[time-nuts] question for expert time guys

Azelio Boriani azelio.boriani at screen.it
Fri Feb 1 04:59:35 EST 2013


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 <hmurray at megapathdsl.net> wrote:

>
> rickharold at 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.
>
>
>
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