Standardization of Definitions for Weapons-Based GPS Receiver Timelines

Walter Trach, Jr., Marty R. McGregor, Jonathan M. Sebast, Ronald L. Lessman

Abstract:	The operating timeline of a GPS receiver – in particular the times associated with receiver start-up and the acquisition of satellite signals – has become a critical requirement for many precision-guided weapons platforms. For the customer seeking a weapons-based GPS receiver, a direct comparison of timelines across receiver models from different vendors can be difficult due to the lack of consistent industry definitions. If such definitions are established, customers can more effectively evaluate the suitability of a particular GPS receiver for a specific weapons application. For weapons applications, there is a pre-launch phase, which is used to initialize the GPS receiver and a postlaunch phase where the GPS receiver uses the initialization data to quickly acquire satellite signals. During the pre-launch phase, there are two important points in time. The first is when the GPS receiver’s Interface is Available for Transaction (IAT), which is the point in time when the GPS receiver can begin receiving initialization data. The second important point in time is when the Initialization Sequence is Complete (ISC). After launch, the GPS receiver must acquire satellites as quickly as possible to maximize the time available for inflight course correction. Depending on the application, there are a number of definable criteria for the GPS receiver to provide a useable solution to the mission computer. Time to First Fix (TTFF) is measured between the point where the receiver is commanded to its navigation mode (i.e. acquire and track GPS satellites) and the point where it outputs a valid navigation solution. TTFF is of special concern when the receiver’s PVT solution is being used directly for weapons guidance. Time to Four Measurements (TTFM) is measured between the point where the receiver enters its navigation mode and the point where at least four valid Line of Sight (LOS) measurements are output. Usually TTFM is of special concern for applications where the platform’s mission computer computes its own navigation solution from the receiver’s measurements. Some customers prefer a measurement of time between when the GPS receiver is supplied with power until the GPS receiver outputs a useable PVT solution (Power to First Fix (PTFF)) or four LOS measurements (Power to Four Measurements (PTFM)). Both of these definitions require that the GPS receiver be Ready for Track Mode (RTM).
Published in:	Proceedings of the 2009 International Technical Meeting of The Institute of Navigation January 26 - 28, 2009 Disney's Paradise Pier Hotel Anaheim, CA
Pages:	1068 - 1072
Cite this article:	Trach, Walter, Jr.,, McGregor, Marty R., Sebast, Jonathan M., Lessman, Ronald L., "Standardization of Definitions for Weapons-Based GPS Receiver Timelines," Proceedings of the 2009 International Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2009, pp. 1068-1072.
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