CA1275479C - Collision avoidance system - Google Patents
Collision avoidance systemInfo
- Publication number
- CA1275479C CA1275479C CA000534898A CA534898A CA1275479C CA 1275479 C CA1275479 C CA 1275479C CA 000534898 A CA000534898 A CA 000534898A CA 534898 A CA534898 A CA 534898A CA 1275479 C CA1275479 C CA 1275479C
- Authority
- CA
- Canada
- Prior art keywords
- station
- ssrs
- identifying
- pulse repetition
- positions
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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Abstract
ABSTRACT
A position-finding collision avoidance system (CAS) at an Own station within the service areas of at least two identified SSRs at known locations derives differential azimuth (A), differential time of arrival (T), identity and altitude data regarding one or more transponder-equipped Other station from standard ATCRBS interrogations and replies. These data are used to compute the positions of Own and Other stations for display at the Own station.
A position-finding collision avoidance system (CAS) at an Own station within the service areas of at least two identified SSRs at known locations derives differential azimuth (A), differential time of arrival (T), identity and altitude data regarding one or more transponder-equipped Other station from standard ATCRBS interrogations and replies. These data are used to compute the positions of Own and Other stations for display at the Own station.
Description
~Z7S479 COLLISION AVOIDANCE SYSTEM
of which the following is a SP~:CIFICATION
BACKGROUND OP THE INVENTION
This invention relates to collision avoidance systems for vehicles such as aircraft, using the standard Aircraft Traffic Control Radar Beacon System (ATCRBS) signals to determine, at an Own station, the positions of Own and any Other transponder-equipped stations within the common areas of two or more secondary surveillance radar (SSR) stationc.
Many collision avoidance systems usin~ the ATCRBS
signals have been devised or proposed. Some simply provide an indication or alarm upon proximity of Own and Other sta-tions; some require active signal transmissions for deter-mination of range; others require uplink data transmissions from ground-based equipment. All are subject, to a greater or lesser extent, to production of false alarms, or missed alarms or radio si~nal interference, such conditions occur-ring frequently under congested airspace conditions where - ~275479 such degradations are least tole~able. Determinations of bearings from Own to Other s~ations, desirable information, have heretofore been difficult ~ o~tain; proposed airborne directional antenna systems for ~his purpose have proven too unreliable and costly to be prac:ical. While North pulses can be used to determine bearinss, this invention avoids the need for so-called North pulse kits to be installed on SSRs.
SU~RY OF TEI ~-_ I NVENT I ON
According to this invention, techniques disclosed in U.S. Patent No. 4,021,~02 anc the patents referred to therein are used with stored data of the locations and sig-natures of all, or an appropriate selection, of existing SSRs to determine passively the geographical locations of an Own station and all Other transponde--equipped stations within an area of interest that is served by two or more SSRs. The needed conditions are generally met quite amply wherever there is enough air traffic to create a need for collision avoidance systems.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram of a preferred embodi-ment of the invention; and Figure 2 is a geometric diagram used in explaining the operation of the system of Figure 1.
~Z7S479 DES~RIPTION OF EXEMPLARY EMBODIMENTS
Referring to Figure 1, the equipment at an Own station, typically aboard an aircraft or other vehicle, includes a standard ATCRBS transponder 1 adapted to receive conventional SSR interrogations at 1030 MHz and to reply thereto at 1090 MHz. It is arranged to provide an output pulse upon receiving each interrogation, in a predetermined time relationship to the received interrogation. A 1090 MHz receiver 2 is adapted to receive the reply signals of any other transponders within its rangP, providing pulse outputs corresponding to such replies. An altimeter and encoder 3 is connected to transponder 1 for including Own's altitude in its replies.
A storage device 4, preferably a non-evanescent register such as a read-only memory tROM) contains an organized listing of all SSRs that might be used with the system, including the signature and geographical location of each. The signature of an SSR is the distinctive combination of main beam rotation period (P) and pulse repetition characteristic ~PRC) assigned to that particular SSR. The term "characteristic" is ~lsed to account for the fact some SSRs are assigned fixed pulse repetition periods, and others are assigned so-called "staggered" pulse repetition periods, wherein the time between successive interrogations varies in a predetermined sequence. For example, an eight-step staggered PRP is repeated continuously, allowing two or more complete sequences to be received during rotational passage of a radar main beam. Since there are only a few thousand SCRs presently installed through the world, it is readily ~;~75~7~3 feasible to store the locations and characteristics of all such radars in the device 4 if desired.
A storage device 5 is adapted to contain data defining Own's estimated position, which can be entered manually or by other external means such as Loran C
equipment, as indicated by the arrow 6. The device 5 is designed to retain the most recently entered data, perhaps when the system is turned off, and to replace said date with revised or updated data when supplied on line 7.
The current Own's estimated position data is supplied to an SSR selector 8, which includes data comparator means arranged in known manner to select, on the basis of their positions as stored in device 4 and Own's estimated position, all SSRs within say 100 miles of Own's position. The selection window may be adjustable, and may be designed to select up to, for example, five of the most favorably located radars. The signatures and locations of the selected radars are supplied to A, T and H computer 9.
The interrogation-related pulses from the receiver of Own's transponder 1, the Other's replies from receiver 2, and encoded Own's altitude from altimeter-encoder 3 are also applied as inputs to computer 9, which may be essentially the same as shown and described in U.S. Patent No. 4,021,802, with reference to the upper three-quarters of Figure 3 thereof, specifically the elements designated therein by the reference numerals 301-304 and 306-319. The PRC selectors, corresponding to elements 301 and 304 of said patent, are adjusted by the SSR selector 8 to accept the interrogations of the detected SSRs and the replies elicited thereby.
of which the following is a SP~:CIFICATION
BACKGROUND OP THE INVENTION
This invention relates to collision avoidance systems for vehicles such as aircraft, using the standard Aircraft Traffic Control Radar Beacon System (ATCRBS) signals to determine, at an Own station, the positions of Own and any Other transponder-equipped stations within the common areas of two or more secondary surveillance radar (SSR) stationc.
Many collision avoidance systems usin~ the ATCRBS
signals have been devised or proposed. Some simply provide an indication or alarm upon proximity of Own and Other sta-tions; some require active signal transmissions for deter-mination of range; others require uplink data transmissions from ground-based equipment. All are subject, to a greater or lesser extent, to production of false alarms, or missed alarms or radio si~nal interference, such conditions occur-ring frequently under congested airspace conditions where - ~275479 such degradations are least tole~able. Determinations of bearings from Own to Other s~ations, desirable information, have heretofore been difficult ~ o~tain; proposed airborne directional antenna systems for ~his purpose have proven too unreliable and costly to be prac:ical. While North pulses can be used to determine bearinss, this invention avoids the need for so-called North pulse kits to be installed on SSRs.
SU~RY OF TEI ~-_ I NVENT I ON
According to this invention, techniques disclosed in U.S. Patent No. 4,021,~02 anc the patents referred to therein are used with stored data of the locations and sig-natures of all, or an appropriate selection, of existing SSRs to determine passively the geographical locations of an Own station and all Other transponde--equipped stations within an area of interest that is served by two or more SSRs. The needed conditions are generally met quite amply wherever there is enough air traffic to create a need for collision avoidance systems.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram of a preferred embodi-ment of the invention; and Figure 2 is a geometric diagram used in explaining the operation of the system of Figure 1.
~Z7S479 DES~RIPTION OF EXEMPLARY EMBODIMENTS
Referring to Figure 1, the equipment at an Own station, typically aboard an aircraft or other vehicle, includes a standard ATCRBS transponder 1 adapted to receive conventional SSR interrogations at 1030 MHz and to reply thereto at 1090 MHz. It is arranged to provide an output pulse upon receiving each interrogation, in a predetermined time relationship to the received interrogation. A 1090 MHz receiver 2 is adapted to receive the reply signals of any other transponders within its rangP, providing pulse outputs corresponding to such replies. An altimeter and encoder 3 is connected to transponder 1 for including Own's altitude in its replies.
A storage device 4, preferably a non-evanescent register such as a read-only memory tROM) contains an organized listing of all SSRs that might be used with the system, including the signature and geographical location of each. The signature of an SSR is the distinctive combination of main beam rotation period (P) and pulse repetition characteristic ~PRC) assigned to that particular SSR. The term "characteristic" is ~lsed to account for the fact some SSRs are assigned fixed pulse repetition periods, and others are assigned so-called "staggered" pulse repetition periods, wherein the time between successive interrogations varies in a predetermined sequence. For example, an eight-step staggered PRP is repeated continuously, allowing two or more complete sequences to be received during rotational passage of a radar main beam. Since there are only a few thousand SCRs presently installed through the world, it is readily ~;~75~7~3 feasible to store the locations and characteristics of all such radars in the device 4 if desired.
A storage device 5 is adapted to contain data defining Own's estimated position, which can be entered manually or by other external means such as Loran C
equipment, as indicated by the arrow 6. The device 5 is designed to retain the most recently entered data, perhaps when the system is turned off, and to replace said date with revised or updated data when supplied on line 7.
The current Own's estimated position data is supplied to an SSR selector 8, which includes data comparator means arranged in known manner to select, on the basis of their positions as stored in device 4 and Own's estimated position, all SSRs within say 100 miles of Own's position. The selection window may be adjustable, and may be designed to select up to, for example, five of the most favorably located radars. The signatures and locations of the selected radars are supplied to A, T and H computer 9.
The interrogation-related pulses from the receiver of Own's transponder 1, the Other's replies from receiver 2, and encoded Own's altitude from altimeter-encoder 3 are also applied as inputs to computer 9, which may be essentially the same as shown and described in U.S. Patent No. 4,021,802, with reference to the upper three-quarters of Figure 3 thereof, specifically the elements designated therein by the reference numerals 301-304 and 306-319. The PRC selectors, corresponding to elements 301 and 304 of said patent, are adjusted by the SSR selector 8 to accept the interrogations of the detected SSRs and the replies elicited thereby.
2~521 ~5~7~
The computer 9 operates in the manner described in said Patent No. 4,021,802 to produce output data representing the differential time of arrival 1', the differential azimuth A, and H, the differenti~l altitude, of each Other station with respect to Own, in associatior, with the respective identlty of the Other, and the identity or signature of the SSR from which it was obtained. Such data will usually appear serially in separate bursLs, in a sequence that depends on the positions of the participating stations and the rotation periods of the SSRs.
The data from computer 9 is stored as it becomes available in a buffer àevice 10, which comprises a group o' registers, each arranged to store associatively the A, T and P~ data relating to an identified Other station, with said l; Other's identity and the location of the SSR from which the data was obtained. As each such set of data is completed, the ~uffer 10 presents it to a position computer 11. When the computer 11 has completed any currently ongoing calcula-tion and is free to do so, it accepts the presented data set and releases the respective buffer register for accumulationof another set.
The computer 11 may be a small general purpose com-puter or a dedicated device, programmed to calculate Own's and Other's geographical positions. One type of program that nas been used successfully for this purpose is based on the "Simplex" algorithm, as described beginning on page 340 of the May 1984 issue of BYTE, a periodical published by McGraw-~ill Inc.
The computer 11 provides outputs representing the positions of Own and an identified Other station in response ~2~7S~7~3 to each data set. Usually it ~ill complete the required calculation before a subsequent data set becomes available. If the calculation requires more time, as when the initial estimate of Own's position is widely erroneous, the data is retained in the buffer until the position computer is ready to accept it.
The Own's and Other's positional data, which may be in latitude-longitude format, for example, with Other's data tagged with its identity code, are applied to a coordinate converter 12 of known type. The converter produces outputs representing range and bearing of the identified Other from Own. A display generator 13, also of known type, uses said outputs to produce signals for controlling a display device 15 such as a cathode ray tube to exhibit Other's range, bearing and identity code. Own's heading, obtained from a device 14 such as a compass, may also be applied to the generator 13 to orient the display with respect to Own's heading.
Figure 2 is a plan or map-like representation of the known positions of two radars SSR 1 and SSR 2 and the (initially unknown) positions of Own and an Other station. The differential azimuths Al and A2 between own and Other with respect to SSR 1 and SSR 2 are determ~ned by computer 9, as are also the differential times of arrival T1 and T2 at Own and Other, from SSR 1 and SSR 2, respectively. The length and direction of the line D
between the radars is known or dire~tly obtainable from the known positions of the radars. R1 and R2 are the lines of position of Own from SSR l and SSR 2, and Sl and S2 are those of Qther from said radars.
~7S~9 Assuming Own to be a~ some estimated position, which usually will not be coincident with Own's true posi-tion, the corresponding direc-ions and lengths of the lines of pssition Rl and R2 are readily determinable. From the directions and the known values of Al and A2, the correspond-ing directions of the lines Sl and S2 may then be calculated.
The position of Other is at the intersection of S1 and S2.
If the initially estimated position of Own were correct, Tl would be l/c (S1 + Y - Rl) after accounting for systemic delays in the transponders, where c is the velocity of radio wave propag~tion.
T2 would be l/c (S2 + Y - R2).
These calculated values of Tl and T2 are compa-ed with the actual values as provided by computer 9. If they agree, the assumption was correct, and the true positions of Own and Other have been determined. If they do not agree, the assumptio~ was incorrect and a new one must be made and the operation repeated.
A recursive algorith~. such as the above mentioned Simplex provides an improved estimate of Own's position with each iteration, converging to one as close as desired to the true position. Once Own's posi~ion is establ~shed by the above algorithm, then a different known algori.hm such as Kalman filtering may be used to update Own's and Other's positlons based on the subsequent data. The number of itera-tions required depends on the degree of accuracy desired, and may be qulte small if the original estimate is reasonably close. Although the operation has been described in an ; -7-~ 7~
environment of two SSRs ar.d one Other station, it is the same with more thar, two SSRs and esser.tially any number of Others, since it requires no radio trar.s~lssions other than those already in use by the exis,ing air traffic control system.
The approximations improve with ~he number of participants.
The computer 11 may be arranged to use the same algorithm by estimating the location of an Other station as the startins point. This mode w~uld be advantageous for example when an Other station is placed at a fixed known location, as on a tower or a mountain top. Then the initial estimate woulc be correct, and Own's location could be deter-minea immediately, without successive approximations. The positions of any additional Othe- stations in the area could then subsequen~ly be determined usins Own's determined posi-tion as its estimated position.
_~_
The computer 9 operates in the manner described in said Patent No. 4,021,802 to produce output data representing the differential time of arrival 1', the differential azimuth A, and H, the differenti~l altitude, of each Other station with respect to Own, in associatior, with the respective identlty of the Other, and the identity or signature of the SSR from which it was obtained. Such data will usually appear serially in separate bursLs, in a sequence that depends on the positions of the participating stations and the rotation periods of the SSRs.
The data from computer 9 is stored as it becomes available in a buffer àevice 10, which comprises a group o' registers, each arranged to store associatively the A, T and P~ data relating to an identified Other station, with said l; Other's identity and the location of the SSR from which the data was obtained. As each such set of data is completed, the ~uffer 10 presents it to a position computer 11. When the computer 11 has completed any currently ongoing calcula-tion and is free to do so, it accepts the presented data set and releases the respective buffer register for accumulationof another set.
The computer 11 may be a small general purpose com-puter or a dedicated device, programmed to calculate Own's and Other's geographical positions. One type of program that nas been used successfully for this purpose is based on the "Simplex" algorithm, as described beginning on page 340 of the May 1984 issue of BYTE, a periodical published by McGraw-~ill Inc.
The computer 11 provides outputs representing the positions of Own and an identified Other station in response ~2~7S~7~3 to each data set. Usually it ~ill complete the required calculation before a subsequent data set becomes available. If the calculation requires more time, as when the initial estimate of Own's position is widely erroneous, the data is retained in the buffer until the position computer is ready to accept it.
The Own's and Other's positional data, which may be in latitude-longitude format, for example, with Other's data tagged with its identity code, are applied to a coordinate converter 12 of known type. The converter produces outputs representing range and bearing of the identified Other from Own. A display generator 13, also of known type, uses said outputs to produce signals for controlling a display device 15 such as a cathode ray tube to exhibit Other's range, bearing and identity code. Own's heading, obtained from a device 14 such as a compass, may also be applied to the generator 13 to orient the display with respect to Own's heading.
Figure 2 is a plan or map-like representation of the known positions of two radars SSR 1 and SSR 2 and the (initially unknown) positions of Own and an Other station. The differential azimuths Al and A2 between own and Other with respect to SSR 1 and SSR 2 are determ~ned by computer 9, as are also the differential times of arrival T1 and T2 at Own and Other, from SSR 1 and SSR 2, respectively. The length and direction of the line D
between the radars is known or dire~tly obtainable from the known positions of the radars. R1 and R2 are the lines of position of Own from SSR l and SSR 2, and Sl and S2 are those of Qther from said radars.
~7S~9 Assuming Own to be a~ some estimated position, which usually will not be coincident with Own's true posi-tion, the corresponding direc-ions and lengths of the lines of pssition Rl and R2 are readily determinable. From the directions and the known values of Al and A2, the correspond-ing directions of the lines Sl and S2 may then be calculated.
The position of Other is at the intersection of S1 and S2.
If the initially estimated position of Own were correct, Tl would be l/c (S1 + Y - Rl) after accounting for systemic delays in the transponders, where c is the velocity of radio wave propag~tion.
T2 would be l/c (S2 + Y - R2).
These calculated values of Tl and T2 are compa-ed with the actual values as provided by computer 9. If they agree, the assumption was correct, and the true positions of Own and Other have been determined. If they do not agree, the assumptio~ was incorrect and a new one must be made and the operation repeated.
A recursive algorith~. such as the above mentioned Simplex provides an improved estimate of Own's position with each iteration, converging to one as close as desired to the true position. Once Own's posi~ion is establ~shed by the above algorithm, then a different known algori.hm such as Kalman filtering may be used to update Own's and Other's positlons based on the subsequent data. The number of itera-tions required depends on the degree of accuracy desired, and may be qulte small if the original estimate is reasonably close. Although the operation has been described in an ; -7-~ 7~
environment of two SSRs ar.d one Other station, it is the same with more thar, two SSRs and esser.tially any number of Others, since it requires no radio trar.s~lssions other than those already in use by the exis,ing air traffic control system.
The approximations improve with ~he number of participants.
The computer 11 may be arranged to use the same algorithm by estimating the location of an Other station as the startins point. This mode w~uld be advantageous for example when an Other station is placed at a fixed known location, as on a tower or a mountain top. Then the initial estimate woulc be correct, and Own's location could be deter-minea immediately, without successive approximations. The positions of any additional Othe- stations in the area could then subsequen~ly be determined usins Own's determined posi-tion as its estimated position.
_~_
Claims (26)
1. Apparatus for determining the positions of a an Own station and an Other transponder-equipped station within the overlapping service area of two or more SSRs at known locations, including at the Own station;
a. means for receiving the interrogations transmitted by said SSRs, b. means for identifying said SSrs, c. means for storing and retrieving the geographical locations of said identified SSRs, d. means for receiving replies transmitted from said Other station in response to said interrogations, e. means for identifying said Other station from its replies, f. means for identifying the SSR eliciting each such reply from the pulse repetition characteristic thereof, g. means for determining from the time relationships between said received interrogations and replies data defining the position of said Other station with respect to the Own station in coordinates of differential azimuth (A) and differen-tial time of arrival (T), and h. means for computing, from said data and the known positions of said SSRs, the positions of said Own and Other stations.
a. means for receiving the interrogations transmitted by said SSRs, b. means for identifying said SSrs, c. means for storing and retrieving the geographical locations of said identified SSRs, d. means for receiving replies transmitted from said Other station in response to said interrogations, e. means for identifying said Other station from its replies, f. means for identifying the SSR eliciting each such reply from the pulse repetition characteristic thereof, g. means for determining from the time relationships between said received interrogations and replies data defining the position of said Other station with respect to the Own station in coordinates of differential azimuth (A) and differen-tial time of arrival (T), and h. means for computing, from said data and the known positions of said SSRs, the positions of said Own and Other stations.
2. The apparatus of claim 1, wherein said means h includes i. means for accepting an initial estimate of the position of one of said Own and Other stations, j. means for computing, from said estimate and said data regarding one of said coordinates and the positions of said SSRs, an estimate of the position of the other of said Own and Other stations, k. means for computing from said latter estimate data regarding the other of said coordinates of said estimated position of said other station, and 1. means for comparing said last mentioned computed data with the actual data regarding said other coordinate to correct said first initial estimate of position.
3. The apparatus of claim 2, wherein said first initial estimate of position is that of said Own station.
4. The apparatus of claim 2, wherein said first initial estimate of position is that of said Other station.
5. The apparatus of claim 2, wherein said one coordinate is differential azimuth (A) and said other coordinate is differential time of arrival (T).
6. The apparatus of claim 1, further including means for displaying the positions of said stations.
7. The apparatus of claim 1, further including means for displaying the range and bearing of said Other station from said Own station.
8. The apparatus of any one of claims 1-3, wherein said means for identifying said SSRs is based on their pulse repeti-tion characteristics.
9. The apparatus of any one of claims 4, 5 or 6, wherein said means for identifying said SSRs is based on their pulse repetition characteristics.
10. The apparatus of claim 7, wherein said means for identifying said SSRs is based on their pulse repetition characteristics.
11. The apparatus of any one of claims 1-3, wherein said means for identifying said SSRs is based on the pulse repetition characteristics and beam rotation periods.
12. The apparatus of any one of claims 4, 5 or 6, wherein said means for identifying said SSRs is based on the pulse repetition characteristics and beam rotation periods.
13. The apparatus of claim 7, wherein said means for identifying said SSRs is based on the pulse repetition charac-teristics and beam rotation periods.
14. A method of determining the positions of an Own station and an Other transponder-equipped station within the overlapping service area of two or more SSRs at known locations, including the steps of a. receiving at the Own station the interrogations transmitted by said SSRs, b. identifying said SSRs, c. retrieving stored geographical locations of said SSRs, d. receiving at the Own station the replies transmitted from said other station in response to said interrogations, e. identifying said Other station from its replies, f. identifying the SSR eliciting each such reply from the pulse repetition characteristic thereof, g. determining from the time relationships between said received interrogations and replies data defining the relative position of said Other station with respect to the Own station in coordinates of differential azimuth (A) and differential time of arrival (T), and h. computing, from said data and the known positions of said SSRs, the positions of said Own and Other stations.
15. The method of claim 14, further including the steps of i. providing an initial estimate of the position of one of said Own and Other stations, j. computing, from said estimate and said data regarding one of said coordinates and the positions of said SSRs, an estimate of the position of the Other of said Own and Other stations, k. computing from said latter estimate data regarding the other of said coordinates of said estimated position of said other station, and l. comparing said last mentioned computed data with the actual data regarding said other coordinate to correct said first initial estimate of position.
16. The method of claim 15, wherein said first initial estimate of position is that of said Own station.
17. The method of claim 15, wherein said first initial estimate of position is that of said Other station.
18. The method of claim 15, wherein said one coordinate is differential azimuth (A) and said other coordinate is differen-tial time of arrival (T).
19. The method of claim 14, further including the steps of displaying the positions of said stations.
20. The method of claim 14, further including the steps of displaying the range and bearing of said Other station from said Own station.
21. The method of any one of claims 14, 15 or 16, wherein the SSRs are identified from their pulse repetition characteris-tics.
22. The method of any one of claims 17, 18 or 19, wherein the SSRs are identified from their pulse repetition characteris-tics.
23. The method of claim 20, wherein the SSRs are iden-tified from their pulse repetition characteristics.
24. The method of any one of claims 14-16, wherein the SSRs are identified from their pulse repetition characteristics and beam rotation periods.
25. The method of any one of claims 17, 18 or 19, wherein the SSRs are identified from their pulse repetition characteris-tics and beam rotation periods.
26. The method of claim 20, wherein the SSRs are iden-tified from their pulse repetition characteristics and beam rotation periods.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000534898A CA1275479C (en) | 1987-04-16 | 1987-04-16 | Collision avoidance system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000534898A CA1275479C (en) | 1987-04-16 | 1987-04-16 | Collision avoidance system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1275479C true CA1275479C (en) | 1990-10-23 |
Family
ID=4135449
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000534898A Expired - Fee Related CA1275479C (en) | 1987-04-16 | 1987-04-16 | Collision avoidance system |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1275479C (en) |
-
1987
- 1987-04-16 CA CA000534898A patent/CA1275479C/en not_active Expired - Fee Related
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