Selective Availability (SA) introduces navigation errors primarily in which data?

• A. Clock and ephemeris data
• B. Receiver clock corrections only
• C. Atmospheric propagation data
• D. Clock and ephemeris data accompanying satellites

Answer: (D)

Selective Availability degrades the information that tells you where a satellite is and what its clock reads. Specifically, it introduces errors in the clock correction data and the ephemeris data broadcast with each satellite. The receiver uses these two data sets—the satellite’s clock bias and its orbital parameters—to compute pseudorange measurements and determine position. When these data are degraded, the calculated position becomes less accurate. Atmospheric propagation data or purely receiver-derived clock corrections aren’t the primary targets of SA, so the main navigation errors come from the clock and ephemeris data accompanying satellites.