Doppler radars work not by measuring the time of
the reflected pulse, but by the phase changes (beat)
between the continuous, (at least during the measuring cycle)
transmitted signal and the reflected signal.
The transmitter signal needs to be present during the
whole of the measuring cycle
Even though the signal is of a high freq. the measure cycle
is very long (relatively)

eg: for 24Gig microwave doppler, resolution 1Kph

    24Gig = wave length approx 10mm,
    1Km per hour = 280mm per second
    Times two for round trip (tx distance + reflected)
    = 480mm per sec

So if you're measuring the beat freq and not the beat phase shift
(more difficult) the beats will come at 48 hz at the lowest speed
(1kph)

Ultrasonic at 40K wave length approx 20,000mm per second
Thats .024Hz beat frequency or 42 seconds per cycle

I think that rules out ultrasonic doppler for land
based vehicles at least with the current speed
limit laws.

--
Peter Cousens
email: p.cousens@cwcom.net  or  p.cousens@virgin.net
smail: 48, Yarmouth Cresent, London, N179PQ, England.