SAASM

An SAASM, for "Selective Availability / Anti-Spoofing Module", is used by military Global Positioning System receivers to allow decryption of precision GPS coordinates, while the accuracy of civilian GPS receivers may be reduced by the US military through Selective Availability.http://www.symmttm.com/pdf/Gps/an_saasm.pdf] However, the US Presidential directive instructing the discontinuation of Selective Availability, along with the directive that no future GPS programs will include selective availability makes any changes to SA unlikely.

SAASM allows satellite authentication, over-the-air rekeying, and contingency recovery. Those features are not available with the similar, but older PPS-SM system. PPS-SM systems require periodic updates with a classified "Red Key" that may only be transmitted by secure means (such as physically taking the receiver to a secure facility for rekeying or having a trusted courier deliver a paper tape with a new key to the receiver, after which that paper tape must be destroyed). SAASM systems can be updated with an encrypted "Black Key" that may be transmitted over unclassified channels. All military receivers that will be newly-deployed after the end of September 2006 must use SAASM..

SAASM does not provide any additional anti-jam capability because it uses the exact same signal-in-space as current GPS signals (power and modulation). Future GPS upgrades, such as M-Code, will provide anti-jam capabilities.

SAASM hardware modules are covered with a sophisticated anti-tampering coating, to deter analysis of their internal operation.

The next generation military signal for GPS, called M-code, will be launched on IIR-M and IIF satellites beginning in 2005. A complete constellation of 18 satellites with M-code capability is planned for 2016.

References

[http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=21878 GPSworld.com article "Saving SAASM"] by Robert Huffman (Webpage, January 2006)