Abstract
Abstract The residual signal in VLBI, SLR, DORIS and GPS station motion, after a linear trend and seasonal components have been removed, is analysed to investigate site-specific and technique-specific error spectra. The study concentrates on 60 sites with dense observation history by two or more space geodetic techniques. Statistical methods include the Allan variance analysis and the three-cornered hat algorithm. The stability of time-series is defined by two parameters, namely the Allan deviation for a one-year sampling time (noise level) and the slope of the Allan variance graph with its spectral interpretation (noise type). The site-specific noise level is found to be in the range 0.5–3.5 mm in either horizontal direction and 1–4.5 mm in height for most sites. The distribution of site-specific noise type includes both white noise and flicker noise. White noise is predominant in the East direction. Both types of noise are found in the North direction, with no particular geographical clustering. In the Up direction, the Northern hemisphere sites seem to be split in two large geographical sectors characterised either by white noise or by flicker noise signatures. Technique-specific noise characteristics are estimated in several ways, leading to a white noise diagnostic for VLBI and SLR in all three local directions. DORIS has also white noise in the horizontal directions, whereas GPS has a flicker noise spectrum. The vertical noise spectrum is indecisive for both DORIS and GPS. The three-dimensional noise levels for the one-year sampling time are 1.7 mm for VLBI, 2.5 mm for SLR, 5.2 mm for DORIS, and 4.1 mm for GPS. For GPS, the long-term analysis homogeneity has a strong influence. In the case of a test solution reanalysed in a fully consistent way, the noise level drops to the VLBI level in horizontal and to the SLR level in vertical. The three-dimensional noise level for a one-year sampling time decreases to 1.8 mm. In addition, the percentage of stations with flicker noise drops to only about 20% of the network.
Highlights
Since the start of the International Earth Rotation and Reference Systems Service (IERS) in the late 1980’s, the International Terrestrial Reference Frame (ITRF) was provided as a combination of sets of station coordinates and linear velocities
Due to the flicker noise spectrum of both GPS solutions, the ranking of GPS with respect to the three other techniques is much changed: the VLBI and SLR single analysis centre noise levels are at the level of the IGS combined solution, and the single analysis centre GPS noise level is close to that of the corresponding DORIS solution. Considering these large differences between short-term and long-term statistical behaviours, we propose in Section 6.1 an alternative weighting scheme for the combination of terrestrial reference frames
Several analysis methods were applied to collocated time-series of positions in 60 sites monitored by the four global geodetic techniques, VLBI, SLR, DORIS and GPS in order to evaluate their long term positioning stability
Summary
Since the start of the International Earth Rotation and Reference Systems Service (IERS) in the late 1980’s, the International Terrestrial Reference Frame (ITRF) was provided as a combination of sets of station coordinates and linear velocities. Good quality series go back to the early or mid-1990’s, depending on the technique The use of these time-series is expected to allow a better geophysical interpretation of the observed motions. Taking advantage of the availability of these new data, the current issue of the ITRF, ITRF2005, is the first attempt to define a multi-technique terrestrial reference frame (TRF) based on time series (Altamimi and Collilieux, 2007). In this context, knowing the quality of station motion measurements, in particular their long term stability for each site and each
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