What are Selective Availability Anti-Spoofing Modules (SAASM)?

Selective Availability Anti-Spoofing Modules (SAASM) are GPS modules used by the United States military that has access to more precise and encrypted P(Y) code signals of the GPS constellation. These modules also use Selective Availability (SA) and anti-spoofing (AS) techniques to reduce the accuracy of civilian GPS receivers and further protect the military GPS modules from purposely induced inaccuracies in positioning.

Civilian GPS receivers use C/A signals of the GPS constellation. P(Y) code signals used by SAASM modules have higher location precision than what is available through the C/A signal. At one time, the US government even introduced errors into the C/A signal, making position location using it less accurate than what was available to P(Y) users. This “Selective Availability” was discontinued by the US Government on May 1, 2000 along with a United States Presidential Directive that no future GPS programs will include it so as to allow civil applications the full benefits of the GPS service.

The encryption of the P(Y) signal provides signal integrity assurance to protect against active spoofing attacks. This encryption is not available in the C/A signal. This makes civilian receivers prone to “spoofing” which is when a device called “spoofer” is used to mimic the normal C/A signal, eventually slightly overcoming the ambient GPS signal strength. The spoofing signal can then be adjusted slowly to make the attacked receiver report an incorrect position and time. Civilian GPS receivers may not be able to recognize the spoofing signal as an anomaly and reject it. But SAASM receivers have access to the P(Y) signal as only they can decrypt these signals and hence will always have access to the “true” GPS signals. Due to this security feature of P(Y) signals, SAASM allows satellite authentication, over-the-air rekeying, and contingency recovery. Simpler SAASMs may not have any additional anti-jam capability however, the higher data (chipping) rate of the P(Y) code means that they can provide a higher processing gain which in turn would provide better tracking performance in a jamming environment.

SAASM is only a part of a layered approach to GPS security. Latest more advanced SAASM receivers can also be coupled with other protection mechanisms like anti-jam antennas and spoofing detection solutions to provide powerful protection against modern threats to GPS services. Future GPS upgrades, such as the deployment of the next generation military signal for GPS called M-code provides additional improvements to the anti-jam capabilities of SAASMs.