The Global Positioning System: A Shared National Asset (1995)
Chapter: Selective Availability
the civil community. Two features were incorporated into the GPS to provide this advantage. The first, SA, degrades the GPS signal so that the unencrypted signal on L1 will provide a stand-alone horizontal accuracy of 100 meters (2 drms).3 The second, A-S (Anti-Spoofing), provides encryption of the P-code on L1, and L2 in order to deny the signal to the adversary and to increase resistance to spoofing.
Selective Availability
Currently, the full accuracy of GPS is denied to stand-alone non-PPS users of GPS for both navigation and time transfer through the implementation of SA. SA comprises two functions: (1) fluctuation of the GPS satellite clock frequency, known as dither, and (2) transmission of incorrect ephemeris parameters in the navigation message, termed epsilon. SA affects all GPS observables, which include the C/A-code and P-code pseudorange measurements and the L1 and L2 carrier phase measurements. SA is discussed in greater detail in Appendix C. The DOD has stated that the degradation produced by SA will be limited to a value that maintains the 100-meter (2 drms) specified stand-alone horizontal accuracy of the SPS. Furthermore, at a recent meeting of the DOD/DOT (Department of Transportation) Signal Specification Issues Technical and Policy Groups, additional specifications were discussed and agreed upon for limits on the individual satellite range rate and acceleration errors, shown in Table 3-2.4
Table 3-2 SA Errors from DOD/DOT Signal Specification Issues Technical Group
|
Type of Error |
Specification |
|
Range Rate Bound |
Not to exceed 2 m/s |
|
Range Acceleration Bound |
Not to exceed 19 mm/s2 |
|
Range Acceleration |
8 mm/s2 (2a) |
Under special circumstances, the level of SA errors can be set to zero or increased to a larger value, but only by the National Command Authority. For example, SA was set at a very low level during the Persian Gulf War and during the initial occupation of Haiti
