Scholarly Communications Project

Atomic Clock Augmentation for Receivers Using the Global Positioning System


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
Hugh VanLandingham
Kent Murphy
Brian Dennison
Frank van Graas

February 7, 1997
Blacksburg, Virginia


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 NameSize (Bytes)
Ch8.pdf3,249,936 Bytes
appa.pdf396,933 Bytes
appb.pdf265,213 Bytes
appc.pdf91,908 Bytes
bib.pdf96,783 Bytes
ch1.pdf88,820 Bytes
ch2.pdf550,724 Bytes
ch3.pdf485,933 Bytes
ch4.pdf1,077,611 Bytes
ch5.pdf667,296 Bytes
ch6.pdf1,378,138 Bytes
ch7.pdf1,210,644 Bytes
ch9.pdf141,604 Bytes
etd.pdf169,298 Bytes
vita.pdf83,974 Bytes

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