Long GPS coordinate time series: Multipath and geometry effects

Within analyses of Global Positioning System (GPS) observations, unmodeled subdaily signals propagate into long‐period signals via a number of different mechanisms. In this paper, we investigate the effects of time‐variable satellite geometry and the propagation of a time‐constant unmodeled multipath signal. Multipath reflectors at H = 0.1 m, 0.2 m, and 1.5 m below the antenna are modeled, and their effects on GPS coordinate time series are examined. Simulated time series at 20 global IGS sites for 2000.0–2008.0 were derived using the satellite geometry as defined by daily broadcast orbits. We observe the introduction of time‐variable biases in the time series of up to several millimeters. The frequency and magnitude of the signal is dependent on site location and multipath source. When adopting realistic GPS observation geometries obtained from real data (e.g., including the influence of local obstructions and hardware specific tracking), we observe generally larger levels of coordinate variation. In these cases, we observe spurious signals across the frequency domain, including very high frequency abrupt changes (offsets) in addition to secular trends. Velocity biases of more than 0.5 mm/yr are evident at some sites. The propagated signal has noise characteristics that fall between flicker and random walk and shows spectral peaks at harmonics of the draconitic year for a GPS satellite (∼351 days). When a perfectly repeating synthetic constellation is used, the simulations show near‐negligible time correlated noise highlighting that subtle variations in the GPS constellation can propagate multipath signals differently over time, producing significant temporal variations in time series.