[mseebach]

You mean pick up the signal from earths GPS? Off the top of my head, it would require earth to be in view, and it would be very inaccurate - GPS is already less accurate the further from equator you get, as all the visible satellites are increasingly on one side of you. You'd have this problem in the extreme, as all the satellites would be clustered in a single dot in the sky. Probably also infeasible to meaningfully correct for the fact that Mars and Earth move very quickly relative to each other.

[jessriedel]

> GPS is already less accurate the further from equator you get, as all the visible satellites are increasingly on one side of you.

Not contradicting you, but just to be clear: GPS satellites are not in geostationary orbits above the equator, or even in geosynchronous orbits. Rather, they are in medium-Earth orbits and their time-averaged density over the Earth is only a little lower over the poles as the equator. At some parts of the month, there are as many visible at the poles as at the equator. See Fig. 4 here:

https://www.researchgate.net/publication/228400966_Analysis_...

The GPS Wikipedia page also has a reasonable illustration of their orbits.

[mseebach]

Hmm, not sure I understand what you're saying. Figure 5 in that paper is the situation I'm describing: at a certain latitude (52 deg north in this case, incidentally that latitude is my 'hood) there are no satellites visible to the north. Figure 2(b) illustrates why that's bad for precision.

[jessriedel]

Sure, that's why I said I wasn't contradicting you, just clarifying. One could mistakenly read your comment and think that the coverage goes to zero at the poles ("the further from the equator you get"), but of course GPS still works pretty well there. When you get far enough north, satellites from the "other side" of the Earth come into view, so the accuracy levels off and (I think) starts increasing again with latitude.

(And I corrected my comment to refer to figure 4, not 2.)