Atmospheric effects and spurious signals in GPS analyses

Improvements in the analyses of Global Positioning System (GPS) obser- vations yield resolvable mm to sub-mm di erences in coordinate estimates, thus provid- ing sucient resolution to distinguish subtle di erences in analysis methodologies. Here we investigate the e ects on site coordinates of using di erent 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 e ect 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.

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