Spurious periodic horizontal signals in sub-daily GPS position estimates

Periodic horizontal signals have been identified in sub-daily estimates of position obtained from GPS data using batch least-squares solutions. The estimation of sub-daily position is commonly used for ocean loading and ice movement studies, for example. These spurious motions are artefacts of the processing methodology due to the presence of unmodelled, tidally- induced vertical signals in the GPS data. The periodic horizontal signals have magnitudes of about 40-50% of the amplitude of the vertical periodic signal. The shape of the periodic horizontal signals is related to the first derivative and the square of the vertical signal. The artefacts are mostly evident in the east-west component and can be removed by fixing the carrier phase ambiguities to integer values. We introduce a sensitivity analysis to study in detail the effects of ambiguity resolution and latitudinal variation of the artefacts. The overall conclusions are that for high precision batch processing of time series of geodetic positions, using sub-daily data spans, care needs to be taken to avoid aliasing of horizontal coordinates due to the vertical motion of the site.