This system of radar is called linear FM CW radar. It is now widely
used (or at least was before ultrasonic methods became dominant). I
think there used to be lots of systems designed to help trucks back up
to loading docks quickly without bumping the dock hard. Gunnplexers
were the devices used for this - they were a short section of
waveguide terminated in a horn antenna. Inside the waveguide, there
was a Gunn diode (CW microwave source), Varactor (for FMing the Gunn
oscillator), and a Schottky mixer diode (provided the heterodyne
between the TX and RX signal).

Sean


On Sat, Oct 23, 2010 at 7:51 PM, RussellMc <apptechnz@gmail.com> wrote:
>> To use RF you need Static RF beacons in the room and compare phase of
>> coded signal on them.
>
> I came to this discussion late. As I skimmed =A0down it seemed that
> phase comparison would be good for what you wanted, as Michael has
> suggested. If you can do phase detection /comparison you can resolve
> to fractions of a wavelength. Distant memory suggests that HP may have
> been doing this for decades with optical systems. Even effectively
> implementing GPS with your own tx may be doable. You may even be able
> to use GPS hardware with a different RX frequency. Implementing the
> phase locked transmitters may become somewhat annoying.
>
> Distant memory also suggest that there may have been people doing
> rocket tracking systems with a multi transmitter phase comparison
> system (or maybe I suggested it to someone for rockets long ago ? :-)
> ).
>
> Long ago the APN1 Radar Altimeter used in Bristol Freighters and
> similar implemented a clever doppler radar system where they linearly
> slewed the transmitter frequency at a given rate and then heterodyned
> the current =A0tx =A0signal with the reflected signal. The difference in
> frequency between tx and rx signals gave the time delay and hence
> distance. Return delay in free space is 6 nS/metre of separation -
> call it 10 nS for convenience. If you slew the tx at 10 Hz/ 10 Ns you
> get 1 Hz heterodyne at 100 mm, 10 Hz at 1m, 100 Hz at 10m.
> That's dependent only on tx slew rate and not tx frequency. 1 Hz/nS is
> 1 Ghz / second slew rate. Large at low frequencies but almost bearable
> at a few Ghz mean tx frequency. If you use phase comparisons on tx and
> rx rather than just frequency you can deal with slower to much slower
> slew rates. Phase inversion and worse at the reflecting surface may be
> 'annoying'. =A0The APN1 managed thousands of feet of altitude
> measurement using push pull Acorn tube TX and RX (I have a circuit
> diagram), with the rx signal having been reflected off whatever ground
> surface was present, so a mere sniff of signal should be OK with
> modern circuitry. (I used to own 2 x APN1's but alas I sold them years
> ago. They would be quite a cute toy for my 'museum' these days.
>
>
>
> =A0 Russell McMahon
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