Mike Schumann wrote:
Would the MTI also filter you out if you had a Mode C transponder?

Mike, AIUI there are two quite different radar types. The originals,
now know as Primary Radar, send out a signal and deduce your range and
direction (azimuth) from a reflection. That sort does not know whether
you have a transponder or not, reflections could be improved by large
enough Luneberg lenses (of dubious benefit for the size we could get
into a glider fuselage), and is subject to MTI if they switch it on.
Also, on a visit to Stansted UK (London's "third" airport) some years
ago, their ATC explained to me that not only did it fail to see slow
moving targets when MTI was on - which it almost always was, to reduce
screen clutter; it was also blind to anything with a radial speed
towards or away from the radar head at certain harmonics of the wave
length - in practice at 37.5 knots, 75 knots, and multiples. So even
with MTI off, thermalling gliders that were good reflectors would be
invisible half the time, and cruising towards or away from them would
also reduce or eradicate their trace of us.

Transponders use SSR (Secondary Surveillance Radar) which sends out an
APB to all transponders within range - "Oi, anyone out there?" to which
they all respond. No transponder, no response, SSR cannot see you.
With Mode S (for Selective), the Controller can then tweak it to say
next time around "Oi, only those transponders I have Selected - where
are you now?". Gliders etc. not so selected will then not be seen by
ATC. This has three effects (at least):

They have less cluttered screens, seeing only those that they want to
for control purposes;

Each Mode S transponder has a unique code identifying it to the
aircraft it is in, so they never run out of codes (unlike Modes A and
C, where Europe already has problems, because the existing 7xxx - type
codes are too few for ATC's purposes);

And transponders NOT selected by ATC would not be made to transmit a
signal zillions of times per second like a Mode A or C is, only say
once per second, so they still show up to TCAS units in the area but
save a lot of power compared with Mode A or C. This seems to be why,
in Europe at least, the authorities are looking at Mode S, rather than
A or C, as a mandatory standard. They know gliders would never have
enough power for Mode A or C with the expected growth in utilisation of
airspace and hence growing number of interrogations making transponders
squawk.

TCAS etc. in airliners etc., however, will still see all transponders
within their range, so will still give collision avoidance in respect
of gliders.

Hope that helps. Any experts care to correct this if necessary?

Chris N.