One of the existing anti-spoofing technologies is the Selective Availability Anti-Spoofing Module (SAASM) available for the GPS constellation. A SAASM-enabled GPS receiver allows an approved government or military user to access the encrypted P(Y) signal transmitted by the GPS constellation. This originally provided two benefits.
First, P(Y) provided higher location precision than what was available through the C/A signal. The U.S. government at one time introduced error into the C/A signal, making position location using it less accurate than what was available to P(Y) users. This “Selective Availability” was turned off by the US Government in 2000 to allow civil applications the full benefits of the GPS service.
The second benefit of SAASM is still available today. The encrypted P(Y) signal provides signal integrity assurance to protect against active spoofing attacks. One of the vulnerabilities of using just the C/A GPS signal is a walk-off attack. A spoofer can mimic the normal C/A signal, eventually slightly overcoming the ambient GPS signal strength. The spoofing signal can then slowly adjust to make the attacked receiver report and incorrect position and time. If done correctly, the C/A GPS receiver will not recognize the spoofing signal as an anomaly and reject it. But a receiver that decrypts the P(Y) signal will always have access to the “true” GPS signal.
SAASM is one part of a layered approach to GPS security. And while it only applies to the GPS constellation, Galileo users will soon have access to the Public Regulated System (PRS) which provides similar integrity assurance through signal encryption. Coupling SAASM receivers with other protection mechanisms like Spectracom anti-jam antennas and Broadshield spoofing detection provides powerful protection against modern threats to GPS services.