[time-nuts] question for expert time guys

[J. L. Trantham]

Just to make sure I understand the 'ground rules', the three base stations
define a plane, unless they are on the same line.  Is the mobile device also
in this plane?  How far apart are the base stations relative to the location
of the mobile device?  Or, better, is the mobile device 'inside' this
triangle or 'outside'?

How precisely can you know the location of the base stations without
violating the 'inexpensive' rule?  Can the base stations 'ping' each other?

Seems to me that an accuracy to 3 feet is going to be a problem without
precise time measurement unless you can use some 'known' reference (like the
precise location of the base stations) and the ability to use that 'known'
to 'calibrate' each measurement, thus minimizing the errors from 'drift',
etc., in the available, 'inexpensive', time references.

Good luck.

Joe

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Rick Harold
Sent: Thursday, January 31, 2013 10:37 PM
To: time-nuts at febo.com
Subject: [time-nuts] question for expert time guys

To time experts/EE's.

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.
These two type of devices (fixed and mobile) are all under my control and
thus customized as needed.

The mobile device (not a phone, custom device) would be the least expensive
item.  I'd like a range of 150 feet or better and accuracy of 3 feet or
better.
When manufactured these items they can be calibrated in order to adjust for
any variation in IC's, discrete components etc...
We can assume for now the temperature is constant 70 degree temperature.
Cost is the key design factor.

The general flow is:

   1. base station 1 indicates we are determining position of device A.
   2. Each base station 1, 2, 3 take turns pinging the device to determine
distance.
   3. A ping consists of (something like, e.g. frequencies as examples)
        -send 915mhz signal from base station to device
        -device response ASAP on different frequency
        -station waits and counts 'time' for return
        -this is repeated N? times to get best avg/accuracy.
        -The mobile device does not move very fast
   4. Since delays of the process on each unit is calibrated the device and
base station would subtrack that time out from the results.
   5. obviously with 3 distances we can determine the 2D position of the
mobile device

I know the time accuracy is the key to count time =  feet, 1ns.  This
overall project is not new concept.   How to make it "inexpensive" is key.
how inexpensive, very ;-)   no OCXO or expensive components like that.

That's my goal, and I'm looking for help on the design/thought process of
getting there.
I am open to a consulting arrangement for a fee, please email if you like.
 I've worked with 'regular' EE's (I'm a software guy) but this time accuracy
is too much for them.
Esp. finding a way to do it inexpensively.

Thanks for any thoughts.
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