On the nature of GPS draconitic year periodic pattern in multivariate position time series

Abstract

AbstractPlate tectonics studies using GPS require proper analysis of time series, in which all functional effects are understood and all stochastic effects are captured using an appropriate noise assessment technique. Both issues are addressed in this contribution. Estimates of spatial correlation, time correlated noise, and multivariate power spectrum for daily position time series of 350, 150, and 50 permanent GPS stations, respectively, collected between 2000–2007, 1998–2007, and 1996–2007 are obtained. The daily GPS global solutions were processed by the GPS Analysis Center at JPL. The detection power of the common‐mode signals is improved by including the time‐ and space‐correlated noise into the least squares power spectrum. Previous signals, such as those with periods of 13.63, 14.2, 14.6, and 14.8 days, are identified in the multivariate analysis. Significant signal with period of 351.6 ± 0.2 days and its higher harmonics are detected in the series, which closely follows the GPS draconitic year. The variation range of this periodic pattern for the north, east, and up components are about ±3, ±3.2, and ±6.5 mm, respectively. Three independent criteria confirm that this periodic pattern is of similar nature at adjacent stations, indicating its independence of the station‐related effects such as multipath. It is likely due to the other causes of the GPS draconitic year period driven into GPS time series. The multivariate power spectrum shows a cluster of signals with periods ranging from 5 to 6 days (quasiperiodic signals). In their aliased forms, the effects are likely partly responsible for the time‐correlated noise and partly for the periodic patterns at lower frequencies.