If all midair collisions were eliminated...

Darryl Ramm wrote:
/snip/

And I forgot to mention Mode S TIS (which is not the same as ADS-B TIS-
B). Which is SSR derived traffic position sent over Mode S to each TIS
capable aircraft. Only available where there is terminal radar with
TIS support. Area radar would update too slowly, so it's limited to
terminal radar coverage and then only if that terminal radar facility
has the extra smarts to do TIS. The TIS ground processor calculates
threats to each aircraft and uplinks that to that aircraft over Mode
S. Obviously the threat aircraft need to have Mode C or S transponders
and be within coverage of that terminal radar. /snip/
Darryl

Which reminds me: I fly out of a civil field in a notch of the zone
round a military base - where a digital radar upgrade has recently
appeared. I notice that with my mode C active, I seem to be able to
hear an interrogation as a click as well as see a visual interrogation
flash.

These days, I don't get a repetitive once a revolution style
interrogation, but rather a string of clicks when I am inbound (i.e a
threat to military types which do have squitter) in my headset tuned to
the CTAF

Brian W

On Feb 14, 10:12*pm, brian whatcott wrote:
Darryl Ramm wrote:

/snip/



And I forgot to mention Mode S TIS (which is not the same as ADS-B TIS-
B). Which is SSR derived traffic position sent over Mode S to each TIS
capable aircraft. Only available where there is terminal radar with
TIS support. Area radar would update too slowly, so it's limited to
terminal radar coverage and then only if that terminal radar facility
has the extra smarts to do TIS. The TIS ground processor calculates
threats to each aircraft and uplinks that to that aircraft over Mode
S. Obviously the threat aircraft need to have Mode C or S transponders
and be within coverage of that terminal radar. /snip/
Darryl

Which reminds me: I fly out of a civil field in a notch of the zone
round a military base - where a digital radar upgrade has recently
appeared. * * I notice that with my mode C active, I seem to be able to
hear an interrogation as a click as well as see a visual interrogation
flash.

These days, I don't get a repetitive once a revolution style
interrogation, but rather a string of clicks when I am inbound (i.e a
threat to military types which do have squitter) in my headset tuned to
the CTAF

Brian W

The click is most likely your transponder replying to an interrogation
and likely just coupling though the avionics power supplies, but it
might also be RF coupling.

Also just FYI on interrogations, each click you hear is likely not a
single interrogation. For an radar/ATCRBS (I just wanted to throw in
another acronym) interrogator the beam will hit your transponder with
an alternating pattern of Mode A and Mode C interrogations at ~few
hundred Hz as the beam sweeps over your aircraft. The exact number and
pattern depends on the type of radar/ATCRBS system.

There is also a remote possibility that what you hear if it is
synchronous with a radar antenna rotation (not this case it seems) is
RF interference picked up from the high-power primary radar beam.
Which is much more powerful than the ATCRBS/transponder interrogator
and usually on a quite different frequency than the 1030MHz
interrogator - usually a few GHz and up. The 1030Mhz interrogator beam
is such low power it's not going to cause interference directly, only
by triggering your transponder to fire.

If you are hearing clicking of order once per second or so then it is
possibly TCAS (and/or TCAD) related. These do a clever Mode C
interrogation sequence (the "Whisper-Shout" sequence) at about 1 Hz.
Each TCAS equipped aircraft close enough will hit you independently
with interrogations each at about 1Hz (and technically each
interrogation may fire rapidly several times but you would only hear
one click).

Another possibility if this is only correlated to changes in military
ground infrastructure is that the higher frequency interrogations not
correlated with a radar rotation could be from ground based Mode C
interrogators that are part of a multilateration (MLAT) systems for
surface or airborne tracking. Maybe that got deployed with the new
radar (or maybe the new "digital radar" is an MLAT system).

And all of these interrogations help paint the position of transponder
equipped aircraft for those passive PCAS systems.


Darryl