System and method for determining the position of an aircraft

Abstract

A system for determining the position of an aircraft comprises an emitter arranged at the aircraft for emitting a signal, at least two receivers arranged at different locations for receiving the signal emitted by the emitter, and an evaluation device which is designed to determine an aircraft position based on the known positions of the receivers at the time of the reception of the signal and on a characteristic of the signal emitted by the emitter and received by the receivers. The invention proposes that at least one of the receivers is located above the aircraft, and that the evaluation means is designed to determine a vertical position (ALT) of the aircraft from the signal received by the receivers and the known positions of the receivers.

Claims

1. A system for determining the position of an aircraft comprising an emitter arranged at the aircraft for emitting a signal, at least two receivers arranged at different locations (P1-4) for receiving the signal emitted by the emitter, an evaluation device which is designed to determine an aircraft position based on the known positions (P1-4) of the receivers at the time of the reception of the signal and on a characteristic of the signal emitted by the emitter and received by the receivers, characterized in that at least one of the receivers is located above the aircraft and at least one of the receivers is located below the aircraft and fixedly arranged in the region of an earth surface, and the evaluation means is designed to determine a vertical position (ALT) of the aircraft from the signal received by the receivers located above the aircraft and the receivers located below the aircraft and the known positions (P1-4) of the receivers, further characterized in that the system is operated with the principles of multilateration, wherein the characteristic of the signal processed by the evaluation means for determining the vertical position of the aircraft is a time of arrival (TOA) of the signal and wherein the receivers each comprise time measuring devices with which a respective time of arrival (TOA1-4) of the signal can be determined in a synchronized time system, wherein the evaluation device is designed to determine time differences of arrival (TDOA1-6) from the determined times of arrival (TOA1-4), and to determine a vertical position (ALT(MLAT)) of the aircraft from the time differences of arrival (TDOA1-6) and the positions of the receivers at the respective times of arrival (TOA1-4).

2. The system according to claim 1, characterized in that the at least one receiver located above the aircraft is an ADS-B receiver.

3. The system according to claim 1, characterized in that it is operated with the principles of bistatic range determination, wherein the characteristic of the signal processed by the evaluation means for determining the vertical position (ALT) of the aircraft is a travel time (ρ.sub.T+ρ.sub.R) of the signal.

4. The system according to claim 1, characterized in that the emitter is a radar transponder of the aircraft.

5. The system according to claim 4, characterized in that at least one of the receivers is capable to interrogate the radar transponder of the aircraft.

6. The system according to claim 1, characterized it is operated both with the principles of multilateration, wherein the characteristic of the signal processed by the evaluation means for determining the vertical position (ALT) of the aircraft is a time of arrival (TOA) of the signal, and with the principles of bistatic range determination, wherein the characteristic of the signal processed by the evaluation means for determining the vertical position (ALT) of the aircraft is a travel time (ρ.sub.T+ρ.sub.R) of the signal.

7. The system according to claim 1, characterized in that it comprises a ground based test emitter or transceiver having a known position and being usable for verifying a clock synchronization of the receivers and/or compensating a clock synchronization error (ε) of the receivers.

8. The system according to claim 1, characterized in that the at least one receiver located above the aircraft is located on board a satellite or another aircraft or a drone or a balloon.

9. The system according to claim 1, characterized in that at least one receiver is a receiver of a secondary radar or an ADS-B receiver.

10. A method for determining a position of an aircraft, comprising the steps: a. emitting a signal by means of an emitter located at the aircraft, b. receiving the emitted signal by means of at least two receivers located at different locations, c. determining an aircraft position based on the known positions of the receivers at the time of the reception of the signal and on a characteristic of the signal emitted by the emitter and received by the receivers, characterized in that it further comprises the steps of d. placing at least one of the receivers above the aircraft and at least one of the receivers below the aircraft, e. determining a vertical position (ALT) of the aircraft from the signals received by the receivers placed above the aircraft and the receivers placed below the aircraft and the known positions of the receivers, f. operating the system with the principles of multilateration, wherein the characteristic of the signal processed by the evaluation means for determining the vertical position of the aircraft is a time of arrival (TOA) of the signal and wherein a respective time of arrival (TOA1-4) of the signal is determined in a synchronized time system with time measuring devices comprised by the receivers; g. determining time differences of arrival (TDOA1-6) from the determined times of arrival (TOA1-4) with an evaluation device; and h. determining a vertical position (ALT(MLAT)) of the aircraft from the time differences of arrival (TDOA1-6) and the positions of the receivers at the respective times of arrival (TOA1-4) with the evaluation device.

11. The method according to claim 10, characterized in that it uses the principles of multilateration, wherein the characteristic of the signal for determining the vertical position (ALT) of the aircraft is a time of arrival (TOA) of the signal.

12. The method according to claim 10, characterized in that it uses the principles of bistatic radar, wherein the characteristic of the signal for determining the vertical position (ALT) of the aircraft is a travel time (ρ.sub.T+ρ.sub.R) of the signal.

13. The method according to claim 10, characterized in that the vertical position (ALT(MLAT)) of the aircraft determined from the signals received by the receivers and the known positions of the receivers is compared by means of an automatic comparison device with at least one vertical position (ALT(BARO)) of the aircraft determined by another method, and that when the difference between the vertical position (ALT(MLAT)) determined from the signals received by the receivers and the known positions of the receivers and the vertical position (ALT(BARO)) determined by the other method reaches and/or exceeds a limit value, an action is initiated by means of an automatic action initiating device.

14. The method according to claim 11, characterized in that the vertical position of the aircraft determined by the other method is at least one of a barometric altitude (ALT(BARO)) and a GPS altitude of the aircraft.

15. The method according to claim 10, characterized in that a GPS-error, barometric altitude error or vertical multilateration altitude error is detected based on a two out of three decision, wherein if one of these parameters deviates from the two other parameters and the two other parameters are at least almost congruent the deviating parameter will be classified as erroneous.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An exemplary embodiment of the invention is now explained with respect to the attached drawing. In the drawing show

(2) FIG. 1 a schematic representation of a multilateration system for determining the vertical position of an aircraft,

(3) FIG. 2 a schematic representation showing the principles of multilateration,

(4) FIG. 3 a schematic representation of a disadvantageous arrangement of receivers,

(5) FIG. 4 a schematic representation of an advantageous arrangement of receivers,

(6) FIG. 5 a schematic representation of the principles of bistatic radar,

(7) FIG. 6 a schematic representation similar to FIG. 5 showing a test emitter for verifying a clock synchronization, and

(8) FIG. 7 a flowchart of a method for determining the vertical position of an aircraft.

(9) Functionally equivalent elements and regions in different embodiments are denoted hereinafter with the same reference numerals.

DETAILED DESCRIPTION

(10) A system for determining the position of an aircraft is generally denoted with reference numeral 10 in FIG. 1. The aircraft itself is denoted with reference numeral 12. In FIGS. 1 to 4, the system 10 uses the principles of multilateration, as will be explained below.

(11) The exemplary multilateration system 10 of FIG. 1 comprises an emitter 14 arranged on board the aircraft 12 for emitting a signal. Such an emitter 14 may be realized by a standard aircraft radar transponder operating with SSR mode A/C, mode S or mode S with extended squitter signals, all such signals comprising also an identifier allowing to clearly identify the emitter and the aircraft, respectively. When operating with SSR mode C, mode S or mode S with extended squitter signals, the emitter 14 may also emit a value of the barometric altitude ALT(BARO) measured with a barometric altitude measuring device such as an altimeter 16.

(12) The identifier may be a 4-digit transponder code (“squawk”) inputted by a pilot of the aircraft 12 by means of an input device 18 into the emitter 14 as requested by air traffic control. In the case of mode S or mode S with extended squitter signals the identifier may be an ICAO 24 bit address (“hex code”) attributed to each aircraft and being unique for almost each aircraft in the world (only military aircraft share mode S addresses). The signal which is emitted by the emitter 14 has the reference numeral 22.

(13) In the present exemplary embodiment the multilateration system 10 further comprises four receivers 20a-d arranged at different locations P1-P4 for receiving the same signal 22 emitted by the emitter 14, the receivers 20a-d each having time measuring devices 24a-d with which a respective time of arrival (TOA) of the signal 22 can be determined in a synchronized time system. The receivers 20a-d do not only receive the signals 22 but also receive the above mentioned identifier. The receiver 20a receives the signal 22 at a time of arrival TOA1, the receiver 20b receives the signal 22 at a time of arrival TOA2, and so on.

(14) The term “synchronized time system” means that the time measuring devices 24a-d all indicate the same time. This means e.g. that the receivers 20a-d of the multilateration system 10 would determine an identical value of time of arrival TOA if the signal was received at all receivers 20a-d exactly at the same point of time, that is if the respective distances between the emitter 14 and the receivers 20a-d were absolutely identical. Therefore, each of the receivers 20a-d may have a clock as a time measuring device 24a-d, and a clock error may be determined and removed in real-time as is well known for example from GPS systems.

(15) Additionally or alternatively time stamping based on a central or master clock at one central location (centralized timing system) may be applied. Also possible is the application of a distributed timing system using a synchronization of the distributed receiver based clocks to a reference time. Amongst others, the time reference distribution can be performed by: RF-synchronization (1090 MHz or via independent PTP link); GPS-sync, reference transmitter on well-known position, ADS-B itself

(16) In addition, the local time keeping clock may be composed of a local precision oscillator (like OCXO) or rubidium frequency standard or cesium clock.

(17) The receivers 20a and 20b are at a respective vertical level which is vertically above the aircraft 12 since they are arranged on board respective satellites 26a and 26b. In another not shown embodiment, one or more of the receivers 20a and 20b or both receivers 20a and 20b are on board another aircraft(s) flying above the aircraft 12 and/or on board one or more drone(s) flying above the aircraft 12 and/or on board one or more balloon(s) flying above the aircraft 12. Such a drone or balloon may be a high flying stratospheric drone or balloon, the balloon especially an electrically powered solar drone, the exact drone/balloon positions being known in real-time. The satellites 26a and 26b may be part of a plurality or even a multitude of satellites forming a satellite network distributed preferably all over the world. The satellites 26a and 26b may be geostationary satellites or may be satellites in a low orbit or any other orbit. Typically the receivers 20a and 20b on board of satellites 26a and 26b are ADS-B receivers.

(18) As can be seen from FIG. 1, “above” the aircraft 12 does not mean that the receivers 20a and 20b are located exactly vertically to the aircraft 12 but rather means that the receivers 20a and 20b are arranged anywhere at an altitude level above the earth surface 36 which is higher than the altitude level of the aircraft 12. As is shown in FIG. 1, the receivers 20a and 20b also have a lateral distance to the aircraft. “Vertical” and “lateral” are related to an earth surface 36. Analogous considerations apply to the term “below” which will be used hereinafter.

(19) The satellites 26a and 26b each comprise an emitter 28a and 28b for emitting a signal 30a and 30b, respectively, to a respective ground receiving station 32a and 32B. As shown in FIG. 1, receivers 20c and 20d are located below the aircraft 12. More specifically, receivers 20c and 20d are ground based receivers of a secondary radar station 24a and 24b (SSR) arranged on the earth surface 36. In a non-shown embodiment one of the ground based receivers or both ground based receivers may be ADS-B receivers. Further, in a non-shown embodiment the ground based receivers may be remote from the secondary radar stations.

(20) Part of the multilateration system 10 is an evaluation device 38 which comprises a first processing means 40 designed to determine time differences of arrival TDOA1-TDOA6 from the times of arrival TOA1-TOA4 determined at the respective receivers 20a to 20d. The time difference of arrival TDOA1 is the numerical difference between on the one hand the time of arrival TOA1 of the signal 22 at the receiver 20a on board satellite 26a and on the other hand the time of arrival TOA2 of the signal 22 at the receiver 22b on board satellite 26b. The time difference of arrival TDOA2 is the numerical difference between on the one hand the time of arrival TOA2 of the signal 22 at the receiver 20b on board satellite 26b and the time of arrival TOA3 of the signal 22 at the receiver 20c at secondary radar station 34a, and so on. With the present amount of four receivers 20a-20d six time differences of arrival TDOA1-TDOA6 may be calculated.

(21) The evaluation device 38 may be a computer and the first processing means 40 may be realized by a software module. The evaluation device 38 may be remote from the ground receiving stations 32a and 32b and from the secondary radar stations and 34a and 34b and may be located for example in an air traffic control center (not shown). It communicates with the ground receiving stations 32a and 32b and the secondary radar stations 34a and 34b preferably by means of a glass fiber data link. Of course, any other type of hard wired or wireless data link is possible.

(22) The evaluation device 38 further comprises a second processing means 42 which also may be realized by a software module in the evaluation device 38 and which determines a vertical position ALT(MLAT) of the aircraft 12 above a standard level of the earth surface 36, e.g. mean sea level (“MSL”, corresponding to an aircraft altimeter setting to a local QNH), using multilateration techniques. Alternatively or additionally, it may determine a vertical position of the aircraft 12 above ground (e.g. corresponding to an aircraft altimeter setting to a local QFE) or above a standard atmosphere (e.g. corresponding to an aircraft altimeter setting to 1013 hPa). The principles of multilateration are well known to the person skilled in the art and will be schematically explained later below with reference to FIG. 2.

(23) For the determination of the vertical position ALT(MLAT) by multilateration techniques the second processing means 42 uses the time differences of arrival TDOA1-TDOA6 calculated in the first processing means 40 and the positions P1-P4 of the receivers 20a-20d at the respective times of arrival TOA1-TOA4 of the signal 22. The positions P3 and P4 of the secondary radar stations 34a and 34b in the present exemplary embodiment are stationary and therefore constant, whereas the positions P1 and P2 of the satellites 26a and 26b are variable and transmitted together with the times of arrival TOA1 and TOA2 by means of the signals 30a and 30b to the ground receiving stations 32a and 32B.

(24) It is evident that the time differences of arrival TDOA1-TDOA6 are normally unequal 0 because the distances between the aircraft 12 and the respective receivers 20a-d are different resulting in different values of times of arrival TOA1-TOA4.

(25) Now, a very short explanation of the principles of multilateration will be given with reference to FIG. 2: multilateration (MLAT) is a surveillance technique based on the measurement of the above mentioned time differences of arrival (TDOA1-TDOA6) of the signal 22 emitted by the emitter 14 at the different receivers 20-20d. When determining e.g. the time difference of arrival TDOA4 between the two receivers 20a and 20c one obtains an infinite number of possible locations of the emitter 14. When these possible locations are plotted as a two-dimensional representation, as is shown in FIG. 2, they form a hyperbolic curve 43a (in the three-dimensional space they form a hyperboloid). When determining e.g. the time difference of arrival TDOA5 between the two receivers 20a and 20d one obtains again an infinite number of possible locations of the emitter 14 forming a hyperbolic curve 43b. The intersection between curves 43a and 43b is the vertical position ALT(MLAT) of the aircraft 12. The accuracy is further enhanced by determining the further time differences of arrival TDOA1-TDOA4 resulting in further hyperbolic curves, these curves however not being shown in FIG. 2.

(26) Of course, with the method described above it is not only possible to determine the vertical position of an aircraft. If respective receivers are not only arranged above and below the aircraft 12 but also beside and/or before and/or after the aircraft 12, also the exact horizontal position of the aircraft 12 may be determined by means of multilateration, resulting in a full three-dimensional position indication.

(27) As is shown in FIG. 1, the evaluation device 38 further comprises an automatic comparison device 44 which also may be realized by a software module and which compares the vertical position ALT(MLAT) of the aircraft 12 determined by multilateration, as shown above, with a vertical position ALT(BARO) determined by the barometric altimeter 16 on board the aircraft 12 and transmitted also by the signal 22 (or, additionally or alternatively, with another vertical position determined by the barometric altimeter 16 when set to standard atmospheric pressure of 1013 hPa or to local QFE or local QNH). In a non-shown embodiment a GPS altitude is used additionally or instead of the barometric altitude.

(28) The comparison in comparison device 44 is executed by calculating a difference between the altitude ALT(MLAT) obtained by multilateration and the barometric altitude ALT(BARO). If the calculated difference reaches or exceeds a limit threshold value, an automatic action initiating device 46 (again realized by a software module) initiates an action, for example issues and alarm provided to an air traffic controller. This allows the traffic controller for example to contact a pilot of the aircraft 12 informing him to check the aircraft altitude.

(29) It is to be understood that the method executed by the comparison device 44 and the action initiating device 46 and/or the overall multilateration method may be executed only at certain time intervals, for example every few minutes or the like, in order to regularly monitor the actual vertical and preferably also the horizontal position of an aircraft indicated otherwise by another means, for example secondary surveillance radar (SSR) or ADS-B. It may be executed in shorter time intervals when the aircraft 12 is on a final approach to a runway of an airport, for example established on an ILS or an RNP approach.

(30) As has been exemplary shown with reference to FIG. 1, the multilateration system 10 combines terrestrial (ground-based) receivers 22c and 22d and space based receivers 22a and 22b, each of the receivers 22a-22d able to receive the same signal 22 from the aircraft 12, and an evaluation means designed to use the times of arrival TOA1-TOA4 determined at the terrestrial and space-based receivers 22a-22d for determining a true high precision vertical and preferably also a horizontal position of the aircraft 12 by means of multilateration.

(31) The representations of FIGS. 3 and 4 help to understand the advantage of the multilateration system 10 explained above. FIG. 3 shows a conventional arrangement of purely terrestrial receivers, amongst them the above mentioned receivers 20c and 20d, which allows to determine a horizontal position of the aircraft 12 by means of multilateration. However, this arrangement does not provide for a sufficient vertical accuracy due to geometrical constraints, because the hyperbolic curves on which the aircraft 12 is located and which are calculated from the time differences of arrival in a vertical plane are almost parallel (small intersection angle) and therefore cannot indicate sufficiently precise the vertical position of the aircraft 12.

(32) In contrast hereto, FIG. 4 shows a combined arrangement of terrestrial and space based receivers, amongst them the above mentioned receivers 20a and 20b on board satellites 26a and 26b, which allows additionally to determine the vertical position of the aircraft 12 by means of multilateration with sufficient accuracy, because the hyperbolic curves on which the aircraft 12 is located and which are calculated from the time differences of arrival in a vertical plane now include hyperbolic curves which intersect each other with a relatively large intersection angle, wherein an intersection angle of 90° would constitute the optimum.

(33) In a non-shown embodiment at least one of the receivers (on ground and/or in space) is capable to interrogate the radar transponder of the aircraft. This means that for example a message may be sent by a satellite to the aircraft transponder in order to trigger a reply message. Based on the time of transmission by the satellite and the time of reception of the reply from the aircraft (plus transponder delay, etc.) a range can be determined. The line of position is then a sphere compared to the hyperboloid resulting from the TDOA-principle. This allows for more degrees of freedom in the vertical plane and allows to get more information from the aircraft. The combination of hyperboloids (from TDOA) and ellipsoids allows for improved geometries.

(34) Reference is now made to FIG. 5 showing the principles of bistatic range determination, more specifically of bistatic radar.

(35) FIG. 5 shows a ground based stationary secondary surveillance radar station 34a which emits a first signal 48 to the aircraft 12. The aircraft is equipped with a non-shown transponder comprising a receiver and an emitter, both not being shown in FIG. 5. The receiver on board the aircraft receives the first signal 48, which triggers the emitter on board the aircraft to send a second signal 22. The sent signal 22 is non-directional and therefore received both at space based receivers 20a and 20b on board respective satellites 26a and 26b. A travel time of the first signal 48 from the secondary surveillance radar 34a to the aircraft 12 is denoted with ρ.sub.T, whereas a travel time of the second signal 22 from the aircraft 12 to the receiver 20b on board satellite 26b is denoted with ρ.sub.R.

(36) A non-shown evaluation device knows the exact time when the first signal 48 was sent from the secondary surveillance radar 34a, and also knows the exact time when the second signal 22 was received at the receiver 20b. The evaluation device now can calculate the overall travel time of the signal and may consider also a so called “transponder replay delay time” which is the time span needed by the transponder to process the reception of the first signal 48 and to initiate sending the second signal 22 which may be encoded with additional information such as transponder code, ICAO address, barometric altitude, et cetera.

(37) On the basis of the calculated travel time, the evaluation device may determine a first elliptical curve 52a, the two foci of which being the locations of the secondary surveillance radar 34a and the receiver 20b on board satellite 26b at the time when the second signal 22 was received. The position of the aircraft 12 is somewhere on this first elliptical curve 52a.

(38) In the same way, a second elliptical curve 52b may be determined on the basis of the same first and second signals 48 and 22, the second signal 22 however being additionally received by a second receiver 20a on board a second satellite 26a. The vertical aircraft position is being defined by the intersection of both elliptical curves 52a and 52b.

(39) FIG. 6 shows an example of a bistatic radar were the clock synchronization of the receiver 20b on board satellite 26b is verified by a test emitter 54. Such a test emitter 54 may be similar to an aircraft based transponder, that is comprise a receiver and an emitter, but is located on the ground (earth's surface) 36 and is stationary, that is does not move with respect to the stationary secondary surveillance radar station 34a. By consequence, the relative position of the test emitter 54 and the secondary surveillance radar station 34a is known. Since the distance r.sub.B between the receiver 20b and the secondary surveillance radar station 34a, the distance ρ.sub.R between the test transmitter 54 and the receiver 20b, and the distance ρ.sub.T between the test transmitter 54 and the secondary surveillance radar station 34a all are known, a clock synchronization error c can be determined and used for the compensation of this clock synchronization error c in order to enhance the position determining accuracy.

(40) FIG. 7 is a flow chart of a method for determining a position of an aircraft. The method is initiated in a start block 56. In a block 58, a signal is emitted by means of an emitter located at the aircraft. In a block 60, one receiver for receiving the emitted signal is placed above the aircraft, preferably in space, for example on board a satellite. Furthermore, in block 60 another receiver for receiving the emitted signal is placed either also above the aircraft or below the aircraft, preferably on an earth surface. In a block 62, the emitted signal is received by means of the two receivers located at different locations. In a block 64, an aircraft position is determined based on the known positions of the receivers at the time of the reception of the signal and on a characteristic of the signal emitted by the emitter and received by the receivers. This characteristic may be a time of arrival and/or a travel time of the signal. In a block 66, a vertical position of the aircraft is determined from the signals received by the receivers and the known positions of the receivers. The method ends in an and block 68.