A) To what extent can two satellites constrain the location of a person holding a reciever?

A) To what extent can two satellites constrain the location of a person holding a reciever? Explain.

B) Can you think of another instrumentation system in the geosciences where three instrument stations are required to pin-point the location of something( a phenomenon)? I can. What is the instrumentation system of which i am thinking? Explain.

[Hint} Although we commonly think of these locations as being on the surface of the earth (2-D) the location of the phenomena that i am thinking of is actually in the subsurface(3-D)