Error Analysis of Carrier Phase Positioning Using Controlled Reception Pattern Antenna Arrays
Type of DegreeMaster's Thesis
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This thesis analyzes and provides algorithms that protect against various carrier phase positioning errors caused by anti-jamming algorithms commonly used to position in a jamming environment. These position errors include effects from the Least-Mean-Square algorithm distorting measurements and the loss of accuracy when an anti-jamming receiver is in the presence of a jammer. While there are a number of different methods in which anti-jamming performance can be accomplished in a GPS receiver, the focus of this thesis is on algorithms that are implemented using multiple antennas, also known as controlled reception pattern array (CRPA) antennas. Carrier phase positioning is a highly accurate position solution, capable of positioning a user down to the centimeter level. However, the increased accuracy also makes the receiver susceptible to new sources of error not typically found when using the ranging code, due to the lower amount of noise on the measurement. Therefore, the effect and magnitude of these errors must be examined. CRPA anti-jam algorithms attenuate interference and/or strengthen desired signals by leveraging the array's spatial, temporal, and frequency difference between each of the antennas in combination with phase shifting and scaling each antenna's received signal. While this operation can remove or weaken the interference signal, algorithms such as Least-Mean-Square can have a biasing effect on a phase lock loop's estimate of the Doppler frequency due to time variant phase shift applied by the algorithm. Depending on algorithm parameters, the time variant phase shift can cause position drifts of up to a centimeter per minute. The use of a normalization process is shown as a method to remove the phase shift, and therefore the position bias, while not degrading the anti-jam performance. The second focus of this thesis is the ability of the CRPA anti-jam receiver to maintain carrier phase position accuracy, on the order of centimeters of less, in a jamming environment. In this experiment, a four element receiver is tested with a wide range of jamming strengths and the standard deviation of Doppler measurements are averaged across all channels to provide an estimate of the accuracy of the carrier phase position solution. Because the tracking loop can be tuned to handle various amounts of noise, the phase lock loop (PLL) bandwidth was varied and individually tested with each jamming strength. At the limits of the CRPA anti-jam receiver's nulling ability, a position solution was computed using a typical receiver PLL bandwidth, a reduced bandwidth, and an extremely low bandwidth with the aiding of an IMU. The results show that the bandwidth must be reduced significantly to maintain carrier phase position accuracy and the tracking loop must be aided by an IMU in order to obtain millimeter accuracy in the high jamming environment.