Anti-Spoofing
The function of anti-spoofing (AS) of the GPS system is designed for an anti potential spoofer (or jammer). A spoofer generates a signal that mimics the GPS signal and attempts to cause the receiver to track the wrong signal. When the AS mode of operation is activated, the P code will be replaced with a secure Y code available only to authorised users, and the unauthorised receiver becomes a single L1 frequency receiver. AS had been tested frequently since 1 August 1992 and formally activated at 00:00 UT on 31 January 1994 and now is in continuous operation on all Block II and later satellites.
The broadcasted ionospheric model (in the navigation message) may be used to overcome the problem of absence of the dual-frequencies, which are originally implemented for eliminating the ionospheric effects. Of course, the method of using the ionospheric model cannot be as accurate as the method of using dual-frequencies data, and consequently the precision is degraded. Carrier phase smoothed C/A code may be used to replace the absence of the P code.
Selective Availability
Selective availability (SA) is a degradation of the GPS signal with the objective to deny full position and velocity accuracy to unauthorised users by dithering the satellite clock and manipulating the ephemerides. In case SA is on, the fundament frequency of the satellite clock is dithered, so that the GPS measurements are affected. The broadcast ephemerides are manipulated so that the computed orbit will have slow variations. Several levels of SA effects are possible. The SA is enabled on Block II and later satellites (Graas and Braasch 1996).
The authorised users may recover the un-degraded data and exploit the full system potential. For doing so they must possess a key that allows them to decrypt correction data transmitted in the navigation message (Georgiadou and Daucet 1990). For high-precision users, IGS precise orbit and forecast orbit data may be used. Using known positions (or monitor stations), the range corrections can be computed. Differential GPS may also eliminate at least a part of the SA effects. SA has been switched off since May 2000.
