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#61
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In article ,
Robert Ehrlich wrote: Eric Greenwell wrote: ... In any case, the bearing info appears to be determined by using two or more antennas. ... More than 2 are necessary. With 2 antennas, you can only measure the time difference between the 2 received signals. This time difference can be translated into a distance difference. The points from where a given difference is observed are on an hyperbola, which can be considered as the same as its 2 asymptotes, as the distance to the antennas is high compared to their mutual distance. But to determine which of the both asymptotes is the correct one, you need some more information, i.e. another antenna. Well, the mode C height information makes this 2 points, and if you turn the aircraft a few degrees, and assume the points are (relatively) stationary, you can distinguish between the two points, right? The mode C is surely there, but the second part seems complex. They must do it some other way... |
#62
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If you are talking about how azimuth is determined by TCAS units, I believe
it is done through the use of a segmented directional receive antenna. Direction info does not need to be precise within a few degrees. bumper "Mark James Boyd" wrote in message news:40289d41@darkstar... In article , Robert Ehrlich wrote: Eric Greenwell wrote: ... In any case, the bearing info appears to be determined by using two or more antennas. ... More than 2 are necessary. With 2 antennas, you can only measure the time difference between the 2 received signals. This time difference can be translated into a distance difference. The points from where a given difference is observed are on an hyperbola, which can be considered as the same as its 2 asymptotes, as the distance to the antennas is high compared to their mutual distance. But to determine which of the both asymptotes is the correct one, you need some more information, i.e. another antenna. Well, the mode C height information makes this 2 points, and if you turn the aircraft a few degrees, and assume the points are (relatively) stationary, you can distinguish between the two points, right? The mode C is surely there, but the second part seems complex. They must do it some other way... |
#63
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I hope you dont mind some info here. I happen to have a little experience
with TCAS and how it works. TCAS only uses slant range (distance from own ship to target) and the reported mode C (or mode S but that is another story) altitiude. From this it creates a TAU value, as stated earlier, which is the range over range rate. When the time to closest approach thresholds are broken depending on own ships altitiude and state (ie gear down) than alerts and or resolution advisories are issued. TCAS is a very complicated system that was carefully designed and test extensively by the FAA. It has several inches of documents to describe in gross detail how it is to work. I have just way simplified what it does. TCAS II antennas are 4 element and are capable of determining bearing and doing directional interrogations. The bearing accuracy is not good enough for alerting so bearing is only used on the cockpit display. This is for the aid to see and avoid part of TCAS. A lot of $$ was spent on trying to make a system that allowed turns to avoid collisions. It is prefered not to have pilots deviate their altitude. Without the up and comming GPS addition to the system there was no way to make a cost effective system with the needed bearing accuracy. The up and comming ADSB systems will change cost and accuracy of availible systems. Bearing is determined by either the difference in phase or amplitude of the received signal depending on manufacturer. It was their choice what they used as long as they passed the tests. Either system works very good. I believe almost all TCAS II systems are installed with a directional antenna top and bottom. One big issue for the TCAS is the fact that a large population of aircraft owners are not maintaining their Mode C encoders and having them check every 2 years. Encoders have been found to be off by as much as 1000 or more feet. If you are flying at 1500 feet and your encoder says you are at 2200 feet and the TCAS equipt aircraft is at 2000 feet guess who will be decending to avoid a target at 2200 feet. Guess who will be in for a big surprise if they get to 1500 feet. I hope that helps. "bumper" wrote in message ... If you are talking about how azimuth is determined by TCAS units, I believe it is done through the use of a segmented directional receive antenna. Direction info does not need to be precise within a few degrees. bumper "Mark James Boyd" wrote in message news:40289d41@darkstar... In article , Robert Ehrlich wrote: Eric Greenwell wrote: ... In any case, the bearing info appears to be determined by using two or more antennas. ... More than 2 are necessary. With 2 antennas, you can only measure the time difference between the 2 received signals. This time difference can be translated into a distance difference. The points from where a given difference is observed are on an hyperbola, which can be considered as the same as its 2 asymptotes, as the distance to the antennas is high compared to their mutual distance. But to determine which of the both asymptotes is the correct one, you need some more information, i.e. another antenna. Well, the mode C height information makes this 2 points, and if you turn the aircraft a few degrees, and assume the points are (relatively) stationary, you can distinguish between the two points, right? The mode C is surely there, but the second part seems complex. They must do it some other way... |
#64
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On Tue, 10 Feb 2004 19:27:31 -0500, "Calergin"
wrote: I hope you dont mind some info here. I happen to have a little experience with TCAS and how it works. snip One big issue for the TCAS is the fact that a large population of aircraft owners are not maintaining their Mode C encoders and having them check every 2 years. Encoders have been found to be off by as much as 1000 or more feet. If you are flying at 1500 feet and your encoder says you are at 2200 feet and the TCAS equipt aircraft is at 2000 feet guess who will be decending to avoid a target at 2200 feet. Guess who will be in for a big surprise if they get to 1500 feet. I hope that helps. If you go to http://www.arinc.com/tcas/ and download Introduction to TCAS II 7.0 you will have a good grasp of TCAS priciples of operation. Mike Borgelt |
#65
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Calergin wrote:
... TCAS only uses slant range (distance from own ship to target) and the reported mode C (or mode S but that is another story) altitiude. From this it creates a TAU value, as stated earlier, which is the range over range rate. When the time to closest approach thresholds are broken depending on own ships altitiude and state (ie gear down) than alerts and or resolution advisories are issued. ... Can you confirm that such a method for resolution advisories may be of little help when the conflicting aircraft is a glider unable to maintain its altitude which will rather seem to vary randomly seen from the TCAS equipped aircraft? |
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