National Academies Press: OpenBook

The Global Positioning System: A Shared National Asset (1995)

Chapter: Signal Processing Improvements

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Suggested Citation:"Signal Processing Improvements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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Page 114

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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 114 GPS components reside in the same box. Embedded architectures combine GPS tracking-loop estimates with INS accelerometer and gyroscope outputs to correct INS biases. This provides fast GPS loop-aiding commands for a 10 dB-15 dB increase in tracking threshold and jamming margins and also supports rapid pull-in after signal blockage. This is referred to as a "tightly-coupled" INS/GPS structure. In less sophisticated aiding systems, often referred to as "loosely-coupled" structures, inertial positions and GPS positions or pseudorange data are merged in a cascaded filter structure, missing the benefits of improved GPS signal tracking margins. However, these loosely-coupled INS/GPS structures do extend the length of time that inertial operations can provide useful accuracy and a GPS integrity check, and also speed GPS signal acquisition. Historically, inertial aiding had been too expensive for many tactical military applications. It was not until the 1980s that less-costly strapdown ring-laser gyroscope technology became common aboard military aircraft. However, in the last 5 years there have been other encouraging developments that could lead to wider implementation of aided GPS in tactical military applications. Fiber-optic gyroscopes and solid-state accelerometer configurations have come into use, and more recently, batch-fabricated quartz rate sensors and quartz and silicon accelerometers have been developed. These technologies should have a major impact on the cost of aided receiver systems. The development of low-cost, solid-state, tightly-coupled integrated inertial navigation system/GPS receivers to improve immunity to jamming and spoofing should be accelerated. Signal Processing Improvements The estimation of path delay and Doppler for all satellites in view is the most fundamental task of any navigation receiver.56 Conventionally, delay and Doppler parameter estimates are extracted in delay-lock and phase-lock tracking loops consisting of dedicated loop software and correlator ASIC channels for each satellite. The resulting pseudorange and carrier phase quantities are then fed to the navigation filter routines, wherein these estimates are combined to produce updated position and velocity. In this traditional setup, predating the availability of fast and inexpensive digital signal processing and reduced instruction set computing (RISC) capable of hundreds of millions of double-precision floating point instructions per second, raw correlator data from a given satellite are processed without reference to state and error from other tracking loops, or from the receiver as a whole. Fast computing permits statistically optimal validation and weighting of correlator data from all satellites early in the processing chain, based upon the full receiver state model. By taking advantage of inter-satellite path correlations, and by rapidly adapting filter gains to encountered signal amplitude and noise fluctuations, tracking thresholds can be improved on the order of 10 dB, and tracking can be made resistant to spoofing, multipath 56 Doppler refers to the relative shift of frequency due to satellite-to-user motion.

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The Global Positioning System (GPS) is a satellite-based navigation system that was originally designed for the U.S. military. However, the number of civilian GPS users now exceeds the military users, and many commercial markets have emerged. This book identifies technical improvements that would enhance military, civilian, and commercial use of the GPS. Several technical improvements are recommended that could be made to enhance the overall system performance.

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