What does Selective Availability (SA) do in GPS?

• A. It improves accuracy of navigation data
• B. It permits the DOD to introduce significant clock and ephemeris error
• C. It disables altitude information
• D. It increases satellite broadcast frequency

Answer: (B)

Selective Availability is the ability to deliberately degrade GPS signals by introducing errors into the satellite timing data that civilian receivers rely on. The navigation solution depends on two key pieces: the precise time the signal was transmitted (satellite clock timing) and the orbital information (broadcast ephemeris) used to know where each satellite is. If those timing and ephemeris values are biased or corrupted, the pseudorange measurements become inaccurate, and the calculated position can drift by a large amount. This approach reduces civilian accuracy without needing hardware changes or altering the basic signal itself. It does not affect altitude data specifically, nor does it increase transmission frequency. In 2000, Selective Availability was turned off, which led to a substantial improvement in civilian GPS accuracy.