Abstracts Computer Science

Carrier-phase wrap-up caused by rotating a global positioning system antenna and its effect on measurements

by Laura L. Boyer

Carrier-phase wrap-up is a result of the interaction between a fight-hand circularly-polarized (RHCP) electromagnetic transmission wave and a similarly-polarized receiving antenna. When the antenna is rotated, the measured phase is altered. This results in a shifted range measurement between the transmitter and receiver, even though the geometric distance is not changing. The Global Positioning System (GPS) signals are RHCP, therefore the effect of wrap-up, specifically on GPS, must be considered. The analysis of carrier-phase wrap-up was conducted using actual GPS data,which were collected during an antenna rotation experiment designed for this study. The experiment consisted of two GPS receivers collecting data simultaneously. One receiver antenna was stationary while the other antenna was rotated at a controlled rate. Based on the computed carrier-phase quantities, it is shown that the wrap-up phenomenon significantly affects GPS measurements; however, the behavior of career-phase wrap-up is predictable and consistent. The observed measurement offsets are all proportional to the GPS signal wavelengths. By using a dual carrier-frequency measurement, it is possible to eliminate the wrap-up-induced range error. The two GPS carrier frequencies wrap up at the same rate when computed in units of cycles; however, when the calculations are done in units of meters, the changes are not equal. This fact can be applied to observe the wrap-up effect on one receiver. By converting the measured range offset to degrees-of-rotation, the GPS receiver can act as a gyroscope.