Benefits of a Space-based Augmentation System for Early Implementation of Gps Modernization Signals

The first space-based GPS augmentation systems were implemented using the L-band geostationary transponders launched on the Inmarsat-3 satellites. The Ground Uplink Station (GUS) hardware and software, which enables these “bent-pipe” transponders to broadcast pseudo-GPS signals, was developed by NAVSYS. The Ground Uplink Station equipment allows the Inmarsat-3 GPS augmentation signals to be precisely locked to GPS time and provide highly accurate, coherent pseudo-range and carrier-phase observations. This hardware and software is now the core of the Federal Aviation Administration’s (FAA) Wide Area Augmentation System (WAAS). The adaptability of the transponder approach is attractive for supporting the development of next generation GPS capabilities as part of the GPS modernization effort. The geostationary augmentation earth station equipment developed by NAVSYS is reprogrammable. While the WAAS signals only use the Course Acquisition or Clear Access (C/A) code, NAVSYS’ Ground Uplink Station hardware can be configured to support C/A code, military encrypted Precision (P(Y) code and also the new Militarycode (M-code) signals. This capability enables a geostationary transponder to be used to support development, test and qualification of new military signals using a space-based augmentation system. In this paper, the geostationary augmentation earth station equipment being developed under contract to Air Force Research Laboratory (AFRL) to support the GPS Modernization signal structure is described and test results of the system operating in a loop-back test-mode are provided.