CA2063003C - Secondary radar system - Google Patents
Secondary radar system Download PDFInfo
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- CA2063003C CA2063003C CA002063003A CA2063003A CA2063003C CA 2063003 C CA2063003 C CA 2063003C CA 002063003 A CA002063003 A CA 002063003A CA 2063003 A CA2063003 A CA 2063003A CA 2063003 C CA2063003 C CA 2063003C
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- signal
- ground
- station
- ground stations
- interrogators
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/87—Combinations of radar systems, e.g. primary radar and secondary radar
- G01S13/878—Combination of several spaced transmitters or receivers of known location for determining the position of a transponder or a reflector
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/74—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems
- G01S13/76—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems wherein pulse-type signals are transmitted
- G01S13/765—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems wherein pulse-type signals are transmitted with exchange of information between interrogator and responder
Abstract
Conventional secondary radar systems use mechanically rotating antennas to radiate a concentrated beam that rotates in a horizontal plane. Data exchange between ground stations and airborne transponders can only take place if the aircraft is struck by the antenna lobe.
The distance from the aircraft to the ground station is determined from the signal transit time.
Directional information is derived from the an-tenna position. The secondary radar system according to the invention uses an omnidirectional antenna, so that data can be exchanged between ground station and air-craft at any time. To determine the position of the aircraft, interrogation signals are transmitted by a single active ground station such that the reply signal from a transponder falls within the common system time frame of the ground station. The times of arrival of the reply signals at different ground stations are then directly proportional to the distances from the aircraft to these ground stations.
The distance from the aircraft to the ground station is determined from the signal transit time.
Directional information is derived from the an-tenna position. The secondary radar system according to the invention uses an omnidirectional antenna, so that data can be exchanged between ground station and air-craft at any time. To determine the position of the aircraft, interrogation signals are transmitted by a single active ground station such that the reply signal from a transponder falls within the common system time frame of the ground station. The times of arrival of the reply signals at different ground stations are then directly proportional to the distances from the aircraft to these ground stations.
Description
~~~'~ ~~' ud.
Secondary Radar System The present invention relates to a secondary radar sys-tem particularly for Mode S operation, as is used for air surveillance and required by the ICAO Specification, Annex 10, Part 1, Chapter 3.8.2.
The construction and operation of secondary radar sys-tems, henceforth called "SSR systems", are well known in the art.
SSR systemsconsist of ground stations and airborne stations. The ground stations send out interrogation sig-nals via a rotating unidirectional antenna. If these in-terrogation signals are received by an airborne station, the latter will transmit reply signals after a fixed de-lay. These reply signals contain, for example, informa-tion on the altitude, temperature, and speed of an air-craft equipped with the airborne station, which is also called "transponder". The ground station locates the aircraft by measuring the elapsed time between the transmission of the interrogation signal and the re-ception of the reply signal and by means of the altitude data contained in the reply signal. Directional information is derived from the position of the rotating antenna. The data-link capability of this system is considerably im-proved if the transponders on the aircraft are addressed by the ground stations selectively, i.e., if the inter-rogation signals transmitted by the ground stations are responded to not by all transponders within the range of a ground station, but only by those whose address was contained in the interrogation signal. The addresses are known to the ground stations either from the flight plan or from squitter signals transmitted by the transponders at intervals of 0.8 to 1.2 seconds.
SSR systems as described above suffer from the drawback that information can be exchanged between ground station and airborne station only if the lobe of the rotating antenna sweeps the area in which the respective aircraft and, hence, the respective transponder are located.
With increasing air traffic density, this is a constraint that may affect safety.
It is the object of the invention to improve an SSR system so that transmission of information between ground station and airborne station is possible at any time.
According to the invention there is provided a secondary radar system for Mode S operation, comprising: a plurality of ground stations, each ground station including a ground-based interrogator which transmits a plurality of interrogation signals and receives a plurality of reply signals; a plurality of antennas respectively connected to said interrogators; at least one airborne station that transmits a reply signal in response to a received interrogation signal; measuring means, connected to each of the int~=rrogators, for measuring a time interval between a transmission of an interrogation signal and a reception of a reply signal; synchronizing means for synchronizing the interrogator in each ground station to a system time within a fixed time frame; said measuring means, including computing means for computing a signal transmit time from a respective one of said interrogators to which a respective measuring means is connected, to the at least one airborne station, and return; and means provided in each of said interrogators, and responsive to the signal transit time computed by said computing means, for adjusting a time of transmission of a further interrogation signal from said respective one of said interrogators such that a further reply signal transmitted by the at least one airborne station, responsive to the further interrogation signal, will be received by said interrogators within the fixed time frame; the further reply signals from the at least one airborne station, responsive to said further interrogation signal, being transmitted at a common time back to said interrogators.
The system according to the invention has the advantage that acquisition of aircraft is possible without transmission of interrogation signals. In an SSR system according to the invention, a position fix is then obtained with the aid of the distance from the aircraft to at least two ground stations and by means of the altitude data contained in the reply signals. This eliminates the need for the costly and failure-prone rotating antennas. In an SSR system of the invention, where no transmission of altitude information to a ground station is required, a third ground station is necessary to obtain a position fix.
The cellular configuration of the network of ground .stations according to a particular embodiment permits surveillance of a Large airspace and, thus, provides increased safety. In another embodiment an advantageous feature of the invention permits distance measurements from aircraft to aircraft by simple means.
A further advantageous feature of the invention makes it possible to check the position fix and to determine any deviation of the delay maintained in the airborne equipment between the receipt of an interrogation signal and the transmission of a reply signal from a predetermined value.
According to another feature of the invention, all interrogators except the one which transmits the interrogation signal may be pure receiver units or operate as such. This is particularly advantageous if the SSR
system according to the invention is to permit precise altitude measurements, e.g., in the approach zones of airports. To increase the accuracy of low-altitude measurements if the interrogators are separated by great distances, low-cost, pure receiver units may be additionally installed at short distances from the active interrogator and used for altitude measurements from the ground.
The SSR system can also be used independently of a cellular network to determine the altitude of airborne-station-equipped aircraft or missiles from the ground.
An embodiment of the invention will now be described with reference to the accompanying drawings, in which:
Fig. la shows a network of ground stations with interrogators;
4a Fig. 1b shows characteristic distances for active and passive operation of interrogators;
Fig. 2 shows the system architecture;
Fig. 3 shows the construction of a ground station with antenna;
Fig. 4a is a timing diagram with the interrogation pulses placed earlier in time (pre-delay);
one airborne station, several ground stations, and Fig. 4b is a timing diagram similar to Fig. 4a;
one ground station, several airborne stations.
In Fig. 1a, a network of ground stations and interrogators, the reference characters 1, 2, 3, and 4 denote :hexagonal cells of a cellular network. The overall network consists of more than four cells, as indicated in the figure by the open cells. At the center of each cell is a ground station. The ground stations are designated 5, 6, 7, and 8.
r~~'~ ~~'!3~.
J ~r7 ~..~ J .
Above the cellular network of ground stations, there are two aircraft 9 and 10. The aircraft 9 is equipped with a transponder 9', and the aircraft 10 with a transponder 10'. The ground stations 5, 6, 7, and 8 contain interrogators which transmit interrogation pulses modulated onto a high-frequency carrier. If these pulses are received by the transponders 9' and 10',,the latter will reply with the transmission of reply signals. In mode S - "S" stands for "selecti.ve" -the interrogation signal transmitted by a ground station includes an address which allows individual aircraft (transponders) to be addressed on a selective basis.
Fig. 1b illustrates the relative proporti.onsof the cells of Fig. 1a. Tn the embodiment being described here, the operational range Ri for active interrogations from a ground station is 120 km, while the reception range Rr is twice as great, i.e., 240 km.
Fig. 2 is a schematic of the system architecture. Re-ference numerals 11 to 15 denote interrogators as are contained in the ground stations 5, 6, 7 and 8 of Fig. 1a.
Associated with each interrogator is an antenna for transmitting the interrogation signals and receiving the reply signals. The antennas are designated 16 to 20. The interrogators 11 to 15 generate the interrogation signals and process the reply signals. Each interrogator is connected to one or more processors 21, 22, 23. In the embodiment shown, the interrogator 13 is connected to the processor 21 by a line 26, to the processor 22 by a Line 28, and to the processor 23 by a line 27. The J ~ y,r 1,7 ..
processors are connected to a common data lima 24. The common data line 24 is connected to a control center 25 by an interconnecting line 32. The system of Fig. 2 operates as follows. At intervals between 0.8 and 1.2.
seconds, the transponders 9° and 10' on the aircraft 9 and 10 (Fig. 1a) send out squitter signals which con-tain the identity of the aircraft in coded form. If at least three ground stations synchronized to the system time, e.g., the ground stations c5, 7, and 8 in Fi.g. 1a, receive a squitter signal from the transponder 9°, they will coarsely determine the transponder's location from the differences between the times of arrival of the signals using the hyperbolic fixing scheme. In Fig. 1a, the aircraft 9 with the transponder 9' is in cell 4, which contains the ground station 8. This cell now becomes an active cell. The interrogator in the ground station sends out an interrogation signal addressed directly to the transponder 9'. The interrogation signal is re-ceived, and after a fixed delay, the transponder 9' re-sponds by transmitting a reply signal. The time interval between the transmission of the interrogation signal and the reception of the reply signal is measured in the ground station. As the propagation velocity and a trans-ponder equipment delay are known, this time interval provides the distance of the aircraft from the ground station.
In the embodiment being described, the interrogator 11 of Fig. 2 is in the ground station 8 of Fig. 1a. After determination of the signal transit time or the distance, the ground station 11 passes this information to several processors, here the processor 21. All interrogators 11 to 15 are synchronized to the same fixed system time.
This system time is transmitted via geostationary satellites, for example.
Fig. 4a shows a two-millisecond frame of the system. time common to all interrogators, which consists of parts I
and II. In part I, the ground stations 'transmit, and in part II, they receive. It is assumed that after coarse location of the aircraft 9, the ground station 8 in cell 4 becomes theactive ground station.
Ground station 8 has sent out an interrogation signal and has determined a distance of 60 km to the air-craft 9 from the time difference between 'transmission of the interrogation signal and reception of the reply signal. With the aid of one of the processors 21, 22, 23, the time of transmission of an interrogation pulse is then placed 328 microseconds from the beginning of part II of the time frame. The black blocks of Fig. 4a designate transmitted pulses, and the hatched blocks received pulses. The narrow blocks are signals sent out by the ground stations, and the wide blocks are signals from the transponders. The transit time of the interro-gation signal transmitted by the ground station 8 is 200 microseconds. After a delay of 128 microseconds, i.e., precisely at the beginning of part II. of the time frame, the transponder 9' transmits a reply signal. This reply signal arrives at the ground station 8, which is 60 km from the ~transponder, after 200 microseconds. The same reply signal arrives at the ground station 7, which is 180 km away, after 600 microseconds. And the ground station 6, which is 150 km away, receives the signal after S00 microseconds. As the time of transmission of the interrogation signal from the ground station 8 is chosen so that the reply signal generated by the trans-ponder is located exactly at the beginning,of part LI
of a time frame, the times of arrival of the reply signals at the ground stations 5, 7, and 8 are a direct measure of e~~l~J ~~~~J
the distances from the ground stations to the trans.ponder 9'. The only active ground station is ground station 8.
The ground stations 6 and 7 are passive and only receive the reply signal. One of the processors 21, 22, 23 to which the distances from the transponder 9' to two ground stations are communicated can immediately determine therefrom the bearing of the aircraft 9 and, together with the altitude data contained in the reply signal, the position of the aircraft. If the transponder 9' did not transmit its reply signal at the beginning of a time frame (or at a fixed reference point within the time frame) at least two ground stations would have to become active to determine the posi.ti:on of the aircraft.
Fig. 4b shows a variant of the timing diagram described with the aid of Fig. 4a. Here, three aircraft are with-in the coverage area of a ground station, which fact is already known to the latter from the above-described coarse location with the aid of the squitter signals. In a first interrogation-reply cycle, the ground station has determined the distances to these aircraft, namely 120 km, 90 km, and 30 km. Then it trans-mits interrogation pulses to the transponders which are located 528 microseconds, 428 microseconds, and 228 microseconds before the beginning of a new time frame.
As a result, all three transponders transmit their reply signals exactly at the beginning of the subsequent time frame. The times of arrival of the reply signals at the ground stations are now directly proportional to the distances to the three transponders. This applies to all ground station in the network. Thus, one active ground station suffices to determine the positions of the aircraft by measuring the times of arrival of the reply signals.
The aircraft position data computed by the processors are placed on the common data line 24 and are thus available for evaluation at the control center 25.
Fig. 3 shows the construction of a ground station. A
multichannel receiver 33 is connected via a set of switches 34, a coupling network 35, and a Butler matrix 36 to an antenna 37 consisting of 16 radiating elements.
The 16 radiating elements of the antenna 37 are arranged on a circle with a diameter of 1 to 2 m. The receiver 33 includes 16 first mixers 38, of which only four are shown here. These first mixers 38 mix the signal from the antenna 37 with the output signal from an oscillator 39. The switch position shown for the switch 34 is the receive position. The outputs of the first mixers 38 are applied to intermediate°frequency stages 40, whose outputs are mixed in mixers 41 with the signal from an oscillator 42. The outputs of the mixers 41 are digitized in an analog-to-digital converter 43 and fed to a digital signal processor 44. The output of the digital signal processor 44 is fed to one or more of the pro-cessors 21 to 23. During transmission, one of the switches 34 is in the switch position not shown. In that case, the antenna 37 is supplied with the output signals from a transmitter 45 for the desired direction. The carrier signal for the transmitter 45 is the output signal from the oscillator 39. The information to be transmitted by the transmitter originates from one of the processors 21 to 23. The overall system can now be operated so that an aircraft whose position has been determined remains in contact with the ground station whose cell it is fil,yi.na over. At the cell boundary, handover from one ground station to the next takes place under control of the data Line 24. As the ground stations use antennas with overlapping, fanlike radiation patterns, information can be exchanged between ground station and transponder at any time. The limitation imposed by a rotating uni-directional antenna is eliminated.
The secondary radar system described can also be used to inform aircraft about distances to other airborne aircraft. This is known under the name "Airborne Colli-sion Avoidance System" (ACAS). It is assumed here that an aircraft was coarsely located with the aid of squitter signals. Then, the ground station in whose cell the aircraft is located sends out an interro-gation signal such that the reply signal transmitted by the aircraft falls within the common system time frame.
Since not only the ground stations but also the trans-ponders are synchronized to the common system time, it suffices to evaluate the times of arrival of the reply signals from the activated transponder in other aircraft if the distance between the aircraft carrying the acti-vated transponder and the other aircraft is to be de-termined.
Secondary Radar System The present invention relates to a secondary radar sys-tem particularly for Mode S operation, as is used for air surveillance and required by the ICAO Specification, Annex 10, Part 1, Chapter 3.8.2.
The construction and operation of secondary radar sys-tems, henceforth called "SSR systems", are well known in the art.
SSR systemsconsist of ground stations and airborne stations. The ground stations send out interrogation sig-nals via a rotating unidirectional antenna. If these in-terrogation signals are received by an airborne station, the latter will transmit reply signals after a fixed de-lay. These reply signals contain, for example, informa-tion on the altitude, temperature, and speed of an air-craft equipped with the airborne station, which is also called "transponder". The ground station locates the aircraft by measuring the elapsed time between the transmission of the interrogation signal and the re-ception of the reply signal and by means of the altitude data contained in the reply signal. Directional information is derived from the position of the rotating antenna. The data-link capability of this system is considerably im-proved if the transponders on the aircraft are addressed by the ground stations selectively, i.e., if the inter-rogation signals transmitted by the ground stations are responded to not by all transponders within the range of a ground station, but only by those whose address was contained in the interrogation signal. The addresses are known to the ground stations either from the flight plan or from squitter signals transmitted by the transponders at intervals of 0.8 to 1.2 seconds.
SSR systems as described above suffer from the drawback that information can be exchanged between ground station and airborne station only if the lobe of the rotating antenna sweeps the area in which the respective aircraft and, hence, the respective transponder are located.
With increasing air traffic density, this is a constraint that may affect safety.
It is the object of the invention to improve an SSR system so that transmission of information between ground station and airborne station is possible at any time.
According to the invention there is provided a secondary radar system for Mode S operation, comprising: a plurality of ground stations, each ground station including a ground-based interrogator which transmits a plurality of interrogation signals and receives a plurality of reply signals; a plurality of antennas respectively connected to said interrogators; at least one airborne station that transmits a reply signal in response to a received interrogation signal; measuring means, connected to each of the int~=rrogators, for measuring a time interval between a transmission of an interrogation signal and a reception of a reply signal; synchronizing means for synchronizing the interrogator in each ground station to a system time within a fixed time frame; said measuring means, including computing means for computing a signal transmit time from a respective one of said interrogators to which a respective measuring means is connected, to the at least one airborne station, and return; and means provided in each of said interrogators, and responsive to the signal transit time computed by said computing means, for adjusting a time of transmission of a further interrogation signal from said respective one of said interrogators such that a further reply signal transmitted by the at least one airborne station, responsive to the further interrogation signal, will be received by said interrogators within the fixed time frame; the further reply signals from the at least one airborne station, responsive to said further interrogation signal, being transmitted at a common time back to said interrogators.
The system according to the invention has the advantage that acquisition of aircraft is possible without transmission of interrogation signals. In an SSR system according to the invention, a position fix is then obtained with the aid of the distance from the aircraft to at least two ground stations and by means of the altitude data contained in the reply signals. This eliminates the need for the costly and failure-prone rotating antennas. In an SSR system of the invention, where no transmission of altitude information to a ground station is required, a third ground station is necessary to obtain a position fix.
The cellular configuration of the network of ground .stations according to a particular embodiment permits surveillance of a Large airspace and, thus, provides increased safety. In another embodiment an advantageous feature of the invention permits distance measurements from aircraft to aircraft by simple means.
A further advantageous feature of the invention makes it possible to check the position fix and to determine any deviation of the delay maintained in the airborne equipment between the receipt of an interrogation signal and the transmission of a reply signal from a predetermined value.
According to another feature of the invention, all interrogators except the one which transmits the interrogation signal may be pure receiver units or operate as such. This is particularly advantageous if the SSR
system according to the invention is to permit precise altitude measurements, e.g., in the approach zones of airports. To increase the accuracy of low-altitude measurements if the interrogators are separated by great distances, low-cost, pure receiver units may be additionally installed at short distances from the active interrogator and used for altitude measurements from the ground.
The SSR system can also be used independently of a cellular network to determine the altitude of airborne-station-equipped aircraft or missiles from the ground.
An embodiment of the invention will now be described with reference to the accompanying drawings, in which:
Fig. la shows a network of ground stations with interrogators;
4a Fig. 1b shows characteristic distances for active and passive operation of interrogators;
Fig. 2 shows the system architecture;
Fig. 3 shows the construction of a ground station with antenna;
Fig. 4a is a timing diagram with the interrogation pulses placed earlier in time (pre-delay);
one airborne station, several ground stations, and Fig. 4b is a timing diagram similar to Fig. 4a;
one ground station, several airborne stations.
In Fig. 1a, a network of ground stations and interrogators, the reference characters 1, 2, 3, and 4 denote :hexagonal cells of a cellular network. The overall network consists of more than four cells, as indicated in the figure by the open cells. At the center of each cell is a ground station. The ground stations are designated 5, 6, 7, and 8.
r~~'~ ~~'!3~.
J ~r7 ~..~ J .
Above the cellular network of ground stations, there are two aircraft 9 and 10. The aircraft 9 is equipped with a transponder 9', and the aircraft 10 with a transponder 10'. The ground stations 5, 6, 7, and 8 contain interrogators which transmit interrogation pulses modulated onto a high-frequency carrier. If these pulses are received by the transponders 9' and 10',,the latter will reply with the transmission of reply signals. In mode S - "S" stands for "selecti.ve" -the interrogation signal transmitted by a ground station includes an address which allows individual aircraft (transponders) to be addressed on a selective basis.
Fig. 1b illustrates the relative proporti.onsof the cells of Fig. 1a. Tn the embodiment being described here, the operational range Ri for active interrogations from a ground station is 120 km, while the reception range Rr is twice as great, i.e., 240 km.
Fig. 2 is a schematic of the system architecture. Re-ference numerals 11 to 15 denote interrogators as are contained in the ground stations 5, 6, 7 and 8 of Fig. 1a.
Associated with each interrogator is an antenna for transmitting the interrogation signals and receiving the reply signals. The antennas are designated 16 to 20. The interrogators 11 to 15 generate the interrogation signals and process the reply signals. Each interrogator is connected to one or more processors 21, 22, 23. In the embodiment shown, the interrogator 13 is connected to the processor 21 by a line 26, to the processor 22 by a Line 28, and to the processor 23 by a line 27. The J ~ y,r 1,7 ..
processors are connected to a common data lima 24. The common data line 24 is connected to a control center 25 by an interconnecting line 32. The system of Fig. 2 operates as follows. At intervals between 0.8 and 1.2.
seconds, the transponders 9° and 10' on the aircraft 9 and 10 (Fig. 1a) send out squitter signals which con-tain the identity of the aircraft in coded form. If at least three ground stations synchronized to the system time, e.g., the ground stations c5, 7, and 8 in Fi.g. 1a, receive a squitter signal from the transponder 9°, they will coarsely determine the transponder's location from the differences between the times of arrival of the signals using the hyperbolic fixing scheme. In Fig. 1a, the aircraft 9 with the transponder 9' is in cell 4, which contains the ground station 8. This cell now becomes an active cell. The interrogator in the ground station sends out an interrogation signal addressed directly to the transponder 9'. The interrogation signal is re-ceived, and after a fixed delay, the transponder 9' re-sponds by transmitting a reply signal. The time interval between the transmission of the interrogation signal and the reception of the reply signal is measured in the ground station. As the propagation velocity and a trans-ponder equipment delay are known, this time interval provides the distance of the aircraft from the ground station.
In the embodiment being described, the interrogator 11 of Fig. 2 is in the ground station 8 of Fig. 1a. After determination of the signal transit time or the distance, the ground station 11 passes this information to several processors, here the processor 21. All interrogators 11 to 15 are synchronized to the same fixed system time.
This system time is transmitted via geostationary satellites, for example.
Fig. 4a shows a two-millisecond frame of the system. time common to all interrogators, which consists of parts I
and II. In part I, the ground stations 'transmit, and in part II, they receive. It is assumed that after coarse location of the aircraft 9, the ground station 8 in cell 4 becomes theactive ground station.
Ground station 8 has sent out an interrogation signal and has determined a distance of 60 km to the air-craft 9 from the time difference between 'transmission of the interrogation signal and reception of the reply signal. With the aid of one of the processors 21, 22, 23, the time of transmission of an interrogation pulse is then placed 328 microseconds from the beginning of part II of the time frame. The black blocks of Fig. 4a designate transmitted pulses, and the hatched blocks received pulses. The narrow blocks are signals sent out by the ground stations, and the wide blocks are signals from the transponders. The transit time of the interro-gation signal transmitted by the ground station 8 is 200 microseconds. After a delay of 128 microseconds, i.e., precisely at the beginning of part II. of the time frame, the transponder 9' transmits a reply signal. This reply signal arrives at the ground station 8, which is 60 km from the ~transponder, after 200 microseconds. The same reply signal arrives at the ground station 7, which is 180 km away, after 600 microseconds. And the ground station 6, which is 150 km away, receives the signal after S00 microseconds. As the time of transmission of the interrogation signal from the ground station 8 is chosen so that the reply signal generated by the trans-ponder is located exactly at the beginning,of part LI
of a time frame, the times of arrival of the reply signals at the ground stations 5, 7, and 8 are a direct measure of e~~l~J ~~~~J
the distances from the ground stations to the trans.ponder 9'. The only active ground station is ground station 8.
The ground stations 6 and 7 are passive and only receive the reply signal. One of the processors 21, 22, 23 to which the distances from the transponder 9' to two ground stations are communicated can immediately determine therefrom the bearing of the aircraft 9 and, together with the altitude data contained in the reply signal, the position of the aircraft. If the transponder 9' did not transmit its reply signal at the beginning of a time frame (or at a fixed reference point within the time frame) at least two ground stations would have to become active to determine the posi.ti:on of the aircraft.
Fig. 4b shows a variant of the timing diagram described with the aid of Fig. 4a. Here, three aircraft are with-in the coverage area of a ground station, which fact is already known to the latter from the above-described coarse location with the aid of the squitter signals. In a first interrogation-reply cycle, the ground station has determined the distances to these aircraft, namely 120 km, 90 km, and 30 km. Then it trans-mits interrogation pulses to the transponders which are located 528 microseconds, 428 microseconds, and 228 microseconds before the beginning of a new time frame.
As a result, all three transponders transmit their reply signals exactly at the beginning of the subsequent time frame. The times of arrival of the reply signals at the ground stations are now directly proportional to the distances to the three transponders. This applies to all ground station in the network. Thus, one active ground station suffices to determine the positions of the aircraft by measuring the times of arrival of the reply signals.
The aircraft position data computed by the processors are placed on the common data line 24 and are thus available for evaluation at the control center 25.
Fig. 3 shows the construction of a ground station. A
multichannel receiver 33 is connected via a set of switches 34, a coupling network 35, and a Butler matrix 36 to an antenna 37 consisting of 16 radiating elements.
The 16 radiating elements of the antenna 37 are arranged on a circle with a diameter of 1 to 2 m. The receiver 33 includes 16 first mixers 38, of which only four are shown here. These first mixers 38 mix the signal from the antenna 37 with the output signal from an oscillator 39. The switch position shown for the switch 34 is the receive position. The outputs of the first mixers 38 are applied to intermediate°frequency stages 40, whose outputs are mixed in mixers 41 with the signal from an oscillator 42. The outputs of the mixers 41 are digitized in an analog-to-digital converter 43 and fed to a digital signal processor 44. The output of the digital signal processor 44 is fed to one or more of the pro-cessors 21 to 23. During transmission, one of the switches 34 is in the switch position not shown. In that case, the antenna 37 is supplied with the output signals from a transmitter 45 for the desired direction. The carrier signal for the transmitter 45 is the output signal from the oscillator 39. The information to be transmitted by the transmitter originates from one of the processors 21 to 23. The overall system can now be operated so that an aircraft whose position has been determined remains in contact with the ground station whose cell it is fil,yi.na over. At the cell boundary, handover from one ground station to the next takes place under control of the data Line 24. As the ground stations use antennas with overlapping, fanlike radiation patterns, information can be exchanged between ground station and transponder at any time. The limitation imposed by a rotating uni-directional antenna is eliminated.
The secondary radar system described can also be used to inform aircraft about distances to other airborne aircraft. This is known under the name "Airborne Colli-sion Avoidance System" (ACAS). It is assumed here that an aircraft was coarsely located with the aid of squitter signals. Then, the ground station in whose cell the aircraft is located sends out an interro-gation signal such that the reply signal transmitted by the aircraft falls within the common system time frame.
Since not only the ground stations but also the trans-ponders are synchronized to the common system time, it suffices to evaluate the times of arrival of the reply signals from the activated transponder in other aircraft if the distance between the aircraft carrying the acti-vated transponder and the other aircraft is to be de-termined.
Claims (23)
1. A secondary radar system for Mode S operation, comprising:
a plurality of ground stations, each ground station including a ground-based interrogator which transmits a plurality of interrogation signals and receives a plurality of reply signals;
a plurality of antennas respectively connected to said interrogators;
at least one airborne station that transmits a reply signal in response to a received interrogation signal;
measuring means, connected to each of the interrogators, for measuring a time interval between a transmission of an interrogation signal and a reception of a reply signal;
synchronizing means for synchronizing the interrogator in each ground station to a system time within a fixed time frame;
said measuring means, including computing means for computing a signal transmit time from a respective one of said interrogators to which a respective measuring means is connected, to the at least one airborne station, and return; and means provided in each of said interrogators, and responsive to the signal transit time computed by said computing means, for adjusting a time of transmission of a further interrogation signal from said respective one of said interrogators such that a further reply signal transmitted by the at least one airborne station, responsive to the further interrogation signal, will be received by said interrogators within the fixed time frame;
the further reply signals from the at least one airborne station, responsive to said further interrogation signal, being transmitted at a common time back to said interrogators.
a plurality of ground stations, each ground station including a ground-based interrogator which transmits a plurality of interrogation signals and receives a plurality of reply signals;
a plurality of antennas respectively connected to said interrogators;
at least one airborne station that transmits a reply signal in response to a received interrogation signal;
measuring means, connected to each of the interrogators, for measuring a time interval between a transmission of an interrogation signal and a reception of a reply signal;
synchronizing means for synchronizing the interrogator in each ground station to a system time within a fixed time frame;
said measuring means, including computing means for computing a signal transmit time from a respective one of said interrogators to which a respective measuring means is connected, to the at least one airborne station, and return; and means provided in each of said interrogators, and responsive to the signal transit time computed by said computing means, for adjusting a time of transmission of a further interrogation signal from said respective one of said interrogators such that a further reply signal transmitted by the at least one airborne station, responsive to the further interrogation signal, will be received by said interrogators within the fixed time frame;
the further reply signals from the at least one airborne station, responsive to said further interrogation signal, being transmitted at a common time back to said interrogators.
2. A secondary radar system as claimed in claim 1, further comprising:
a plurality of cells formed into a cellular network, each of said plurality of cells having a respective one of said interrogators positioned therein;
said respective one of said interrogators in a respective cell in which the airborne station is located and said computing means of said measuring means connected to said respective one of said interrogators, computing said signal transit time from said respective cell to said at least one airborne station, and return.
a plurality of cells formed into a cellular network, each of said plurality of cells having a respective one of said interrogators positioned therein;
said respective one of said interrogators in a respective cell in which the airborne station is located and said computing means of said measuring means connected to said respective one of said interrogators, computing said signal transit time from said respective cell to said at least one airborne station, and return.
3. A secondary radar system as claimed in claim 1, wherein:
the plurality of antennas are positioned at the interrogators; and each antenna comprises a plurality of radiating elements fed through at least one network in such a way that a fan of overlapping radiation patterns is produced in a horizontal plane.
the plurality of antennas are positioned at the interrogators; and each antenna comprises a plurality of radiating elements fed through at least one network in such a way that a fan of overlapping radiation patterns is produced in a horizontal plane.
4. A secondary radar system as claimed in claim 1, wherein:
said computing means of said measuring means is connected to at least three interrogators for computing a position of said at least one airborne station from the reply signal transmitted from said at least one airborne station to said at least three interrogators, within the fixed time frame.
said computing means of said measuring means is connected to at least three interrogators for computing a position of said at least one airborne station from the reply signal transmitted from said at least one airborne station to said at least three interrogators, within the fixed time frame.
5. A secondary radar system as claimed in claim 1, further comprising a cellular network having a plurality of cells, each of said plurality of cells having at least one of said interrogators therein.
6. A secondary radar system as claimed in claim 1, further comprising:
means in each of said plurality of ground stations and each of said at least one airborne stations for exchanging information.
means in each of said plurality of ground stations and each of said at least one airborne stations for exchanging information.
7. A secondary radar system as claimed in claim 1, wherein each airborne station includes a computing means for computing a plurality of distances from the reply and further reply signals radiated within the system time frame.
8. A secondary radar system as claimed in claim 1, wherein:
the plurality of antennas are positioned at the interrogators; and each antenna comprises a plurality of radiating elements fed through at least one network in such a way that a fan of overlapping radiation patterns is produced in a horizontal plane. station equipped with a means for transmitting reply signals in response to the received interrogation signals.
the plurality of antennas are positioned at the interrogators; and each antenna comprises a plurality of radiating elements fed through at least one network in such a way that a fan of overlapping radiation patterns is produced in a horizontal plane. station equipped with a means for transmitting reply signals in response to the received interrogation signals.
9. A secondary radar system as claimed in claim 1, further comprising at least one geostationary satellite for synchronizing the interrogators to the system time.
10. A secondary radar system as claimed in claim 1, further comprising:
at least one additional interrogator connected to at least one measuring means having a computing means for computing a distance to an airborne station whose position was previously computed by at least three other computing means; and said computing means of said at least one measuring means computing a difference between a previously computed distance value to the previously computed position of the airborne station and a distance value calculated from a plurality of coordinates of a present position of the airborne station, and based on the thus computed difference, said computing means of said at least one measuring means computes a temporal correction value which represents a deviation of an elapsed time between receipt of an interrogation signal and transmission of a reply signal from a predetermined value.
at least one additional interrogator connected to at least one measuring means having a computing means for computing a distance to an airborne station whose position was previously computed by at least three other computing means; and said computing means of said at least one measuring means computing a difference between a previously computed distance value to the previously computed position of the airborne station and a distance value calculated from a plurality of coordinates of a present position of the airborne station, and based on the thus computed difference, said computing means of said at least one measuring means computes a temporal correction value which represents a deviation of an elapsed time between receipt of an interrogation signal and transmission of a reply signal from a predetermined value.
11. A secondary radar system as claimed in claim 10, wherein all interrogators but one are pure receiver units or operate as pure receiver units.
12. A secondary radar system as claimed in claim 10, wherein the system includes means for determining an altitude of the airborne station equipped with a means for transmitting reply signals in response to the received interrogation signals.
13. In a secondary radar system for Mode S operation, having:
a plurality of ground stations, each ground station including a clock and a ground-based interrogator which transmits a plurality of interrogation signals and receives a plurality of first reply signals;
a plurality of antennas respectively connected to said interrogators;
at least one airborne station that transmits a first reply signal in response to a received interrogation signal;
measuring means, connected to each of the interrogators, for measuring a time interval between a transmission of an interrogation signal and a reception of a first reply signal;
synchronizing means for synchronizing the interrogator in each ground station to a system time within a fixed time frame;
said measuring means including computing means for computing a signal transit time from a respective one of said interrogators to which a respective measuring means is connected, to the at least one airborne station, and return;
and means provided in each of said interrogators, and responsive to the signal transit time computed by said computing means, for adjusting a time of transmission of a further interrogation signal from said respective one of said interrogators such that a further first reply signal transmitted by the at least one airborne station, responsive to the further interrogation signal, will be received by said interrogators within the fixed time frame;
the further first reply signals from the at least one airborne station, responsive to said further interrogation signal, being transmitted at a common time back to said interrogators;
a method of correcting measurement errors caused by clock deviations in the plurality of ground stations, comprising the steps of:
transmitting toward said at least one airborne station from all the ground stations (S1-S4) which are used to determine a position of the at least one airborne station, a plurality of transmit interrogation signals in a cyclic sequence and at fixed points in time within another time frame that is predetermined by the clock of one of said plurality of ground stations (S1);
receiving second reply signals from said at least one airborne station at all said ground stations, said second reply signals being responsive to said plurality of transmit interrogation signals;
determining at each ground station receiving said second reply signals, a time of reception of said second reply signals;
forming said plurality of ground stations into different respective pairs of ground stations;
each of said different respective pairs of ground stations comprising one of said ground stations and one of a master ground station and another one of said ground stations;
exchanging data between the ground stations of a selected pair of ground stations, said exchanged data representing an elapsed time that is calculated between a transmission of a transmit interrogation signal by one of said selected pair of ground stations and a reception by the other one of said selected pairs of ground stations of a second reply signal from said airborne station that is responsive to said transmit interrogation signal transmitted by said one of said selected pair of ground stations;
calculating a first elapsed time for said selected pair of ground stations beginning at a first time when said one ground station of said selected pair of ground stations transmits a first transmit interrogation signal and ending at a second time when the other one of said selected pair of ground stations receives a second reply signal responsive to the first transmit interrogation signal transmitted by said one ground station;
calculating a second elapsed beginning at another first time when said other one of said ground stations of said selected pair of ground stations transmits a second transmit interrogation signal and ending when the one ground station of said selected pair of ground stations receives another second reply signal from said airborne station that is responsive to said second transmit interrogation signal transmitted from said other ground station; and wherein one-half of an absolute value of a difference between the first and second calculated elapsed times is representative of a deviation between the clocks of the ground stations of said selected pair of ground stations;
and then adjusting a determination of a position of said at least one airborne station based on said deviation between said clocks.
a plurality of ground stations, each ground station including a clock and a ground-based interrogator which transmits a plurality of interrogation signals and receives a plurality of first reply signals;
a plurality of antennas respectively connected to said interrogators;
at least one airborne station that transmits a first reply signal in response to a received interrogation signal;
measuring means, connected to each of the interrogators, for measuring a time interval between a transmission of an interrogation signal and a reception of a first reply signal;
synchronizing means for synchronizing the interrogator in each ground station to a system time within a fixed time frame;
said measuring means including computing means for computing a signal transit time from a respective one of said interrogators to which a respective measuring means is connected, to the at least one airborne station, and return;
and means provided in each of said interrogators, and responsive to the signal transit time computed by said computing means, for adjusting a time of transmission of a further interrogation signal from said respective one of said interrogators such that a further first reply signal transmitted by the at least one airborne station, responsive to the further interrogation signal, will be received by said interrogators within the fixed time frame;
the further first reply signals from the at least one airborne station, responsive to said further interrogation signal, being transmitted at a common time back to said interrogators;
a method of correcting measurement errors caused by clock deviations in the plurality of ground stations, comprising the steps of:
transmitting toward said at least one airborne station from all the ground stations (S1-S4) which are used to determine a position of the at least one airborne station, a plurality of transmit interrogation signals in a cyclic sequence and at fixed points in time within another time frame that is predetermined by the clock of one of said plurality of ground stations (S1);
receiving second reply signals from said at least one airborne station at all said ground stations, said second reply signals being responsive to said plurality of transmit interrogation signals;
determining at each ground station receiving said second reply signals, a time of reception of said second reply signals;
forming said plurality of ground stations into different respective pairs of ground stations;
each of said different respective pairs of ground stations comprising one of said ground stations and one of a master ground station and another one of said ground stations;
exchanging data between the ground stations of a selected pair of ground stations, said exchanged data representing an elapsed time that is calculated between a transmission of a transmit interrogation signal by one of said selected pair of ground stations and a reception by the other one of said selected pairs of ground stations of a second reply signal from said airborne station that is responsive to said transmit interrogation signal transmitted by said one of said selected pair of ground stations;
calculating a first elapsed time for said selected pair of ground stations beginning at a first time when said one ground station of said selected pair of ground stations transmits a first transmit interrogation signal and ending at a second time when the other one of said selected pair of ground stations receives a second reply signal responsive to the first transmit interrogation signal transmitted by said one ground station;
calculating a second elapsed beginning at another first time when said other one of said ground stations of said selected pair of ground stations transmits a second transmit interrogation signal and ending when the one ground station of said selected pair of ground stations receives another second reply signal from said airborne station that is responsive to said second transmit interrogation signal transmitted from said other ground station; and wherein one-half of an absolute value of a difference between the first and second calculated elapsed times is representative of a deviation between the clocks of the ground stations of said selected pair of ground stations;
and then adjusting a determination of a position of said at least one airborne station based on said deviation between said clocks.
14. The method of claim 13, wherein:
the step of forming different pairs of ground stations comprises forming each of said pairs so that one (S1) of the ground stations in each different pair of ground stations is the master ground station; and further comprising determining any deviation of the clocks of any selected pair of ground stations from each other in said master station; and wherein:
said exchanging step comprises communicating data from the master ground station to said other one of said ground station (S2, S3 or S4) of said selected pair, said exchanged data being used by said other one of said ground stations to correct the clock therewithin.
the step of forming different pairs of ground stations comprises forming each of said pairs so that one (S1) of the ground stations in each different pair of ground stations is the master ground station; and further comprising determining any deviation of the clocks of any selected pair of ground stations from each other in said master station; and wherein:
said exchanging step comprises communicating data from the master ground station to said other one of said ground station (S2, S3 or S4) of said selected pair, said exchanged data being used by said other one of said ground stations to correct the clock therewithin.
15. The method of claim 13, further comprising transmitting a clock signal of one of the ground stations over a plurality of data links (V) to all of a remaining plurality of ground stations.
16. The method of claim 14, further comprising transmitting a clock signal of one of the ground stations over a plurality of data links (V) to all of a remaining plurality of ground stations.
17. The method of claim 13, further comprising:
receiving at all of said plurality of ground stations a time standard signal transmitted from a public time standard transmitter; and adjusting the clock in each ground station to said time standard signal.
receiving at all of said plurality of ground stations a time standard signal transmitted from a public time standard transmitter; and adjusting the clock in each ground station to said time standard signal.
18. The method of claim 14, further comprising:
receiving at all of said plurality of ground stations a time standard signal transmitted from a public time standard transmitter; and adjusting the clock in each ground station to said time standard signal.
receiving at all of said plurality of ground stations a time standard signal transmitted from a public time standard transmitter; and adjusting the clock in each ground station to said time standard signal.
19. The method of claim 13, further comprising:
forming each second reply signal to include data identifying the ground station that transmitted a transit interrogation signal to which said second reply signal is responsive.
forming each second reply signal to include data identifying the ground station that transmitted a transit interrogation signal to which said second reply signal is responsive.
20. The method of claim 14, further comprising:
forming each second reply signal to include data, identifying the ground station that transmitted a transit interrogation signal to which said second reply signal is responsive.
forming each second reply signal to include data, identifying the ground station that transmitted a transit interrogation signal to which said second reply signal is responsive.
21. A secondary radar system for Mode S operation, comprising:
a plurality of ground stations, each ground station including a clock and a ground-based interrogator which transmits a plurality of interrogation signals and receivers a plurality of first reply signals;
a plurality of antennas respectively connected to said interrogators;
at least one airborne station that transmits a first reply signal in response to a received interrogation signal;
measuring means, connected to each of the interrogators, for measuring a time interval between a transmission of an interrogation signal and a reception of a first reply signal;
synchronizing means for synchronizing the interrogator in each ground station to a system time within a fixed time frame;
said measuring means including computing means for computing a signal transit time from a respective one of said interrogators to which a respective measuring means is connected, to the at least one airborne station, and return;
means provided in each of said interrogators, and responsive to the signal transit time computed by said computing means, for adjusting a time of transmission of a further interrogation signal from said respective one of said interrogators such that a further first reply signal transmitted by the at least one airborne station, responsive to the further interrogation signal, will be received by said interrogators within the fixed time frame;
the further first reply signals from the at least one airborne station, responsive to said further interrogation signal, being transmitted at a common time back to said interrogators; and correcting means for correcting pairs of clocks of said ground stations, said correcting means including:
deviation determining means for determining any deviation between the clocks of selected pairs of said ground stations; and means for correcting a determination of a position of said at least one airborne station based on the deviation determined by said deviation determining means.
a plurality of ground stations, each ground station including a clock and a ground-based interrogator which transmits a plurality of interrogation signals and receivers a plurality of first reply signals;
a plurality of antennas respectively connected to said interrogators;
at least one airborne station that transmits a first reply signal in response to a received interrogation signal;
measuring means, connected to each of the interrogators, for measuring a time interval between a transmission of an interrogation signal and a reception of a first reply signal;
synchronizing means for synchronizing the interrogator in each ground station to a system time within a fixed time frame;
said measuring means including computing means for computing a signal transit time from a respective one of said interrogators to which a respective measuring means is connected, to the at least one airborne station, and return;
means provided in each of said interrogators, and responsive to the signal transit time computed by said computing means, for adjusting a time of transmission of a further interrogation signal from said respective one of said interrogators such that a further first reply signal transmitted by the at least one airborne station, responsive to the further interrogation signal, will be received by said interrogators within the fixed time frame;
the further first reply signals from the at least one airborne station, responsive to said further interrogation signal, being transmitted at a common time back to said interrogators; and correcting means for correcting pairs of clocks of said ground stations, said correcting means including:
deviation determining means for determining any deviation between the clocks of selected pairs of said ground stations; and means for correcting a determination of a position of said at least one airborne station based on the deviation determined by said deviation determining means.
22. The system of claim 21, further comprising data links (V) for forwarding a clock signal of one of the ground stations to all of a remaining plurality of ground stations.
23. The system of claim 21, wherein said plurality of antennas receive at all of said plurality of ground stations, a time standard signal transmitted from a public time standard transmitter; and the clock in each ground station being adjusted to said time standard signal.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE4109981A DE4109981A1 (en) | 1991-03-27 | 1991-03-27 | SECONDARY RADAR SYSTEM |
| DEP4109981.8 | 1991-03-27 |
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| CA2063003A1 CA2063003A1 (en) | 1992-09-28 |
| CA2063003C true CA2063003C (en) | 2002-12-31 |
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| CA002063003A Expired - Lifetime CA2063003C (en) | 1991-03-27 | 1992-03-13 | Secondary radar system |
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| CA (1) | CA2063003C (en) |
| DE (2) | DE4109981A1 (en) |
| ES (1) | ES2090385T3 (en) |
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| US5363109A (en) * | 1991-03-27 | 1994-11-08 | Alcatel Sel Aktiengesellschaft | Method of correcting measurement errors caused by clock deviations in a secondary radar system |
| DE4224645A1 (en) * | 1992-07-25 | 1994-01-27 | Sel Alcatel Ag | Method for correcting measurement errors caused by time deviations of clocks in a secondary radar system |
| DE19512787A1 (en) * | 1995-02-19 | 1996-09-12 | Horn Wolfgang | Location detector using synthetic aperture, for locating e.g. microwave interference source |
| JP2991710B1 (en) * | 1998-10-30 | 1999-12-20 | 運輸省船舶技術研究所長 | SSR device and aircraft secondary monitoring network |
| US8203486B1 (en) | 1999-03-05 | 2012-06-19 | Omnipol A.S. | Transmitter independent techniques to extend the performance of passive coherent location |
| US7889133B2 (en) | 1999-03-05 | 2011-02-15 | Itt Manufacturing Enterprises, Inc. | Multilateration enhancements for noise and operations management |
| US7570214B2 (en) | 1999-03-05 | 2009-08-04 | Era Systems, Inc. | Method and apparatus for ADS-B validation, active and passive multilateration, and elliptical surviellance |
| US8446321B2 (en) | 1999-03-05 | 2013-05-21 | Omnipol A.S. | Deployable intelligence and tracking system for homeland security and search and rescue |
| US7908077B2 (en) | 2003-06-10 | 2011-03-15 | Itt Manufacturing Enterprises, Inc. | Land use compatibility planning software |
| US7782256B2 (en) | 1999-03-05 | 2010-08-24 | Era Systems Corporation | Enhanced passive coherent location techniques to track and identify UAVs, UCAVs, MAVs, and other objects |
| US7777675B2 (en) | 1999-03-05 | 2010-08-17 | Era Systems Corporation | Deployable passive broadband aircraft tracking |
| US7667647B2 (en) | 1999-03-05 | 2010-02-23 | Era Systems Corporation | Extension of aircraft tracking and positive identification from movement areas into non-movement areas |
| US7739167B2 (en) | 1999-03-05 | 2010-06-15 | Era Systems Corporation | Automated management of airport revenues |
| DE19910715C2 (en) * | 1999-03-10 | 2002-09-26 | Deutsch Zentr Luft & Raumfahrt | Process for autonomous driving robotic vehicles in halls and radar station for carrying out the process |
| DE10330321B4 (en) * | 2003-07-04 | 2006-08-17 | Siemens Ag | Plant for managing a parking space for vehicles |
| DE102005041705A1 (en) | 2005-09-02 | 2007-03-15 | Oerlikon Contraves Ag | Method for room / air space monitoring |
| US7965227B2 (en) | 2006-05-08 | 2011-06-21 | Era Systems, Inc. | Aircraft tracking using low cost tagging as a discriminator |
| DE102010014814A1 (en) | 2010-04-13 | 2011-10-13 | Eads Deutschland Gmbh | Antenna arrangement for use in secondary radar system utilized in trilateration system to support approach for landing of aircraft, has mast for supporting vertical dipoles arranged in array, where each dipole is connected with mast |
| RU2750741C1 (en) * | 2020-05-25 | 2021-07-05 | Быков Андрей Викторович | Radar system |
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| US4197536A (en) * | 1978-10-30 | 1980-04-08 | International Telephone And Telegraph Corporation | Airport surface identification and control system |
| DE2911313C3 (en) * | 1979-03-22 | 1981-11-05 | Siemens AG, 1000 Berlin und 8000 München | Airport surveillance system |
| US4812852A (en) * | 1987-02-20 | 1989-03-14 | Scientific Development Corporation | Locating system and method |
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1991
- 1991-03-27 DE DE4109981A patent/DE4109981A1/en not_active Withdrawn
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- 1992-03-11 ES ES92104119T patent/ES2090385T3/en not_active Expired - Lifetime
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| EP0505827B1 (en) | 1996-06-12 |
| CA2063003A1 (en) | 1992-09-28 |
| DE59206523D1 (en) | 1996-07-18 |
| ES2090385T3 (en) | 1996-10-16 |
| EP0505827A1 (en) | 1992-09-30 |
| DE4109981A1 (en) | 1992-10-01 |
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