Paul A. Kline
PhD Dissertation submitted to the Faculty of the Virginia Tech in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
Bradley Department of Electrical Engineering
Timothy Pratt, Chair
Frank van Graas
February 7, 1997
For receivers using the Global Positioning System (GPS), it is standard procedure to treat the receiver clock bias from GPS time as an unknown. This requires four range measurements to the satellites in order to solve for three dimensional position and clock offset. If the receiver clock could be synchronized with GPS time, the extra range measurement would not be necessary. To achieve this synchronization, a stable frequency reference must be incorporated into the GPS user set. This concept is known as clock aiding or clock augmentation of GPS receivers.
Clock augmentation increases the availability of the navigation function because only three GPS satellites are required. Also, it is shown that clock augmentation improves vertical accuracy by reducing the vertical dilution of precision (VDOP), which is a unitless multiplier that translates range measurement error into vertical position error. This improvement in vertical accuracy is particularly beneficial for applications involving final approach and landing of aircraft using GPS, because GPS typically provides better horizontal accuracy than vertical accuracy.
The benefits of atomic clock augmentation are limited by factors that cause a loss of synchronization either between the receiver and GPS time, or between ground station and airborne receivers processing GPS data in differential mode (DGPS). Among the error sources that cause a clock offset are antenna rotation, hardware drifts due to temperature variations, and relativistic effects for GPS receivers on moving platforms. Antenna rotation and temperature effects are addressed and supported by experimental data. It is shown that two particular relativity terms thought to be missing from GPS receiver algorithms are not evident in data collected during a flight test experiment.
Upon addressing the error sources, the dissertation concludes with analysis of DGPS data collected during a flight test at the Federal Aviation Administration (FAA) Tech Center in Atlantic City, during which external rubidium oscillators were used by airborne (Boeing 757-B) and ground station GPS receivers. A new method of clock modeling is introduced, and this clock model is used to demonstrate the improvement in vertical accuracy, as well as three-satellite navigation.
List of attached files
File Name Size (Bytes) Ch8.pdf 3,249,936 Bytes appa.pdf 396,933 Bytes appb.pdf 265,213 Bytes appc.pdf 91,908 Bytes bib.pdf 96,783 Bytes ch1.pdf 88,820 Bytes ch2.pdf 550,724 Bytes ch3.pdf 485,933 Bytes ch4.pdf 1,077,611 Bytes ch5.pdf 667,296 Bytes ch6.pdf 1,378,138 Bytes ch7.pdf 1,210,644 Bytes ch9.pdf 141,604 Bytes etd.pdf 169,298 Bytes vita.pdf 83,974 Bytes
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