Rover, now go find Beagle:
NASA's commands to Spirit's rover this morning included not only "turn right and go down the ramp," but also "and when you're done, look up."
The Mars Express orbiter (of Beagle 2 fame) will pass overhead, and will be looking down at the landing area. By design, the two craft have similar sensors onboard, and they will hopefully look through some of the same portions of the tenuous martian atmosphere.
When you look down, it's very hard to tell what part of the signal comes from the ground, and what part comes from the atmosphere in-between. For example, if you are interested in how much dust there is in the air, but you are looking down at a dusty surface, you can't tell how much of the "dust signal" is in the air, and how much is coming from the ground:
Express Signal = Ground Signal + Atmosphere Signal
If you look up, you can measure what's in the air, but you can only do it in one place at a time:
Spirit Up-look Signal = Atmosphere Signal
The best thing to do is to combine these techniques, and get an idea of what the correct value is for at least one spot, use that to validate the measurement, and then calibrate your space-borne sensors so they can be used over wide areas. When Spirit looks down, it will also be able to determine what is actually on the ground, rather than the combination Express sees:
Spirit Down-look Signal = Ground Signal
You can use your high school algebra to see that this system of equations overdetermines the individual signals - this is what gives us some bounds on errors.
And that is what Spirit will help Express to do today.
You might think this kind of thing is done all the time on Earth, but you'd be wrong. This type of ground truthing calibration/validation exercise is very expensive. The ground truth that is used to calibrate sensors here on Earth is actually quite sparse. Most of the remotely sensed data of the Earth is not fully understood, and very few of the tens of thousands of maps produced from satellite data even have rigorous error estimates for the signal interpretation.
I'm still chewing on President Bush's "Infinity and Beyond" speech from yesterday, and trying to find U.S. budget figures for 1962. Stay tuned.