"SA" is "Selective Availability" - basically something added to the signals from the satellites so that the gps positioning wasn't too accurate. Idea was it was to stop hostile artillery being ranged in too quickly on "friendly" positions. Even if you were stationary the gps unit would show you moving in a random pattern. Meant you knew where you were within maybe 50 yards but no more. US govt turned off SA years ago though so it's all a bit academic... Should expect accuracy to be within 20 - 50 feet now.