Investigation of site-dependent GPS errors and monument stability using a short-baseline network of braced monuments Emma Hill, Jim Davis, Pedro Elosegui, Brian Wernicke, Nathan Niemi, and Eric Malikowski

Introduction All monuments are deep-braced except REP2 REPO-REP2 ~10 meters REPO-REP3 ~100 meters REPO-REP4 ~1000 meters All monuments are deep-braced except REP2

East time series RMS about a model of the seasonal cycle: 0.06 to 0.15 mm Annual cycle amplitudes: REP2: 0.22 ± 0.01 mm REP4: 0.34 ± 0.01 mm These signals look really huge, but you need to look at the scale. Even if we ignore the seasonals and calculate RMS about the mean, the values for the stations that are close together are around 0.08 mm ~50% of noise at REP3 is likely to be due to receiver RMS for a zero-length baseline (SLI4-SLID): 0.03 mm

North time series RMS about a model of the seasonal cycle: 0.06 to 0.16 mm Annual cycle amplitudes: REPO: 0.13 ± 0.01 mm Similar story to east component. Note that there’s no coherence in the seasonal signals from site to site… REPO had a much smaller seasonal for the east. REP4 has a dominant semi-annual cycle. RMS for a zero-length baseline (SLI4-SLID): 0.03 mm

Up time series RMS about a model of the seasonal cycle: 0.14 to 0.51 mm RMS for a zero-length baseline (SLI4-SLID): 0.08 mm Surprisingly, annual signal not so clear (does show up in spectral analysis). RMS values are still *very* small.

Monument motion Baseline length difference for REP2-REP3 Maximum ‘velocity’ estimate for any baseline component: 0.23 ± 0.02 mm/yr Previously estimated random walk noise for the BARGEN sites: ~1 mm/yr1/2 (Williams et al., 2004) Already seen that RMS values are on the level of <0.1-0.2 mm, and we also see that rates are very small. Too early to get reliable rates, but they’re on the order of <0.2 mm/yr (but there is a hint that we do see rates). Williams et al number gives ~1.2 mm over 1.5 years… however you cut it, we just don’t see that. REP2 about the same, but it does have a little more noise (and see next slide). REP2 (shallow-braced) behaves similarly to REPO and REP3 (deep-braced)

Seasonal cycles GPS Multipath GPS Temperature (scaled) Estimation of multipath from SNR for station REPO (technique from Bilich [2006]) GPS Temperature (scaled) NB: ONLY 10-30 deg data for multipath. Some components of the multipath correction do show seasonal signals. But these are preliminary results and we still have work to do. Correlation coeff with temperature with seasonal removed: 0.5 for REP2 east, next best 0.3 for REP4 east, REP2 north When seasonal cycle not removed, correlation coeff for REP2 east is 0.9. Have some other stations at 0.5-0.7. The two may not be unrelated… for example, Christine’s paper on multipath and soil moisture. Comparison of GPS and (scaled) temperature data at REP2

Summary The time series are remarkably stable, with RMS values at the level of microns (even for the shallow-braced monument). Our results appear to limit the level of monument wander at these sites to <0.1 mm. The time series do have some structure, particularly in the form of clear seasonal signals. Multipath may be a cause. Big picture important point is that we limit site-dependent error for these braced monuments to a very small number.. Implications in terms of putting error bounds on velocities. Baseline-dependent errors are probably leaking into prior estimates of monument motion (and we see a hint of the effect of baseline length with REP4). Zero-baseline … RMS about mean of horizontal components is 0.3 mm. So about half of what we see at REP3 is probably receiver noise. The seasonal signals remain a bit of a puzzle, but we show that multipath may be a factor, and correlations with temperature do exist.

Thanks!