2006 AGU Fall Meeting. 14 Dec. 2006, San Francisco – Poster #G43A-0985 Jim Ray (NOAA/NGS), Tonie van Dam (U. Luxembourg), Zuheir Altamimi (IGN), Xavier Collilieux (IGN) Abstract. Prior studies of the power spectra of GPS position time series have found pervasive seasonal signals against a power-law background of white noise plus flicker noise [e.g., S. Williams et al., JGR, 109, B03412, 2004]. D. Dong et al. [JGR, 107(B4), 2002] estimated that less than half the observed GPS seasonal power can be explained by redistributions of geophysical fluid mass loads. Much of the residual variation is probably caused by unidentified GPS technique errors and analysis artifacts. Among possible mechanisms, N. Penna and M. Stewart [GRL, 30(23), 2003] have shown how unmodeled analysis errors at tidal frequencies (near 12- and 24-hour periods) can be aliased to longer periods very efficiently. Signals near fortnightly, semi-annual, and annual periods are expected to be most seriously affected. We have examined spectra of the 167 sites of the International GNSS Service (IGS) network having at least 200 weekly measurements during 1996.0-2006.0. The non-linear residuals of the weekly IGS solutions that were included in ITRF2005 have been used. To improve the detection of common-mode signals, the normalized spectra of all sites have been stacked, then smoothed with a boxcar filter (0.03 cpy window), for each local N, E, and H component. Anomalous Harmonics in the Spectra of GPS Position Estimates 1. Spectra of Raw GPS Time Series – Stacked & Smoothed The stacked, smoothed spectra are similar for all three components. Peaks are evident at harmonics of about 1 cpy up to at least 6 cpy, but the peaks are not at strictly 1.0 cpy intervals. Based on the 6th tones of the spectra, which are among the sharpest peaks, and assuming a linear overtone model, then a common fundamental of 1.039 ± 0.005 cpy can explain all peaks well. A flicker noise power-law continuum describes the background spectrum down to periods of a few months, after which the residuals become whiter. Similar sub-seasonal tones are not apparent in the residuals of available SLR and VLBI sites, which are both 10 times less numerous and dominated by white noise. There is weak evidence for a few isolated 1 cpy overtones in the spectra of geophysical loadings, but these are much noisier than for GPS positions. Alternatively, as pointed out by U. Hugentobler (TU Munich), the period of the 1.039 cpy frequency, about 351.5 days, is very close to a "GPS year"; i.e., the interval required for the constellation to repeat its inertial orientation with respect to the sun. This could indicate that the harmonics are a type of systematic error related to the orbits. Mechanisms could involve modeling defects or aliasing of site-dependent positioning biases. 2. Filtered GPS Spectra – Remove Seasonal Fits 3. Compare to SLR Spectra • since signals at 1.0 & 2.0 cpy are likely due to geophysical & technique sources, fit & remove those first • then recompute stacked, smooth spectra as before • harmonics at >2 cpy are still apparent • assume linear overtone model for harmonics • average of narrow 6 th N,E,H peaks gives fundamental tone of 1.039 ± 0.005 cpy • model fits observed peaks well, except seasonal peaks affected by pre- filtering (see red dashed lines) • flicker noise describes background spectra down to periods of a few months • at shorter intervals, residuals become whiter • stacked spectra (Lomb periodogram) for 167 IGS sites with >200 weekly points during 1996.0 - 2006.0 • using non-linear residuals from the ITRF2005 combination • smoothed using boxcar filter with 0.03 cpy window • spectra are very similar for all 3 components • harmonics seen near 1 cpy up to at least ~6 cpy • frequencies for harmonics >2 cpy are definitely not at even 1.0 cpy intervals • compare observed spectral peaks with 1.0 intervals (marked with red dashed lines) • contributions also probable at 1.0 & 2.0 cpy due to geophysical signals & Raw SLR Spectra • use ITRF2005 non-linear residuals for 18 sites with >200 weekly points • ~1 cpy peaks seen in all 3 components • no sub-seasonal harmonics seen • spectra are dominated by white noise • insufficient sensitivity to detect harmonic signals if they are present (e.g., geophysical) Raw GPS Spectra Filtered GPS Spectra