"Jonathan Silverlight" wrote
in message ...
In message , Andrew Yee
writes
ESA News
http://www.esa.int

24 December 2003

Mars Express status report ...

As of 2200 CET [2100 UTC] on 24 December, Mars Express is 66800 km from
Mars and the mission is proceeding as planned. ESOC Mission Control
data on the velocity of Mars Express show that the pull of Martian
gravity is continually increasing.

This gravitational influence is as predicted, and serves as an
independent confirmation that the spacecraft is on its planned course.

Am I missing something here?

They're not checking the planet, they're checking the
location of the craft given the knowledge of the planet.

The gravitational influence of Mars has been very well mapped by
spacecraft over the last 40 years, and I doubt it's changed recently :-)
And do they have a really accurate measurement of the position of Mars
Express relative to Mars, or is the Doppler shift the only way to
calculate where it is?

Certainly Doppler, possibly range, but three checks is
better than two ;-) I might guess the way the check the
gravitational influence is by rate of change of Doppler
but it would be better to see if there is something about
navigation techniques on the web site.

Merry Christmas Jonathan.

George