[tarre]

I think even one satellite is enough, if you have enough data points. You can tune the clocks based on the signals captured, when the location of the plane was known. After that each signal gives you a sphere, on which the plane is supposed to be. With reasonable assumptions about the altitude and speed of the plane, you should be able to locate the plane with enough accuracy to determine, if it was in the middle of an ocean or close to land.