Long GPS coordinate time series: Multipath and geometry effects

Abstract

[1] 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.

Improved Constraints on Models of Glacial Isostatic Adjustment: A Review of the Contribution of Ground-Based Geodetic Observations

“…Ostini et al (2007) demonstrated that there are regional correlations in amplitude and phase between these periodic functions detectable in the coordinate time series. This implies that at least a part of these periodic components are very likely caused by orbit modelling deficiencies [see also and King and Watson (2010)]. Slabinski (2006) analyzed the discontinuities of the GPS orbits at the day boundaries of the IGS final orbits and has found non-random behaviour.…”

“…They also found surprisingly large secular rates, up to 0.3 mm/year, compared with previous estimates of tectonic activity for the area. In a further study, King and Watson (2010) examined the effects of radiating near field multipath. They showed that time-constant multipath combined with the GPS satellite constellation, which has evolved over time, is sufficient to produce substantial signal across the entire frequency spectrum, including apparent offsets, velocity biases of several tenths of a millimetre per year and time-correlated noise.…”

“…Ambiguity free (lighter shades) and ambiguity fixed (darker shades) solutions are shown. Reproduced from King and Watson (2010) et al. 2002).…”

Assessment of modernized GPS L5 SNR for ground-based multipath reflectometry applications

“…The antenna receives energy directly from the satellite transmitter and also from the ground-reflected signals. This multipath reception condition is one of the main error sources in high-precision GPS studies (Elósegui et al, 1995;King and Watson, 2010).…”

Groundwater-induced vertical movements in Tsukuba revisited: installation of a new GPS station

“…4 are not fully understood. Using numerical simulations, King and Watson (2010) showed that even time-constant multipath effects can result in time-correlated multipath-induced errors under time-variable GPS geometry, as seen in reality. It is also probable that time-variable surface reflectivity changes due to soil moisture (Larson et al, 2010) or vegetation growth (Small et al, 2010) cause temporal changes in the multipath effects and hence result in timecorrelated multipath-induced errors.…”