Atmospheric Effects and Spurious Signals in GPS Analysis

Improvements in the analyses of Global Positioning System (GPS) obser- vations yield resolvable mm to sub-mm dierences in coordinate estimates, thus provid- ing sucient resolution to distinguish subtle dierences in analysis methodologies. Here we investigate the eects on site coordinates of using dierent approaches to modelling atmospheric loading deformation (ATML) and handling of tropospheric delays. The rig- orous approach of using the time-varying VMF1 mapping function yields solutions with lower noise at a range of frequencies compared with solutions generated using empiri- cal mapping functions. This is particularly evident when ATML is accounted for. Some improvement also arises from using improved a priori zenith hydrostatic delays (ZHD), with the combined eect being site speci c. Importantly, inadequacies in both mapping functions and a priori ZHDs not only introduce time correlated noise but signi cant pe- riodic terms at solar annual and semi-annual periods. We nd no signi cant di erence between solutions where non-tidal ATML is applied at the observation level rather than as a daily-averaged value but failing to model diurnal and semi-diurnal tidal ATML at the observation level can introduce anomalous propagated signals with periods that closely match the GPS draconitic annual (351.4 d) and semi-annual period (175.7 d). Ex- acerbated by not xing ambiguities, these signals are evident in both stacked and sin- gle site power spectra, with each tide contributing roughly equally to the dominant semi- annual peak. The amplitude of the propagated signal reaches a maximum of 0.8 mm with a clear latitudinal dependence that is not correlated directly with locations of maximum tidal amplitude. This is the first evidence of aliased signals being produced from tidal ATML deformations.