Android Start Line Apps Anyone?

>>the same satellites. That is how Differential GPs gets much better accuracy.

Not quite correct. If I may explain...

 

Differential GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements. The stationary receiver is the key. It ties all the satellite measurements into a solid local reference.

Remember that GPS receivers use timing signals from at least four satellites to establish a position. Each of those timing signals is going to have some error or delay depending on what sort of perils have befallen it on its trip down to us. Since each of the timing signals that go into a position calculation has some error, that calculation is going to be a compounding of those errors.

Luckily the sheer scale of the GPS system comes to our rescue. The satellites are so far out in space that the little distances we travel here on earth are insignificant. So if two receivers are fairly close to each other, say within a few hundred kilometers, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and so will have virtually the same errors.

Differential GPS can eliminate all errors that are common to both the reference receiver and the roving receiver. These include everything except multipath errors (because they occur right around the receiver such as masts and sails etc) and any receiver errors (because they're unique to the receiver).
That's the idea behind differential GPS: We have one receiver measure the timing errors and then provide correction information to the other receivers that are roving around. That way virtually all errors can be eliminated from the system, even the pesky Selective Availability error that the DoD can put on if they so wish...

Hope that helps...

Indeed - even differential GPS can involve two moving receivers.