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.
NPO-20819
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Automatic Bias Compensation in GPS Receivers
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Overview
The document discusses a novel technique for automatic bias compensation in Global Positioning System (GPS) receivers, developed by researchers George H. Purcell and Jeffrey Tien at NASA's Jet Propulsion Laboratory (JPL). This technique addresses errors caused by variations in electronic components, particularly in receivers that utilize the coarse/acquisition (C/A) GPS code. These errors, known as sampler biases, can lead to the introduction of spurious signals, which can confuse GPS receivers, especially at high Doppler shifts encountered by receivers on fast-moving vehicles, such as spacecraft.
The automatic bias compensation technique is implemented in the digital signal-processing section of GPS receivers. It involves measuring the digital samples of the amplified received signal plus noise for any bias. Corrections are then computed and applied to the incoming samples to drive the resulting bias to zero, thereby reducing the signal-to-noise ratio. This process is managed by a feedback loop that adapts automatically to variations in the level of uncorrected bias from the sampler.
In addition to the automatic bias compensation method, the document mentions alternative strategies for reducing these errors. These include using higher-quality components with lower manufacturing tolerances and performing more extensive manual adjustments during the integration and testing phases of GPS receiver production.
The significance of this work lies in its potential to enhance the performance and reliability of GPS receivers, which are critical for various applications in both government and commercial sectors. The document emphasizes that the errors associated with sampler biases have not been widely understood, despite the large market for GPS technology.
The research was conducted under a contract with the National Aeronautics and Space Administration (NASA) and highlights the collaborative efforts of JPL and Caltech. The document also includes a disclaimer stating that references to specific commercial products or services do not imply endorsement by the U.S. Government or JPL.
Overall, this document presents a significant advancement in GPS technology, offering a solution to a previously underappreciated problem that affects the accuracy and reliability of GPS systems, particularly in high-speed applications.
