Hello Des:

The TDR approach requires very fast circuitry.
The swept VSWR approach using a scalar network analyzer required doing an FFT an
d is picky in the step size and bandwidth, but it works without a DC path that t
he TDR requires.

The telephone companies use a capactive method that works quite well.  You need
to know the capacitance between a couple of wires for some distance.  The length
 of an open pair can then be calculated.  Of course there are already boxes that
 do this.  In the case of a short knowing the Ohms/mile you can
calculate the distance to the short.  The surplus ZM-x series LCR bridges contai
n instructions for using these kinds of calculations to find opens and shorts.

I worked on a system that used "modal decomposition".  It measured the standard
unbalanced s-parameters of balanced components, converted them into unbalanced Z
-parameters, and these into balanced Z-parameters, and finally into balanced s-p
arametes.  This was rather expensive, but it could measure the
length of coax and balanced lines with  millimeter resolution.

Have Fun,

Brooke