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...