john smith wrote:

In article . com,
wrote:


wrote:

john smith wrote:


but a TCAS II equipped aircraft can see you.

Only if you have a Mode-S transponder.

Nope, all they need is mode C.

Actually, they don't even need mode C, now that I think about it.
You get different levels of service depending on what the target has.

If they've got Mode A you'll get an callout.
If they've got Mode C then you'll get an RA (climb / descend)
If they've got Mode S and they've also got TCAS, the two TCAS's
will boogie with each other to coordinate the RA.

So really mode S by itself does nothing for TCAS II unless you've
also got a TCAS as well.


My understanding is that the Mode-S provides the communication
information. Mode C may contain altitude, but there is no distance or
direction information.


That's correct. Mode C only transmits either the squawk code or the
encoded altitude, depending on the specific interrogation. However,
TCAS is an active system that sends an interrogation and listens for the
reply. It uses the time delay between the interrogation it sends and
the reply it recieves back to accurately measure range. Many use an
antenna array and beamforming techniques to determine azimuth as well.
Therefore, it can determine both distance and direction, and of course
it gets the altitude directly from the reply.

The passive TCAD boxes do not transmit interrogations. Instead they
listen for replies to other interrogations and use some signal
processing to glean rough range information from the dispersion and
amplitude of the received pulse. Considering the lack of a hard
measurement, they do a pretty decent job at estimating range. Strike
finders work under a similar principle. In the case of a transponder
reply, the job is easier because the transmit pulse shape and amplitude
are pretty narrowly defined for an in-spec transponder. Most of the
TCAD boxes do not provide azimuth info. The ones that do also use a
phase array of antennas and signal processing to do beamforming to
resolve the azimuth.