and even a more crazy idea: use a phasing array for the directional antenna.
On Fri, Feb 1, 2013 at 10:45 AM, Hal Murray <hmur...@megapathdsl.net> wrote: > > rickhar...@gmail.com said: > > I would like to triangulate a position of a device which moves using 3 > fixed > > positions devices of known location. The idea is to have these operate on > > 915mhz or 434mhz or 2.4ghz or appropriate frequency. > > > I'd like a range of 150 feet or better and accuracy of 3 feet or better. > > > I know the time accuracy is the key to count time = feet, 1ns. > > > How to make it "inexpensive" is key. how inexpensive, very ;-) > > I think it's going to be tough to do it at low cost. > > The low cost transmit/receive chip sets you can get for 915 MHz are low > bandwidth. That will turn into noise on the on/off transition that you > need > for timing. > > I'd suggest getting a pair of demo boards and running some experiments. > > The first simple sanity check would be to wire them up to a couple of > uProcs > and send messages and see if they work at your required range. Play around > to learn the error rate vs baud rate. > > > > -device response ASAP on different frequency > > Not with low cost. But you don't need to use a second frequency. > > My straw man would be something like this: > > Only one station transmits at a time. > Wire up the transmit and receive lines to counter/timers. > On both the transmit and recv side, grab the time on each data-level change > and average them to figure out when the packet started. > > Look at the NTP protocol. It collects 4 time stamps. From that, you can > compute the time of flight. The time at the remote server drops out. > > The sequence would be something like this: > > fixed->remote: long timing packet. > (with lots of 1/0 transitions to feed data to the timer hardware) > remote->fixed: long timing packet. > remote-> fixed: packet with 2 time stamps > first from the receive side, second from the transmit side > > I'd put a CRC on the packets as a sanity check. > > Handwave time: > Assume the counter/timers run at 100 MHz. That's 10 ns. > So you'll have to do lots of averaging to get below 3 ns. > But that's assuming the RF units work well and that the averaging works. > You could build a (much) faster counter/timer in a FPGA. > I'm not sure that will help. > > Get a pair (or 2 pairs) of units and see how well they work. > > I'd expect FSK to work slightly better than ASK (on/off), but > I'm not enough of an RF geek to turn than into numbers. > > I'll say more if that will help. We should probably take it off list. > > ------------ > > One probably crazy idea... > > Setup a directional antenna with a motor to aim it. Scan for the best > signal. > > > -- > These are my opinions. I hate spam. > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.