> 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 down 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 tx signal 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'. The 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. Russell McMahon --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .