, and United Kingdom Application GB 0812096.6, filed Jul. 3, 2008. Benefit of the filing date of each of these prior applications is hereby claimed. Each of these prior applications is hereby incorporated by reference in its entirety.


This invention relates to a multilateration system for aircraft location.
Many types of aircraft transmit coded signals for example Secondary Surveillance RADAR (SSR) codes such as a so-called mode A, C or S codes which may be used by ground based receivers to determine the aircraft's position. The position is determined from noting the time of arrival at the receivers and by using this with knowledge of the positions of the receivers themselves. GB2250154A and GB 2349531A disclose such multilateration systems. These systems utilise four receiver stations controlled from one master station in order to establish the aircraft's position in three dimensions.
The present invention arose from a consideration of situations when one receiver station fails to receive the transmitted code, the code is garbled or when one receiver station develops a fault. Consideration was also given to situation where aircraft are unable to transmit mode A codes.
According to the invention there is provided a multilateration system comprising a plurality of receiver stations for receiving signals from aircraft and a controller to apply a multilateration process to outputs of the receiver stations, indicating receipt of the signal, to derive a position of the aircraft characterised in that the controller determines the number of active receiver stations receiving the code, determines the type of code and performs a multilateration process in accordance with the determination to provide a position of the aircraft.
In certain situations it will be appreciated that there may be insufficient receiver stations receiving the transmitted code to determine the location with a great deal of accuracy. For example, three receiver stations will be able to provide a two dimensional position which may in some circumstances be useful.
Some aircraft are equipped with mode A SSR transponder but are able to transmit other codes for example mode C. Mode A codes include a unique aircraft identifier and thus can distinguish a mode A code transmitted by one aircraft from second mode A code transmitted by another. In some cases the mode A transmission may be corrupted and hence not usable. Other codes may not include such a unique identifier. Preferably, in such a case the multilateration process will include a reference to tracking system to distinguish between possible sources. In the tracking system, a table is produced on the basis of the returned signals which is revised over time.
It will be appreciated that a multilateration process involves significant computational resources and it will be advantageous in some applications to perform the different multilateration processes available according to the accuracy required. Preferably, this is selected on the basis of the source aircraft's height. This has been found to be advantageous since the uncertainty in position in terms of ground position of the aircraft will increase with an increase in height. Hence, when the aircraft is at a high altitude full three dimensional multilateration will be required whereas at a relatively low altitude two dimensional multilateration will suffice. In the described embodiment, for heights between high and the low altitude thresholds a two dimensional multilateration is performed which is augmented with the height of the aircraft.
The transmitted code may include data concerning the height of the aircraft. This may be determined by the aircraft itself or by ground based means. In the case of Secondary Surveillance Radar (SSR) codes, a mode C code includes height information. In this case, the controller may perform a two dimensional multilateration process using some of the receivers and using the value of the height to arrive at a three dimensional location.
The invention also provides a multilateration method.



A specific embodiment of the invention will now be described by way of example only with reference to the drawing in which:
FIG. 1 shows in schematic form a multilateration system 1 operating in accordance with the invention; and
FIGS. 2 to 7 are explanatory drawings.



As is shown in FIG. 1, a multilateration system 1 includes a plurality of receiver stations 2 to 6 positioned at a number of locations on the ground. These receive a transmitted signal include a code from a transponder mounted on an aircraft 7. The code is a Secondary Surveillance RADAR code which may be a mode A, mode S, mode C or may be an unknown mode of code.
Receiver station 4 is termed a master station because it includes a controller which uses the data from the receivers to perform the multilateration process. (In alternative embodiments it need not be co-located with the receiver.) The data from the receiver stations 2, 3, 5 and 6 is passed to the master station 4 over data links 8. The controller 20 is microprocessor based and is shown in greater detail in FIG. 2. It includes a number of input ports 21 to 24 linked to the data links 8 and hence to the receiver stations. The input ports are connected to a correlator 25 which forms the data into sets which originate for particular transmissions of codes or events. The events are correlated by reference to time. Thus, if a mode A, mode C, mode S or an unknown mode code arrive within a certain time frame then they are considered to originate from the same aircraft. The correlator also ties up Time Of Arrival information from all receivers for a given transmission as a so-called TOA Vector which may be of arbitrary length dependent upon the receivers that received a particular emission of a code. The vectors are then stored in memory 26 as tables of times of arrival and associated codes.
The vector held in the memory 26 is then accessed by a locator 27. This includes decision logic 28 which analyses the data to determine for each vector a number of criterion, as will be described later, and then to select the appropriate multilateration process to be applied to the data. The vector together with an instruction as to the process to be applied is then passed to a multilateration processor 28 and the pertinent multilateration process applied. The position is then used to populate an entry in a track table 29. This is shown in FIG. 3. Each row of the table is a termed a “track” and includes the codes whether mode A, C or S Airframe Address, and a position expressed as co-ordinates x, y and z. The track table is accessible to a plot association block 30. This is able to associate different responses from the aircraft to form a single track entry in the track table 30 from multiple track entries.
The plot association block 30 provides an output to a formatter 31 which places the tracks into the correct format for input into a tracker system 32. The tracker system 32 provides an output to an air traffic control system 33 for displaying the tracks to a human air traffic control officer.
The selection criterion referred to above include the following:

    1. Mode type, whether the received signal is mode A, S or C or an unknown mode.
    2. Number of receiver stations providing data to the data set.
    3. Whether the data indicates height or height is available from another system or height may be assumed.
    4. Desired accuracy for the positioning


The height may be determined in a number of ways. If the received code is mode C then this includes a height value provided by the aircraft itself by use of an onboard altimeter for example. (In some embodiments, height may be provided from an earlier multilateration on the same aircraft or from knowledge of the aircrafts flight path which may require the use of a particular altitude for example.)
As is shown in explanatory FIG. 4, the aircraft's height has an uncertainty and this will translate to an uncertainty in the corresponding ground position. Arising from an appreciation of this, the inventors have determined that a satisfactory multilateration at low levels may be achieved by assuming that the aircraft is at zero altitude and co-planar with the receivers and hence a two dimensional multilateration process may suffice. For intermediate levels, a two dimensional multilateration augmented with height information may be used and a three dimensional multilateration using four or more receivers will be required at high altitudes. This banding is shown schematically in FIG. 5.
The decision logic 28 considers each vector in the manner illustrated in FIG. 6. The TOA vector is accessed and, in decision step 60, the vector length is considered. If the vector length is four returns or more then branch 61 is followed to the next step 62.
In step 62, the vector is considered and the receivers making the returns determined. It will be appreciated that even though four returns are available in a practical system these may not be ideally spread. Hence, if the geometry of the group of receivers providing the returns in the vector are not such as to give sufficient accuracy then a negative branch 63 is followed to step 64. If the geometry does offer sufficient accuracy then branch 65 is followed to step 66. In step 66 a full three dimensional multilateration is instructed.
Returning to step 60, if the vector length is less than four then negative branch 67 is followed to step 64. In step 64, a decision is made as to whether or not a two dimensional multilateration or an augmented two dimensional multilateration is to be performed using the barometric height indicated in a mode C emission. The step is divided into various choices dependent upon both the SSR code associated with the data input and the certainty of which type the code might be. The following cases exist:

    
    
        1. Mode S
        2. Mode A
        3. Mode C
        4. Either Mode A or C (i.e. not known which)
    
    


Note that military variants have been ignored for clarity.
Each of these cases has its own decision logic. The logic used is dependent upon the extent of information in the Track Table. The process is illustrated for three cases as shown in FIG. 7 a to d. 
In FIG. 7a, the first case process is represented for an examination of the received code in which the code is a mode S code. In a first consideration step 70, the received code has an airframe address which is compared with the tracks in the track table 29 to see if the airframe address is already present as an entry. If it is, then the process follows branch 71 to the next consideration step 72 in which the track table is examined for the availability of the mode C altitude for the particular track. If the mode C altitude is present, then the branch 73 is followed and the next consideration step 74 made. In step 74, consideration is made as to whether or not the altitude is contained in the current mode S received code. If it is, then branch 75 is followed and the track is used in process 76 to perform a two dimensional multilateration using the altitude in the mode S code and the track in the track table is updated with the position in terms of x,y and z co-ordinates.
If in the step 74, the altitude is not contained in the current mode S then the negative branch 77 is followed to process 78. In this process a multilateration is performed with the altitude from the track table available from the last mode C return. If the result is then passed to a matching process 79 which compares the result with one extrapolated for the track. If there is a match within a certain threshold, then the track in the track table is updated with the new position. If there is no match, then the negative branch 80 is followed to process 81. (In essence this will be because the received code is a new aircraft entering the air traffic control area.) The step 81 results in a multilateration process being performed at zero feet and the track table is updated to include a new track bearing a flag indicating that it is not to be output from the system as it has insufficient positional accuracy. This track will be updated as more codes are received and when the accuracy is acceptable the flag will be brought down permitting the track to be output from the system.
Returning back to step 70, if the airframe address is not present in the track table then negative branch 82 is followed to the step 74. If the altitude is contained in the current mode S code, then the positive branch 83 is followed to step 76. If the attitude is not contained then a negative branch 84 is followed to process 81.
Returning to step 72 if the result of the consideration of the mode C altitude being in the track table is negative then branch 85 is followed to step 74.
In step 78, the multilateration process is performed using the altitude in the track table. However in step 76 the multilateration process will be carried out on the basis of that in the current mode S code. The multilateration process to be applied whether 2d or 2d augmented is done with a consideration of the bandings of FIG. 5. If the height is between Hmin and H3d then a 2d assisted multilateration process is followed and a flag is added indicating reduced accuracy. If the height is below hmin then the multilateration process is a 2d process and the result is marked by a flag as full accuracy. If the height is above h3d then the locator does not provide an output and the track table is not updated.
In case 2 shown in FIG. 7b the currently received code is a mode A code. This code is compared in step 90 with each track in the track table. The matches are then considered in turn in step 91 as to whether or not the track includes a Mode C altitude. If it does then branch 92 to process 93 is followed or if not branch 94 to process 95. If there are no matches then branch 96 is followed to process 97.
In process 93, the height is compared with the banding as before to select the multilateration process to be applied. If the height is between hmin and h3d then a 2d assisted multilateration process is applied using the altitude from the track table for the matching track flagging the result as reduced accuracy. If the height from the track table is below hmin then a 2d multilateration is performed marking the results as full accuracy. if the height is above h3d then there is no output from the locator.
In process 95 a 2d multilateration is carried out and the result flagged as not to be output from the system. The results for position from process 93 and 95 are passed to a comparison step 96. In this comparison step the position in terms of x,y and z co-ordinates is compared with an extrapolated position for the track. If there is a match the results are used to update the track in the track table if there is no match then the loop branch to the step 90 is followed. (Matching may be done in terms of x,y and z or x,y and a z determined from a mode C transmissions in some embodiments.)
In the case of no matches in the code or on the x, y, z co-ordinates in process 96, a 2d multilateration process is carried out and the results marked as not to be output from the system.
In the case of the received code being a mode C code the steps are shown in FIG. 7c. In a first step 100, the matches for the received code in the track table are identified. Then each match has a multilateration process 101 applied to it using the altitude from the track entry in the track table. If the altitude is between hmin and h3d then a 2D assisted multilateration is carried out with the results mark as reduced accuracy. If the altitude is below hmin then a 2d multilateration process is carried out and marked as full accuracy. If the altitude is over h3d then no output results from the locator. The resultant multilateration position is compared with process 102 to an extrapolated position for the particular track form the track table. If there is a match then the track is updated. If there is not a match, then the negative branch is followed back to the step 100.
If there are no matches for the code or co-ordinates then process step 104 is carried out for the received mode C code as it is being received from an aircraft entering the monitored airspace. In process 104 a 2 d or 2d assisted multilateration process is carried out on the basis of the altitude in the received mode C code. In this process if the altitude is between hmin and h3d a 2d assisted multilateration process is carried out with the result marked as reduced accuracy. If the altitude is below hmin then a 2d multilateration process is carried out and the results marked as full accuracy. If the height is above h3d then there is no output from the locator.
The 3D multilateration process will involve four or more of the receivers as disclosed in GB225014 or GB 239531 for example. However, if the height is known, only two time difference of arrival figures need be determined.

## Claims

1. A multilateration system comprising:
a plurality of receiver stations configured to receive aircraft signal data; and
a controller configured to derive aircraft positional data by applying a multilateration process to at least some of the aircraft signal data, wherein the controller is configured to
i) determine an altitude associated with the aircraft signal data;
ii) choose a multilateration process based on the determined altitude; and
iii) perform the chosen multilateration process to determine an aircraft position.

2. The system of claim 1, wherein the controller is configured to perform the multilateration process using the aircraft signal data and the determined altitude.

3. The system of claim 1, wherein the controller is configured to:
use the aircraft signal data to create aircraft track data; and
store the aircraft track data to a memory.

4. The system as claimed in claim 3, wherein the aircraft track data comprises at least one of a mode type indicator, a mode code, and a position.

5. The system as claimed in claim 3, wherein the controller comprises a locator having logic configured to:
compare i) the determined aircraft position provided by the chosen multilateration process with ii) a predicted aircraft position extrapolated from the aircraft track data; and
update the aircraft track data stored in the memory with the determined aircraft position if the comparison is within a certain threshold.

6. The system of claim 1, wherein the controller comprises a locator having control logic configured to:
i) determine a number of the receiver stations that provide the aircraft signal data to the controller,
ii) determine from the aircraft signal data a geometry of the receiver stations that provide the aircraft signal data to the controller,
iii) determine whether the geometry will support a three dimensional multilateration,
iv) perform the three dimensional multilateration if the geometry is determined to support the three dimensional multilateration; and
v) perform a two dimensional or two dimensional augmented multilateration if the geometry is not determined to support the three dimensional multilateration.

7. An air-traffic control system comprising:
a multilateration system that includes:
a plurality of receiver stations configured to receive aircraft signal data, and
a controller configured to derive aircraft positional data by applying a multilateration process to at least some of the aircraft signal data, wherein the controller is configured to
i) determine an altitude associated with the aircraft signal data,
ii) choose a multilateration process based on the determined altitude, and
iii) perform the chosen multilateration process to determine an aircraft position; and


a tracker system configured to display to a system operator aircraft track data provided by the chosen multilateration system.

8. A multilateration method comprising:
receiving an aircraft code at a plurality of receiver stations; and
deriving aircraft positional data by applying a multilateration process to the received aircraft code, wherein the deriving includes:
determining an altitude associated with the received aircraft code;
choosing a multilateration process based on the determined altitude; and
performing the chosen multilateration process to determine an aircraft position.

9. The method of claim 8 wherein the determined altitude is a barometric altitude obtained from the received aircraft code.

10. A method of claim 8, wherein the choosing of the multilateration process comprises comparing the determined altitude with one or more threshold values to determine the multilateration process.

11. A method of claim 8, further comprising:
storing data associated with the received aircraft code as aircraft track data in a memory.

12. The method of claim 11, further comprising:
comparing the determined aircraft position provided by the chosen multilateration process with an extrapolated position from the aircraft track data; and
updating the aircraft track data in the memory with the determined aircraft position if the comparison results in a match.

13. The method of claim 8, further comprising:
displaying the determined aircraft position to an operator.