On Mon, 26 Jan 2004 16:55:44 -0800, "Tim Wescott"
wrote:

That's basically how GPS works, except that you need more than one
transmitter, or veddy expensive clocks in the receiver. Basically with GPS
you receive accurate time signals from four satallites. Since your receiver
doesn't know the time or it's position you have four equations (from four
satellites) in four unknowns and voila! you have an answer. You can do
differential GPS down to millimeter accuracy if you sense the phase of the
carrier.

Since the measurement should be done in a confined space, why not
switch the roles and use one transmitter on the moving object and four
receivers on known fixed locations around the perimeter of that space?

With the receivers connected by cables receiving a common clock
signal, the accuracy of the clock is not important, contrary to the
situation in GPS, in which the four satellites must have atomic clocks
to have a common time base.

Loran uses the same basic concepts (synchronized transmitters, speed of
light, yadda yadda), but it asks you to sense the phase difference between a
master and a slave TX, and it isn't nearly so accurate.

Space probe ranging is done by sending a high data rate pseudo noise
sequence from earth to the probe, in which a simple frequency
translation is done with a high accuracy local oscillator to a
different frequency and sent back to earth. From this, the total
earth-probe-earth phase difference of the PRN sequence (and possibly
also the total RF phase difference) can be determined and hence also
the distance.

However, in this case the accuracy requirement was 1 cm, so I guess
that at least 10 GHz (3 cm) radiation should be used.

Paul