In article ,
Jonathan Silverlight wrote:
In addition to measuring the motion-generated frequency shift of
Huygens' radio signal, radio telescopes also were used to make
extremely precise measurements of the probe's position...
...employing the technique of Very Long Baseline Interferometry (VLBI).

Probably a dumb question, but is that done by triangulation on the
transmitter?

Speaking without wading through the numbers, I *think* Saturn is a bit too
far to do triangulation even with VLBI. They will, I expect, have been
just determining the direction to Huygens very precisely, and fitting that
into models of exactly where Titan is to determine location.

VLBI determines direction by looking at the phase difference in the
received signal at receivers thousands of kilometers apart. The signal
arrives at different receivers at slightly different times because they
are at slightly different distances from the source. Precisely measuring
the differences in arrival times tells you the differences in distance,
and given the known 3D positions of the receivers, those differences tell
you which direction the signal must be coming from. With modern timing
technology, this can be done very precisely indeed.

If you put the receivers near each other, the distance differences can be
measured in real time -- this is what multi-antenna radio telescopes like
the VLA do -- but then they're necessarily pretty small. But with atomic
clocks as time references, you can *record* the signal at receivers
anywhere on Earth, and put the recordings together afterward. That's what
VLBI does.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |