Quoting RussellMc : > ... 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. > ... Not clear here. As this is indoors, are you suggesting I launch my =20 own satellites on the ceiling? :) > 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.... Pretty neat idea. > 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. This sounds nice, except that (a) this sounds like a relatively large =20 system for a simple copter project, (b) it seems that reflections may =20 not be what I need due to obstacles, etc, but rather having other =20 transcievers receive and re-send a signal, and (c) I have no clue how =20 to implement something like this. Cheers, -Neil. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .