Filtered and semi-continuous single epoch GPS for deformation monitoring

Abstract

Multipath is a major sidereally-repeating error affecting GPS. The repetition of satellite-receiver geometry approximately every sidereal day enables filtering to minimize multipath. Computing the exact error repeat interval using the day-to-day autocorrelation with a 10-hour window yields a consistent and steady value of 23h 55m 54s. A 2-hour window gives fluctuating lag values with >97% of the optimal lag's correlation, suggesting little advantage in using a satellite-specific or timevarying lag in double-difference processing. GPS data are filtered by stacking at the optimum lag, and applying, either coordinate residuals ("coordinate filtering"), or double-difference carrier phase residuals ("observation filtering"). Coordinate filtering yields better coordinate repeatability than observation filtering, but with similar hour-to-hour consistency. The variance reduction in a high multipath environment over a 24-hour dataset reaches 73%, using a 3-day coordinate-filter in a high multipath enviromnent. However, observation filtering requires less processing time to generate and apply the filter. As the time gap betweent he days generatingth e filter and the applied day increases, the variance reduction worsens gradually, reaching zero at a gap of -30 days. The optimal variance reduction (61% in a low multipath environment) is achieved by stacking seven days immediately before the applied day, but a stack of 3-9 days is acceptable. A switched-antenna array system may be adopted instead of continuous GPS for cost minimization. According to the used receivers' hardware, the optimum session interval to use is 119 seconds, which provides sufficient usable epochs per switch interval and is synchronized with the "sidereal" lag. Semi-continuous sidereallyfiltered GPS is efficient in detecting horizontal and vertical deformations in nearstatic enviromnents with high multipath to the millimetre level. In addition, the precision of the deforming stations improved, reaching to a precision better than 5 mm and 8 mm in the horizontal and vertical plane respectively. Hence, this provides the capability of monitoring slow moving deformations using a quick, cost-effective and precise GPS technique.