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**Derek Lyons**

(Henry Spencer) wrote:

In article ,
Danny Dot wrote:
Does anyone know how we navigate our deep space probes? I know we use radar
and GPS in LEO. What do we use for deep space? My guess is we use Doppler
shift a lot -- but that is just a guess.

The dominant method is radio tracking -- radar, but with the assistance of
the probe to repeat the signal back. (Normal radar is useless for such
small objects at such distances.) Round-trip time gives range, and Doppler
shift gives range rate (velocity along the probe-Earth line); these can
both be measured extremely accurately. More subtly, the *change* in
Doppler shift as the Earth rotates can be used to determine direction,
although not as accurately. These measurements are combined using a
sophisticated estimating process that looks at how the measurements
*should* be changing for a given trajectory, and solves simultaneously
for the most likely probe position/velocity and the most likely values of
other parameters (masses of planets, errors in measurements, etc.).

Some other techniques sometimes get added. If multiple ground antennas
are available *and* the probe's signal is a strong one, interferometry can
be used to measure direction *extremely* accurately. Maneuvers can be
measured by onboard accelerometers for comparison with tracking data. And
if the probe has a suitable camera, images showing planet(s) against a
star background can be used to establish position; this "optical
navigation" is especially useful when approaching a planet, because it
gives an independent check on position relative to the planet, which is
what you mostly care about.

Damm - it's amazing how close that description is to the methods we
used to track targets using sonar. (Though our targets were usually
assumed to be non-cooperative.)

D.
--
Touch-twice life. Eat. Drink. Laugh.

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL