WO1998005977A1

WO1998005977A1 - A process for location of objects, mainly aircraft, and a system for carrying out this process

Info

Publication number: WO1998005977A1
Application number: PCT/CZ1997/000027
Authority: WO
Inventors: Vladimír KUBEC^¿EK; Pavel S^¿T^¿ERBA; Miroslav Maly; Oldr^¿ich HEJLEK; Jir^¿í FAJMON; Petr Skalicky; Petr Svoboda
Original assignee: Era A.S.
Priority date: 1996-08-01
Filing date: 1997-08-01
Publication date: 1998-02-12
Also published as: AU3618497A; EP0853767A1

Abstract

A process and a system for location of objects, mainly aircraft (7), where at least three receiving stations (1 to 3) spaced apart from each other on places with known co-ordinates are received signals (6) emitted from object's emitter, these signals (6) are retransmitted in real-time from all receiving stations (1 to 3) into one processing station (5) placed on place with known co-ordinates, in which selection and mutual association of signals emitted by individual emitters and measurements of time delay (τij) between signals belonging to individual emitter which come from individual receiving stations (1 to 3) is carried out and where from measured time delays (τij) and known locations of receiving stations (1 to 3) and processing station (5) location of object emitting received signals (6) is evaluated.

Description

A process for location of objects, mainly aircraft, and a system for carrying out this process

Field of invention

The invention relates to a process for location, mainly aircraft (airplanes), used mainly m a r traffic surveillance and control and to a system for carrying out this process.

Background of invention

In present time the location of airplanes is based mainly on exploitation of primary surveillance radars and ATCRBS (Air Traffic Control Radar Beacon System) Secondary Surveillance Radars (SSR) . The disadvantage of this prior art is a high procurement and exploitation cost of these radars, high power supply requirements, pollution of environment by high intensity microwave emission and high density of interrogations in area where there s high density of interrogators, what decrease efficiency of secondary surveillance radar systems (ATCRBS - A r Traffic Control Radar Beacon System) .

In technical and patent literature there are known aircraft locating systems, based on multilateration T me difference of Arrival (TDOA) principle and processing signals of SSR transponders. The EP 0 385 600 A2 describes „A system for accurately monitoring aircraft position during training exercises". This system uses squitter transmitter on aircraft to interrogate airborne SSR transponder, it means that there is additional electronic equipment on aircraft board to ensure function of the system. Beside that the system uses direct measurement of signal time of arrival in each receiving station by clock synchronized by clock synchronization data distributed to all remaining receiving stations via communication link from one receiving station.

A system HMU - Height Monitor Unit presented by Roke Manor Research also uses multilateration TDOA principle for SSR transponder signals. It consists of at least four receiving stations and evaluates space co-ordinates (x,y, z) of aircraft. The primary function of the system is very accurate measurement of geometric height of aircraft and checking of barometric altimeters of aircraft. Again, the system uses direct measurement of signal time of arrival in each receiving station by clock synchronized by clock synchronization data distributed to all remaining receiving stations via communication link from one (master) receiving station.

Similar embodiment has CAPTS (Cooperative Area Precision Tracking System) of Cardion, Inc. which is dedicated for location and labeling of aircraft in airport area (Airport surveillance function) . The system is based on multilateration TDOA principle and use SSR transponder's signals for its function. Again, the system uses direct measurement of signal time of arrival in each receiving station by clock synchronized by signals of reference transponder which is located on place with known coordinates . All above mentioned systems are characterized in that they require very precise synchronization of clocks in plurality of distributed receiving stations and some additional technical means to ensure this synchronization what is drawback of this type of technical solution.

Summary of the invention

The task of present invention is to remove above given drawbacks of prior art. This task is solved by a process for location of objects, mainly aircraft, where at least at three receiving stations spaced apart from each other on places with known co-ordinates are received signals emitted from object's emitter, these signals are retransmitted in real-time from all receiving stations into one processing station with known co-ordinates, where selection and mutual association of signals emitted by individual objects and measurement of time delay among signals of individual objects which come from individual receiving stations is carried out and where from measured time delays and known locations of receiving stations and processing station location of object emitting received signals is evaluated.

Preferably, signals of secondary surveillance radar transponders in modes 3/A and/or C and/or 1 and/or 2 and/or 4 (IFF) and/or in Mode S are received and processed.

Alternatively, signals emitted by radar and/or navigation mean and/or jammer located on the object are received and processed. Preferably the content of signals or parameters of signals received by each receiving station is evaluated and used for selection and mutual association of signals emitted by individual emitters and measurement of time delay among signals belonging to individual emitter which come from individual receiving stations, mainly m case of dense signal scenario, and s also used for identification of object (s), whereby the mutal association is carried out in accordance with pre-deterrαined limits of time delays.

More preferably, automatic tracking of objects is carried out .

The aim of the present invention is also solved by a system comprising at least three receiving stations and one processing station, where receiving stations are placed on places with known co-ordinates and arranged for receiving signals of said emitters, each of said receiving stations is connected to a processing station, placed on place with known co-ordinates and is common to all receiving stations, is arranged to carry out selection and mutual association of signals emitted by individual objects and measurement of time delay among signals belonging to individual objects and coming from individual receiving stations, and for evaluation of location of object (s) emitting received signals from measured time delays and known locations of the receiving stations and the processing station.

Preferably the emitters are secondary surveillance radar transponders transmitting replies in modes 3/A and/or C and/or 1 and/or 2, and/ore 4 (IFF) and/or Mode S. Another embodiment of the invention is such, that emitter is a radar and/or navigation mean and/or jammer located on the object.

Preferably, the processing station is arranged for evaluation of content of signals or parameters of signals received by each receiving station what is used for selection and mutual association of signals emitted by individual emitters and measurement of time delay among signals belonging to individual emitter which come from individual receiving stations and is also used for identification of object.

The receiving stations are preferably equipped with receiving antennas, receivers of signals emitted by emitters and transmitters of communication links used for transmission of received signals, and the processing station is equipped with receivers of said communication links, and measuring unit for measurement of time of arrival of signals which come from individual communication links and for decoding of the content of these signals or evaluation of parameters of these signals, and a computer for selection and mutual association of signals emitted by individual emitters and belonging to individual emitter which come from individual receiving stations, evaluation of time delays from measured times of arrival, evaluation of location of the object and for identification of object according to content of the signal or parameters of the signal and for tracking of objects. Preferably, the system comprises three receiving stations spaced apart from each other and spaced apart from processing station, or comprises at least four receiving stations spaced apart from each other and spaced apart from processing station.

Particularly preferable is embodiment of the system, where processing station is located at the same place as one of the receiving stations.

Preferably receiving stations contain further receivers for receiving signals of radar and or navigation mean and or jammer located on the oDject and processing station is correspondingly adapted for selection and mutual association of signals, evaluation of time delays of said signals and for evaluation of the signal's parameters.

For the system comprising the three receiving stations (2D system) receiving antennas are directional and directed into area of interest, where said antennas cover azimuth sector approximately 120° for removing object location ambiguity following from the general existence of two intersections of lines of position - hyperbolae.

An advantage of presented invention is lower cost in comparison with primary or secondary radar, low exploitation expenditures and power supply requirements, minimal pollution of environment by microwave emission and the fact that the system according to this invention does not increase by any way density of interrogations in area where there is high density of interrogators. There is not any requirement for additional emitter to be installed on surveyed objects, the system exploits signals of emitters yet installed on aircraft, e.g. signals/replies of (ATCRBS) transponders of secondary surveillance radar system of mode 3/A and/or C and/or 1 and/or 2, 4 (IFF) and/or Mode S, and/or signals of airborne radar and/or other airborne emitters. Primary advantage of the process and the system according to presented invention is the fact that it creates another independent source of location and surveillance data, what brings increase of air traffic safety. Another feature of the system is ability to survey high number of aircraft in large surveillance area.

Description of the Figures

The invention will be described in more details by examples of embodiments depicted on Figures of which

Fig. 1 illustrates schema of basic 2D system embodiment,

Fig. 2 illustrates an example of received signals in the processing station in case of the system according to Fig. 1,

Fig. 3 illustrates schema of basic 2D system embodiment with display of geometrical distances,

Fig. 4 illustrates schema of basic 2D system embodiment with processing station here located on the place of central receiving station, Fig. 5 illustrates an example of time diagram of received signals in processing station case of the system according to Fig. 4,

Fig. 6 illustrates layout of receiving stations and processing station,

Fig. 7 embodiment of the system with four receiving station with display of geometrical distances.

Description of preferred embodiments

Block schema of the first embodiment of the process and system using secondary surveillance radar (SSR) transponders is depicted on the FIG.l. In this example the system consists of three receiving stations 1_, 2_ and 3_ located in different places. The number of the receiving stations can be higher, as described in connection with FIG.7. Individual receiving stations 1 , 2_ and 3_ receive signals _6 emitted by an object, here an aircraft ^and received signals _6 are transmitted m real time into the processing station 5_.

The time delays τ,, between signals coming from receiving stations _1, 2_ and 3_ are measured in the processing station 5_. Based on the measured time delays and known locations of the receiving stations _1, 2_ and 3_ and the processing station 5_ is determined the instantaneous location of the aircraft that emits received signal 6. Such system for location of the aircraft 1_ can be used especially m the field of the Air Traffic Surveillance and Control m the airspace and the airport area, but it is suitable for location of the another objects equipped by the corresponding emitter.

Equations for location of the object for the basic 2D solution according FIG. 1,3, i.e., with processing station 5 placed n a place different from the receiving stations 1_, 2_, 3_ and with three receiving stations 1, 2_, 3_ have the form

/^•,+/?,-(/,+ ) _r,-r₃ R_l-R₂ _ ~r r,, = t-α_r 1 r-σ,, = r- >₁₂ c c c ^" c r,+R₃-(r₂+R₂) _r₃-r₂ &-R. _r₃-r₂ τ_zl = +d₃₂ + + d_n + >₃₂ c c c c

where

Ti2, τ₃₂ = measured delays, i.e., delays evaluated in computer 5_5 from Time Of Arrival of individual signals which come from receiving stations _1, 2_; or 3, 2 respectively, measured in measuring unit 5_4_ i2, d₃ = fixed time delays originating in receivers _ 2 and communication links _14_, 24_, 3_4_ (the difference of the cables' length, in the group delay, etc.)

^r,

distance between aircraft 1_ with coordinates (x,y) and receiving station 1 with co-ordinates

(^χχ,y R_i = J(x, - x_p)² + (y, ~ y_pΫ distance between receiving station 1 with co-ordinates (X Yi) and processing station 5_ with coordinates (x_p,y_p) c = velocity of the light.

Through the solution of above given set of equations for unknown x, y we obtain the co-ordinates of the aircraft 7. Above written set of equations have generally two solutions, correspondingly to two intersections of the hyperbolae. This problem is solved by receiving antennas IA with azimuth limited antenna pattern. Antennas 1_1 are pointed into area of surveillance (area of interest), where exists only one solution. The problem is formulated fully in the plane m this case (receivers 1_2 and aircraft 1_ in the same plane). The situation is different in general. In this case the barometric altitude H evaluated from code C is used for correction of the error caused by the plane representation. Basic set of equation can be defined in different way (e.g., for τ_i3, τ₃₂) . Nevertheless, only two independent equation exist m all cases.

Possible values of hyperbolic delay τ₁₃ are limited by intervals :

.?,,

12 + D, '. > 12

* 32 V ^T ^32 ' ^T ^32

\ C C I

where Rj_.- = distance between receiving stations 1 and j These limits are effectively used in the processing station _5 for association of signals belonging individual emitter which come from individual receiving stations 1_ to 3 to the processing station 5_^

Receiving station 1_, 2 , 3 consists, according FIG.6, of: receiving antenna _1_1 for receiving of the signals 6_ transmitted by SSR transponders with fixed antenna pattern that is optimised for coverage of the area of surveillance, - in the case of the 2D system the pattern is limited in the azimuth to the sector approximately 120° to eliminate the ambiguity following from this embodiment; m the case of the 3D system the antenna pattern is omnidirectional, receivers 1_2 of the signals _6 of the SSR transponders providing video-signal on its output, that is led to the input of the transmitter 1_3 of the communication links 14 , 24_, 34_, communication transmitter 13_ with antenna, if the antenna is necessary, for the considered type of the communication links 1_4_, 2_4_, 34.

Communication links 1_4_, 2_4_, 3_4_ can be made in a different way, e.g., as a metallic l nk (coaxial cable), microwave link, optical fibre link or open space laser link.

Communication lonks _!_ , 2_4, 3_4_ serves for real-time retranslation (relay) of detected video-signal to processing station 5_. Communication links _14_, 2_4_, 3_4 have to be wideband to preserve the shape of the video-signal £> during transmission via communication means 14, 24, 34. Leading and trailing edge of the pulses carry the basic information on Time Of Arrival of the signal 6 to the receiving station 1 , _2, _3- This communication links 14, 24, 34 have to have good transmission delay stability. Transmitter _13_, including antenna if t is necessary, of communication links 1_4, 2_4_, 3_4 is a part of receiving station 1_, 2 , 3_. Receivers 5_1 of communication links 14, 24, 34 are a part of processing station 5_ (including antenna, if it is necessary) .

For the sake of simplicity, only the block scheme of the receiving station 1 is depicted m the FIG.6, further receiving stations 2 , 3_ are arranged in the same way.

Processing station 5_ consists of: measuring unit 5_4 having one channel for each receiving station 1 to 3 that is connected to the computer 55 via fast data channel (e.g., DMA channel) computer 5_5 with proper software.

Measuring unit 5_4_ in each channel : detects presence of replies of the SSR transponders, measures Time Of Arrival of each reply; Time Of Arrival is measured by leading edge method. Time Of Arrival of selected pulse of SSR reply - e.g., frame pulse of standard reply in mode 3/A and/or C and/or 1 and/or 2 or selected pulse of preamble of Mode S reply represents Time Of Arrival of the whole reply. More sophisticated method can be used, when Time Of Arrival of several or all pulses in the reply is measured and these values are averaged to reach minimal random Time Of Arrival error, decodes content SSR reply code, includes degarbling circuitry to detect and process non-overlaping but interleaved replies (what frequently occur in case of higher number of aircraft 1_ ^and higher number of SSR interrogators), includes circuitry to check conformity of the replies to check if the pulses in the code have proper position and pulse width according to ICAO standards), nonconformity is a symptom of replies interference and such replies are either excluded from other processing or are processed in the computer 5_ with special care, transmits Time Of Arrival and content of the codes from each channel in the form of digital words into the computer 5_5 - e.g., via DMA channel.

Measuring unit 5_4 has only one reference clock to measure Time Of Arrival in all channels. The time stability of this clock has not to be to high, sufficient value is about 10^"5. It means that it is not necessary to precisely synchronise clocks at each - geographically distributed - receiving station 1., 2_, 3_.

Computer 55 : associates codes coming from individual channels of the measuring unit 5_4_ according the content of the code and according time relations limiting possible values of Time Difference Of Arrival, evaluates hyperbolic delays (τ₁₃) (by subtracting corresponding Time Of Arrivals) from associated data and from these time delays evaluates Cartesian co-ordinates (x,y) of the aircraft ^{for 2D} system or co-ordinates (x,y,z) for 3D system, carries out tracking of aircraft's 1_ trajectory based on the processing of the sequence of replies from the each individual aircraft 1_, determines and identifies individual modes of the replies, i.e., mode 3/A and/or C and/or 1 and/or 2 by the processing of higher number of replies from the aircraft 7, and detects presence of mode 4 (IFF) evaluates a barometric altitude H of the aircraft 7 from mode C (with resolution 100 ft), evaluates 24 bit aircraft identity and barometric altitude (with resolution 25 ft) and another information from short (56 bits) and long (112 bits) code of Mode S replies, builds up a real-time air-picture and displays it, transmits data on air-picture to the systems for air situation processing and displaying (Air Traffic Control centre, etc. ) .

Hereinafter the example of the embodiment of the process and system with four receiving stations 1 to 4 is described (see FIG.7).

In this example the system for location of aircraft 2 consists of four receiving stations 1_ to 4_ and one processing station _5.

Individual receiving stations 1_ to are placed m terrain in places with known co-ordinates and creates irregular quadrangle. Co-ordinates of individual receiving stations 1 to 4 and processing station Ξ> are known in the processing station 5_. Each of the receiving stations 1 to 4 is equipped accordingly with omnidirectional receiving antenna 1., receiver Y2_ for receiving of the signals §_ from the SSR transponders operating m Mode S and communication transmitter 33 of the communication links li_, 2_4, 3_4_ for real time transmission of the signals _6 received from the aircraft 1_ to the processing station 5_.

Processing station _5 is accordingly preceding embodiment equipped with receivers 5_1, .. of communication links 1 _, ... , measuring unit 5_4_ for measuring of the Time Of Arrival of the signals _6 received from individual communication transmitters _13 and for decoding of the contents of the received signals _6. Further part of the processing station 5_ is the computer 5_5 that is final processor of all obtained data. The result of the processing in the computer 5_5 is information on immediate locations of the identified aircraft's 1_ . This information describing instantaneous air-picture is transmitted in prescribed time intervals for further use, e.g., in Air Traffic Control.

Operation and function of individual parts of the equipment for immediate location of the aircraft 1_ m this embodiment can be described in this way:

Signals _6 from the SSR transponder operating in Mode S transmitted by the aircraft 1_ are continuously received by omnidirectional receiving antennas 1 and receivers 12_ in each of four receiving stations _1 to 4_. Via communication transmitters 13 of the receiving stations 1 to 4 the signals 5 are transmitted in real time to the corresponding communication receiver 5_1 to 5_3 of the processing station 5, where their individual Time Of Arrival is measured and content of Mode S replies is decoded.

Measuring unit 5_4_ measures Time Of Arrival and in the same time decodes contents of the signal 6_. These measured and processed data input into computer 5_5 of the processing station 5_, that at the first selects and associates signals 6 according their decoded contents and time relation limits and then evaluates Time Difference Of Arrival of individual signals _6 from individual communication transmitters 1_3 of receiving stations 1_ to 4_ and then from evaluated Time Differences Of Arrival and known co-ordinates of receiving stations _1 to _4_ and processing station _5 determine instantaneous location of the aircraft 1_ and from decoded content of signal _6 determines aircraft's 1_ Mode S 24-bit address or barometric altitude or other information contained in Mode S reply.

Equations describing system consists of four receiving stations 1_ to 4_, .e., for 3D system are

_r_l+R_] -(r₂+R₂) _r-r -R, , _r,-r₂

¹12 ^— c ^τu12 ^— c ^ c ^"ru12 ^— c ^"r L-Υl r₃-r-^-(r₂+R,) _r,-r₂ R-R, _r₃-r₂ c c c c

The solution of these equations for x_, _, _z are the co-ordinates of the aircraft 7. Possible values of hyperbolic delay τ_± are limited by intervals :

where where Ri_j = distance between receiving stations i and j •

These limits are effectively used in the processing station _5 for association of signals _6 belonging individual emitter which come from individual receiving stations 1 to _4 to the processing station _5

It has to be remarked that generally exists only one solution (x, y, y) of this set of equations (in the upper hemisphere regarding Earth surface) and that is why that this 3D system is omnidirectional - it can use (and use) omnidirectional antennas 11.

Generally it could be used more than four receiving stations 1 to 4_. In this case the set of equation is over- determined and only 3 of them, ensuring the most precise location, can be selected or all equations can be used and solved by least square method.

Processing station _5 can be placed in different place as receiving stations 1 to 4 or it can be placed in common place with one of them (see FIG.7). In this case it is suitable to select "centre" station 1 to 4 in such way to minimise range of the communication links 1 _, 2_4_, 3_4_ and to simplify the whole system in particular by cut down the number of the communication means 1_4, 2_4_, 3_4 comparing implementation according FIG.l and FIG.3.

Such implementation is depicted on the FIG.4 illustrating the principle of operation of 2D system with processing station 5> placed together with centre receiving station 2_ . This type of the system can be implemented in 2D and 3D version.

Location equations for 2D version (see FIG.5) have in this case following form

⁼ - c χτl(^χ-^χι)^{2 +}(^y-^{y 2 +}V^X? ⁺JΪ -J^χZ +^y2)

Receiving station 1 to 4 can include receivers for another navigation and/or radar and/or communication signals and or for the signals of jammers and the processing station 5_ is extended in this case to select and associate signals for the purpose of determination of the time delays and evaluation of parameters of the signals.

At the end we can make following conclusion of the subject of the invention. It is necessary to emphasise the application of the system according this invention based on exploitation of Mode S replies which are transmitted by transponder spontaneously, it means without being interrogated by interrogator. This spontaneous transmission is used in 3 different regimes according Annex 10 to ICAO standard

1. Mode S short air-air surveillance reply or Mode S long air-air surveillance reply used for Traffic advisory and Collision Avoidance System (TCAS)

2. Mode S short squitter reply

3. Mode S long squitter reply used for Automatic Dependent Surveillance - Brodcast (ADS-B) where GPS navigation data are coded into replies

The regimes 1 and 2 are used in present days as a standard. In western Europe about 50 % of commercial aircraft are equipped with this regime and since 1999 this will be obligatory for all aircraft of commercial aviation. In present time in the USA regime 1 is obligatory for aircraft with given number of passengers.

Automatic Dependent Surveillance - Brodcast (ADS-B) concept seems to be main concept of ATC for next century. It is supposed that ADS-B will be used in "en route" environment; ADS-B will be used with secondary radar backup in the terminal area; and ADS-B will be used with primary radar backup within the Airport Surface Traffic Automation (ASTA) system on airport's surface environment. In all these areas the system according this invention can be used as ground based complement of ADS-B and in case of requirement for low-cost solution can replace corresponding radars both primary and secondary.

From the point of view of the presented invention it is substantial, that presented solution does not require any external clock synchronisation tool to synchronise clocks in plurality of distributed receiving stations as it is according to prior art required in practically all designs concerning TDOA locating systems, the solution covers full range of SSR reply signals (classical replies in modes A and/or 3/C and/or 1 and/or 2 and/or IFF mode 4 and all Mode S replies), the system practically does not requires any additional equipment on aircraft, the solution can be applied for long-range (400 km), mid-range and short-range (airport surface) surveillance purposes, the solution, by decoding and processing SSR replies identifies aircraft, provides barometric altitude and other data encoded in replies for each aircraft,

- the has very high capacity in the sense of number of tracked aircraft (200 and more),

- the solution is low-cost and has very low exploitation requirements.

At the end it is necessary to note, that the invention is not limited by presented preferred embodiments, but on the base of presented preferred embodiments it s possible to create various combinations according given claims.

Claims

C L A I M S

1. A process for location of objects, mainly aircraft, characterized in that at least at three receiving stations (1 to 3) spaced apart from each other on places with known co-ordinates are received signals (6) emitted from object's emitter, these signals (6) are retransmitted in real-time from all receiving stations (1 to 3) into one processing station (5) placed on place with known coordinates, where selection and mutual association of signals emitted by individual objects and measurement of time delay

among signals of individual objects which come from individual receiving stations (1 to 3) is carried out and where from measured time delays (τ_Xj) and known locations of receiving stations (1 to 3) and processing station (5) location of object emitting received signals is evaluated.

2. The process as claimed in claim 1, characterized in that signals (6) of secondary surveillance radar transponders are received and processed.

3. The process as claimed in claim 2, characterized in that signals (6) of secondary surveillance radar transponders in Mode S are received and processed.

4. The process as claimed in claim 2, characterized in that signals (6) of secondary surveillance radar transponders in mode 3/A and/or C and/or 1 and/or 2 and/or 4 (IFF) are received and processed.

5. The process as claimed in claim 1, characterized in that signals (6) emitted by radar and/or navigation mean and/or jammer located on the object are received and processed.

6. The process as claimed in claims 1 to 5, characterized in that a content of signals (6) or parameters of signals (6) received by each receiving station (1 to 3) is evaluated and used for selection and mutual association of signals (6) emitted by individual emitters and measurement of time delay (τ_lα) among signals belonging to individual emitter which come from individual receiving stations, and is also used for identification of object, whereby the mutal association is carried out in accordance with pre-determmed limits of time delays (τ_i3) .

7. The process as claimed m claims 1 to 6, characterized in that automatic tracking of objects is carried out.

8. A system for carrying out the process as claimed in any one of preceding claims, characterized n that said system comprises at least three receiving stations (1 to 3) and one processing station (5), where receiving stations (1 to 3) placed on places with known co-ordinates and arranged for receiving signals (6) of said emitters each of said receiving stations (1 to 3) are connected to the processing station (5), placed on place with known co-ordinates and is common to all receiving stations (1 to 3), is arranged to carry out selection and mutual association of signals (6) emitted by individual objects and measurement of time delay (τ_x, ) among signals belonging to individual objects and coming from individual receiving stations (1 to 3), and for evaluation of location of object (s) emitting received signals (6) from measured time delays (τ_ια) and known locations of receiving stations (1 to 3) and processing station (5) .

9. The system as claimed in claim 8, characterized in that emitters are secondary surveillance radar transponders .

10. The system as claimed in claim 9, characterized in that emitters are secondary surveillance radar transponders transmitting Mode S replies.

11. The system as claimed in claim 9, characterized in that emitters are secondary surveillance radar transponders transmitting replies in mode 3/A and/or C and/or 1 and/or 2, and/or 4 (IFF) .

12. The system as claimed in claim 8, characterized in that emitters are radar and/or radionavigation mean and/or jammer emitting signals (6).

13. The system according to any one of claims 8 to 12, characterized in that the processing station (5) is arranged for evaluation of content of signals (6) or parameters of signals (6) received by each receiving station what is used for selection and mutual association of signals emitted by individual emitters and measurement of time delay among signals belonging to individual emitter which come from individual receiving stations and is also used for identification of object (7).

14. The system according to any one of claims 8 to

13, characterized in that the receiving stations (1 to 3) are equipped with receiving antennas (11), receivers (12) of signals (6) emitted by emitters and transmitters (13) of communication links (14) used for transmission of received signals (6), and the processing station (5) is equipped with receivers (51 to 53) of said communication links, and measuring unit (54) for measurement of time of arrival of signals (6) which come from individual communication links and for decoding of the content of these signals (6) or evaluation of parameters of these signals (6), and a computer (55) for selection and mutual association of signals (6) emitted by individual emitters and belonging to individual emitter which come from individual receiving stations (1 to 3), evaluation of time delays (τ_ι ) from measured times of arrival, evaluation of location of the object and for identification of object according to content of the signal or parameters of the signal and for carrying out of tracking of objects.

15. The system according to any one of claims 8 to

14, characterized in that the system comprise the three receiving stations (1 to 3) spaced apart from each other and spaced apart from processing station (5).

16. The system according to any one of claims 8 to 14, characterized in that the system comprise at least four receiving stations (1 to 4) spaced apart from each other and spaced apart from the processing station (5).

17. The system according to any one of claims 8 to 14, characterized in that the one of the receiving stations (1 to 4) is located at the same place as the processing station (5) .

18. The system according to any one of claims 8 to 17, characterized in that the receiving station (1 to 4) contains optional receivers of signals of radar and/or radionavigation mean and/or jammer located on the object and processing station (5) is arranged for selection and mutual association of signals, evaluation of time delays

(ti_j) of said signals (6) and for evaluation of the signal's parameters .

19. The system according to claim 15, characterized in that receiving antennas (11) are directional and directed into area of interest, where said antennas cover azimuth sector approximately 120° for removing object location ambiguity following from the general existence of two intersections of lines of position - hyperbolae.

AMENDED CLAIMS

[received by the International Bureau on 7 January 1998 (07.01.98), original claims 1-19 replaced by amended claims 1-19 (5 pages)]

1. A process for location of objects, mainly aircraft in which at least m three receiving stations (1 to 3) spaced apart from each other in places with known coordinates, signals (6) are received emitted from an object's emitter, these signals (6) are retransmitted from all receiving stations (1 to 3) into one processing station (5) positioned at a place with known co-ordinates where measurement of time αelay (τ_ι)) among signals of individual objects whicn come from individual receiving stations (1 to 3) is carried out and where from measured time delays (τ_xl ) and known locations of receiving stations (1 to 3) and a processing station (5) location of object emitting received signals is evaluated, characterized in that received signals (6) emitted from the object's emitter are retransmitted into the processing station (5) in real-time, and in the processing station (5) selection and mutual association of signals emitted by individual objects s carried out.

2. A process according to claim 1, characterized n that signals (6) of secondary surveillance radar transponders are received and processed.

3. A process according to claim 2, characterized in that signals (6) of secondary surveillance radar transponders in Mode S are received and processed.

4. A process according to claim 2, characterized in that signals (6) of secondary surveillance radar transponders in mode 3/A and/or C and/or 1 and/or 2 and/or 4 (IFF) are received and processed.

5. A process according to claim 1, characterized in that signals (6) emitted by radar and/or navigation means and/or a jammer located on the object are received and processed.

6. A process according to any one of claims 1 to 5, characterized in that the content of signals (6) or parameters of signals (6) received by each receiving station (1 to 3) is evaluated and used for selection and mutual association of signals (6) emitted by individual emitters and measurement of time delay (τ_ιrι) among signals belonging to individual emitter which come from individual receiving stations, and is also used for identification of an object, whereby the mutal association is carried out in accordance with pre-determined limits of time delays (τ_1D).

7. A process according to any one of claims 1 to 6, characterized in that automatic tracking of objects is carried out.

8. A system for carrying out the process according to any one of preceding claims comprising at least three receiving stations (1 to 3) and one processing station (5), where receiving stations (1 to 3) positioned at places with known co-ordinates and arranged for receiving signals (6) of said emitters each of said receiving stations (1 to 3)

AMENDED SHEET (ARTICLE 13) is connected to the processing station (5), positioned at place with known co-ordinates and being common to all receiving stations (1 to 3) and arranged to carry out measurement of the time delay (τ_Xj) among signals belonging to individual objects and coming from individual receiving stations (1 to 3), and for evaluation of location of object (s) emitting received signals (6) from measured time delays (T ) and known locations of receiving stations (1 to 3) and the processing station (5), characterized in that the processing station (5) is arranged to carry out selection and mutual association of signals (6) emitted by individual objects and retransmited in real time by the receiving stations (1 to 3) into the processing station (5) .

9. A system according to claim 8, characterized in that emitters are secondary surveillance radar transponders.

10. A system according to claim 9, characterized in that the emitters are secondary surveillance radar transponders transmitting Mode S replies.

11. A system according to claim 9, characterized in that the emitters are secondary surveillance radar transponders transmitting replies in mode 3/A and/or C and/or 1 and/or 2, and/or 4 (IFF) .

12. A system according to claim 8, characterized in that the emitters are radar and/or radionavigation mean and/or jammer emitting signals (6).

13. A system according to any one of claims 8 to 12, characterized in that the processing station (5) is arranged for evaluation of the content of signals (6) or parameters of signals (6) received by each receiving station, which is used for selection and mutual association of signals emitted by individual emitters and measurement of the time delay among signals belonging to individual emitter which come from individual receiving stations and is also used for identification of an object (7).

14. A system according to any one of claims 8 to 13, characterized in that the receiving stations (1 to 3) are equipped with receiving antennas (11), receivers (12) of signals (6) emitted by emitters and transmitters (13) of communication links (14) used for transmission of received signals (6), and the processing station (5) is equipped with receivers (51 to 53) of said communication links, and a measuring unit (54) for measurement of the time of arrival of signals (6) which come from individual communication links and for decoding of the content of these signals (6) or evaluation of parameters of these signals (6), and a computer (55) for selection and mutual association of signals (6) emitted by individual emitters and belonging to individual emitter which come from individual receiving stations (1 to 3), evaluation of time delays (τ^ ) from measured times of arrival, evaluation of location of the object and for identification of an object according to the content of the signal or parameters of the signal and for carrying out of tracking of objects.

15. A system according to any one of claims 8 to 14, characterized in that the system comprises the three receiving stations (1 to 3) spaced apart from each other and spaced apart from the processing station (5) .

16. A system according to any one of claims 8 to 14, characterized in that the system comprise at least four receiving stations (1 to 4) spaced apart from each other and spaced apart from the processing station (5) .

17. A system according to any one of claims 8 to 14, characterized in that one of the receiving stations (1 to 4) is located at the same place as the processing station (5) .

18. A system according to any one of claims 8 to 17, characterized in that the receiving station (1 to 4) contains optional receivers of signals of radar and/or radionavigation mean and/or a jammer located on the object, and the processing station (5) is arranged for selection and mutual association of signals, evaluation of time delays (tij) of said signals (6) and for evaluation of the signal's parameters.

19. A system according to claim 15, characterized in that receiving antennas (11) are directional and directed into an area of interest, where said antennas cover an azimuth sector of approximately 120° for removing object location ambiguity following from the general existence of two intersections of lines of position - hyperbolae. Statement under article 19

DI (US 3,518,674) describes a system for locating mobile units (for example aircraft) in a surveillance area based on a similar process and system as the present invention. However, this objected to solution uses preset "different fixed delays" at each receiving station to achieve a prescribed sequence of signals from different receiving stations on the input of computer means regardless of the location of mobile unit. This solution does not completely solve the problem of mutual interference of signals coming from different mobile units. According to col. 7, 1. 69 - 74 such measuring is rejected. Due to this it can not be guarantied uninterrupted surveillance of objects, mainly in a high density traffic area.

In contrast to the present invention this objected to solution does not transmit signals to computer means "in real time" - additional delays are selectively introduced in particular receiving stations. Moreover, in the computer means (processing station) there is not carried out "selection and mutual association of signals emitted by individual objects" what is necesary when signals comming from different mobile units appear interleaved in time.

D2 (US 4,215,345) describes a system for determining the position of an emitter using a clock in each receiving station together with the necessity of a calibration beacon being common to all receiving stations. Moreover, the time of arrival is already measured directly at each receiving station, in the processing station there is not processed only a retransmited signal from the respective receiving station but a different one - a transformed signal - precise time-formated (col.l, 1. 53), and the system is designed for particular signals (col. 9, 1. 32 - 38) . In contrast, our invention requires neither a clock at each receiving station or the respective calibration beacon as it is actually a task of our invention to avoid their necessity.

From the above stated comparison it is clear that neither of the objected to solutions transmits signals "in real time". Therefore, neither does the combination of both "Y" documents involve all features claimed in main claims, and thus the invention cannot be considered as not involving an inventive step.

The present invention involves an inventive step as not all features are published in the objected to documents. Moreover, there is no logical link as to why a person skilled in the art would necessarily combine both solutions (see also the age of both objections) . As the main claims involve an inventive step the inventivness of dependent claims does not come in question.

That being the closest prior art is the solution described in DI, as it tries to solve the same problem by similar but not identical technical means. Therefore, we emphasized this by transferring all known features into the respective preamble and after the dividing formula leaving only a combination of new features.

On the grounds of these amendments, both objected to solutions will be added into "background of invention". "Summary of the invention" will also be amended respectively.

We believe that on the ground of this reasoning and amendments it will be found that our invention meets all requirements including an inventive step.