Richard Clark wrote:
On Thu, 17 Mar 2011 12:52:28 -0700, Jim Lux
wrote:

Certainly he predicts that the temporal dispersion is going to be 0.1ps
for near IR, which is, shall we say, challenging to measure.

Why?


measuring things to tenths of a picosecond, repeatably, can be tricky..
That's like measuring the phase difference between two 10 GHz signals
to 0.3 degrees. Or, another way to look at it is 1 picolightsecond is
about a third of a millimeter.

You're looking at
a) figuring out how to generate two signals at near IR that has a
frequency offset that can be accurately controlled. Probably some sort
of heterodyne mixing scheme would be easiest.
b) sending those two signals over the optical path through
interplanetary space.
c) recovering the signals, measuring the propagation time variation
(say, by looking at the phase difference between the modulation
signals), and then removing atmospheric effects.
d) it's probably going to be a pretty weak signal, so you'll need to
average. That means your measurement system has to be picosecond stable
over the averaging interval.

None of those steps are particularly simple or easy.

I've worked on systems to measure the (microwave) distance to Jupiter
and back with an accuracy of around 1 part in 1E15 at 32 GHz,
integrating over 1000 seconds. That's tenths of a picosecond out of 1000
seconds. It's challenging.

http://en.wikipedia.org/wiki/Juno_%28spacecraft%29
http://juno.wisc.edu/spacecraft_instruments_GSE.html