Effects of azimuthal multipath heterogeneity and hardware changes on GPS coordinate time series

1. Effects of azimuthal multipath heterogeneity and hardware changes on GPS coordinate time series Sibylle Goebell, Matt King Email:sibylle.goebell@ncl.ac.uk, m.a.king@ncl.ac.uk • Introduction • GPS signals may travel along several paths before reaching the receiving antenna due to signal multipath (MP) as they are reflected off surrounding buildings, structures, or the ground. This multipath bias may be partially mitigated - in the field by careful site selection and monument design (e.g., avoiding large diameter pillars), in the data analysis by using an appropriate elevation cut-off angle or through elevation-dependent weighting. However, these approaches are not able to remove all multipath signal and since geodetic analysis soft-ware does not model it these errors may propagate into the GPS solutions. King and Watson (2010) have shown that, together with subtle changes in the GPS observation geometry, unmodelled multi-path may produce time-correlated noise, including anomalous harmonics, which will be largely site specific. • Method • To investigate the effect of artificially generated multipath on time series we use a multipath model which is based on the model by Elosegui et al. (1995) although extended to be appropriate for the antenna near field and include the effects of antenna gain (T.A. Herring, pers. comm., 2009). The model is shown in Figure 1 alongside that of Elosegui et al. (1995) (King and Watson, 2010). • We examine the effect of azimuthally symmetric and asymmetric multipath at a range of sites, including one site (PLAT) with sub-stantial obstructions and which also undergoes hardware changes. • Processing Strategy • Precise Point Positioning (PPP) with GIPSY v5, reprocessed JPL • GPS orbits, ambiguity resolution using ambizap (Blewitt, 2008), • absolute antenna phase centres, 7º cut-off angle. • Offshore site PLAT(form) and IGS sites ONSA, NTUS, NYAL • (Figure 2). • Several PPP runs for each site: one base run with the original • AOAD/M_T antenna, i.e. no multipath induced (MP00), and a • series of runs perturbed by the Herring model - multipath reflector • at 0.1m (MP0.1), 0.2m (MP0.2), 1.5m (MP1.5) below AOAD/M_T • antenna – inducing azimuthally symmetric multipath (Figure 4). • Additionally, asymmetric multipath is modelled for different 45º sectors of the sky MP45 (0º-45º), MP90 (45º-90º), MP135 (90º-135º), MP180 (135º-180º), MP225 (180º-225º), MP270 (225º-270º), MP315 (270º-315º), MP360 (315º-360º) based on the model with reflector at 0.1m (Figure 6). Symmetric MP Hardware change Figure 4: Effect of multipath reflector at 0.1m, 0.2m, 1.5m below antenna on raw time series (compared to multipath-free model MP00) for offshore station PLAT and IGS sites NYAL, ONSA, NTUS (hardware change at vertical line). Figure 5: Amplitude spectra of time series based on the Lomb-Scargle periodogram method for offshore station PLAT and IGS sites NYAL, ONSA, NTUS . Asymmetric MP Figure 6: Effect of multipath reflector at 0.1m for different 45º sectors of the sky on 91-day smoothed time series (compared to multipath- free model MP00) for offshore station PLAT and IGS sites NYAL, ONSA, NTUS (hardware change at vertical line). • Results and Conclusions • Study of azimuthally symmetric multipath supports conclusions of King and Watson (2010) ▪ Height time series is less noisy for PLAT after the receiver change, presumably due to improvements in observation geometry (Figure 4) ▪ Multipath propagation changes as the satellite geometry changes (e.g. due to hardware changes), with each multipath scenario giving a slightly different offset magnitude (see vertical line for PLAT in Figure 4 and Figure 6) ▪ Large periodic signals are evident at near-annual signal (ONSA and NTUS, Figure 5) and semi-annual (PLAT). The latter disappears after the receiver change (Figure 5). • Asymmetric MP results in a complex relationship between different models for height, longitude and latitude components (Figure 6) ▪ Much larger effect on time series than azimuthally symmetric MP, including site velocity biases (Figure 4 and Figure 6) ▪ Longitude and height component effects are largest; latitude slightly less affected. • Multipath is likely to be a significant contributor to GPS noise, and changes in its nature (e.g., horizontal distribution) or hardware changes leading to tracking changes may also change the coordinate time series. Figure 1: Ionospheric-free (LC) range bias with different model settings for the Elosegui et al. (1995) model (left) and the model used here (right) for H=0.1m (grey), H=0.2m (cyan) and H=1.5m (magenta). Elosegui et al (1995) T.A. Herring References Blewitt, G. (2008), Fixed point theorems of GPS carrier phase ambiguity resolution and their application to massive network processing: Ambizap, J. Geophys. Res., 113, B12410, doi:10.1029/2008JB005736. Elosegui, P., Davis, J. L., Jaldehag, R. K., Johansson, J. M., Niell, A. E., Shapiro, I. I. (1995), Geodesy Using the Global Positioning System: The Effects of Signal Scattering, J. Geophys. Res., 100, pp. 9921-+. King, M.A., and C.S. Watson (2010), Long GPS coordinate time series: multipath and geometry effects, J. Geophys. Res., doi:10.1029/2009JB006543. Figure 2: GPS sites. Figure 3: Skyplots showing number of observations before and after receiver change for station PLAT. Elosegui et al (1995) T.A. Herring