A technique of automatic bias compensation has been devised to correct errors caused by variations among electronic components in Global Positioning System (GPS) receivers that use the coarse/acquisition (C/A) GPS code. Even though there are large government and commercial markets for such GPS receivers, these errors have not been generally understood. [Alternatively or in addition to the automatic-bias-compensation technique, the errors can be reduced by (1) building GPS receivers from components of higher quality (that is, components that have lower manufacturing tolerances and are less susceptible to aging) and (2) performing more extensive manual adjustments during integration and testing of GPS receivers.]
The errors in question are sampler biases, which can interact with GPS signals in such a way as to introduce spurious signals that can confuse affected receivers. The effect of these errors is more pronounced at the high Doppler shifts in signals received by a GPS receiver aboard an orbiting spacecraft or other high-speed vehicle. The automatic-bias-compensation technique is implemented in the digital signal-processing portion of a GPS receiver. The digital samples of amplified received signal + noise are measured for a bias. Corrections are computed and written over the incoming samples to drive the resulting bias to zero, which also reduces the signal-to-noise level. This process is controlled by a feedback loop to adapt automatically to variations in the level of uncorrected bias coming from the sampler.
This work was done by Lawrence Young, Jeffrey Tien, and George Purcell of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.
This Brief includes a Technical Support Package (TSP).
Automatic Bias Compensation in GPS Receivers
(reference NPO-20819) is currently available for download from the TSP library.
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