Which statement about a double-difference GPS solution is true?

• A. It uses the difference of pseudorange measurements between two receivers for the same satellite.
• B. It uses the difference of measurements between two satellites observed at the same receiver.
• C. It yields the baseline vector directly without any bias.
• D. It cancels both receiver and satellite clock biases by differencing measurements between two satellites observed at two receivers.

Answer: (D)

Double-difference uses measurements from two receivers and two satellites and cancels the clock biases by two levels of differencing. First, take the difference of the receivers’ measurements for the same satellite; this removes the satellite clock error because both receivers see the same satellite clock. Then take the difference of those results for two different satellites; this cancels the receiver clock error as well. What’s left depends primarily on the geometry between the two receivers and the baseline between them, plus some residual noise and atmospheric effects. This is why the method is so effective for relative positioning: clock biases don’t pollute the baseline estimate, which is obtained by solving a small linearized system. The other statements miss this two-step cancellation, or imply you get the baseline directly without estimation, which isn’t how the process works.