Architecture and performance of a new GPS time transfer and positioning receiver

This paper describes the Interstate Electronics 4200 GPS Receiver System that has been developed for time transfer and low dynamic positioning applications. The receiver employs the NAVSTAR Global Positioning System (GPS) L1 CIA-code and has three optional solution modes for the clock/navigation state estimation: A time transfer-only mode while tracking at least one satellite, with a solution set containing user-clock and oscillator-bias states. Typically this mode can be used for about 12 hours per day with the current NAVSTAR GPS satellite constellation. A two-dimensional (2-D) navigation-plus-time transfer mode, with a solution set containing clock and oscillator bias, plus user latitude and longitude states. This mode is operable on three satellites simultaneously, which makes it usable for about 4 hours per day with the current GPS constellation. A three-dimensional (3-D) navigation-plus-time transfer mode, with a solution set as for 2-D except for the addition of an altitude state, making it usable for about 2 hours per day at present. The system consists of a separate preamplifierlantenna and processing unit (5-114 inches high by 17-114 inches wide by 24 inches deep). The processor has the following four circuit board assemblies: (I) an RF converter that converts the preamplifier output to baseband for digitizing; (2) a digital signal processor containing a digital correlator, numerically controlled oscillators, and a code generator; (3) a tracking controller equipped with a user clock, digital phase-stepper that fully synchronizes the 1-pps output signal to the nanosecond level, and a microprocessor that operates in conjunction with the signal processor to close the tracking loops; and (4) a data processor for executive control, input/output management, and clock/navigation state estimation.