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SAASM On Trial

A Test Regime for Integrated GPS Military Systems

Testing a military-grade GPS receiver by itself is one thing; testing its operation after integration on a platform — vehicle, aircraft, or munition — is something else. It is more of a challenge when the receiver in question incorporates a Selective Availability Anti-Spoof Module (SAASM) for which on-orbit signals have not yet been activated. Staff members of the 746th Test Squadron at Holloman Air Force Base designed and developed a mobile test bed that incorporates a GPS signal simulator and makes realistic performance evaluation of integrated SAASM possible.

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Selective Availability Anti-Spoof Module (SAASM), which among other things enables decryption of precise GPS satellite signals, is the newest generation of security architecture for all GPS Precise Positioning Service (PPS) users.

Combined with navigation warfare (NavWar) protection technologies, SAASM enhances a combatant commander’s ability to use GPS precise position, velocity, and time (PVT) in all environments. The module combines the new capabilities of unclassified black keys, multiple user networks, and extended GPS functions.

Collectively, SAASM extended functions provide the military user with authenticated GPS data necessary for continuous PPS operations without the periodic need to return to base. This affords the warfighter more secure military operations, simplified handling of cryptovariables — the bits that enable PPS operations, and additional operational capabilities, all while preserving GPS’s accuracy performance.

. . .Several navigation and weapon platforms with SAASM receivers are now operating in the field. However, due to a test capability gap, many of these SAASM-integrated systems have not been fully exercised at the operational system level to verify their integrated functionality.

This article describes the development of an innovative test bed that performs end-to-end functional tests on weapon systems that have integrated SAASM-based receivers within the platform itself. . .

. . . SAASM Testing: The History
Although this level of integrated platform testing is a recent innovation, we do not mean to suggest that SAASM itself has not been tested. Sandia National Laboratories (SNL) designed and rigorously tested the SAASM architecture using a disciplined and repeatable laboratory test process. This test process included a thorough validation of SAASM’s Key Data Processor (KDP), and test results clearly show that the KDP’s performance works as advertised within the SAASM design.

. . .However, this robust test capability still does not guarantee the overall performance of larger systems integrated with SAASM-based receivers.

[We] responded to this concern of system integrators by developing an innovative test bed called the SAASM Integrated System Evaluator and Reporter (SAASM-ISER). The SAASM-ISER is a mobile laboratory asset that fills the testing gap by performing end-to-end functional tests on weapon systems with integrated SAASM-based receivers within the platform and/or its munitions.

This test resource provides the capability to generate the appropriate GPS signals and collect data from the weapon system platform. It then verifies the fidelity of the SAASM functions and the data transferred from a stand-alone or embedded GPS receiver (EGR) to other system components.
Hence, the SAASM-ISER can provide a real-time functionality assessment of a platform at the receiver, the cockpit, and a weapons pylon, if the system passes GPS information among these subsystems. Additionally, SAASM-ISER provides a means to conduct SAASM anomaly investigation and resolution regardless of on-orbit signal status.

Because the SAASM-ISER is a deployable resource, testing with this asset can be accomplished frugally on a platform in its operational configuration at virtually any location.

The SAASM-ISER consists of a multi-channel GPS satellite simulator and control network, a data acquisition system, an antenna hood, a bus interface compatible with RS-422 and MIL-STD 1553, as well as a MIL-STD 1760 connector for the collecting the bit stream which is internally sent to the platform pylon. . .

(For the complete article, including figures, charts, and images, please download the PDF version at the link above. )


The multi-channel GPS satellite simulator used in the SAASM-ISER is the STR4760 from Spirent Communications, Crawley, United Kingdom. The Quality Control Monitor receiver used in the SAASM-ISER is the Defense Advanced GPS Receiver (DAGR) from Rockwell Collins, Cedar Rapids, Iowa.

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