Coordinate kinematic models in the International Terrestrial Reference Frame releases

International audienceIn the latest International Terrestrial Reference System realization (ITRF2014) combination model, new types of displacements have been introduced by means of mathematical functions. The addition of these functions has led to the implementation of new constraints to define the reference frame. This work was anticipated by A. Dermanis (2008) who derived constraint equations for different kinematic models. This paper presents the fundamental theoretical concepts that have been used to derive the latest International Terrestrial Reference Frame (ITRF). A new physical interpretation of the partial inner constraints involving transformation parameters is presented to supplement earlier work. By reviewing the various possibilities that could have been implemented to enhance the ITRF coordinate variations, this paper justifies the ITRF2014 chosen kinematic model and why it still does not include functions suggested by Dermanis (2008)

Sea level: measuring the bounding surfaces of the ocean

The practical need to understand sea level along the coasts, such as for safe navigation given the spatially variable tides, has resulted in tide gauge observations having the distinction of being some of the longest instrumental ocean records. Archives of these records, along with geological constraints, have allowed us to identify the century-scale rise in global sea level. Additional data sources, particularly satellite altimetry missions, have helped us to better identify the rates and causes of sea level rise and the mechanisms leading to spatial variability in the observed rates. Analysis of all of the data reveals the need for long-term and stable observation systems to assess accurately the regional changes as well as to improve our ability to estimate future changes in sea level. While information from many scientific disciplines is needed to understand sea level change, this paper focuses on contributions from geodesy and the role of the ocean’s bounding surfaces: the sea surface and the Earth’s crust

ANALYTICAL QUALITY ASSESSMENT OF ITERATIVELY REWEIGHTED LEAST-SQUARES (IRLS) METHOD

The iteratively reweighted least-squares (IRLS) technique has been widelyemployed in geodetic and geophysical literature. The reliability measures areimportant diagnostic tools for inferring the strength of the model validation. Anexact analytical method is adopted to obtain insights on how much iterativereweighting can affect the quality indicators. Theoretical analyses and numericalresults show that, when the downweighting procedure is performed, (1) theprecision, all kinds of dilution of precision (DOP) metrics and the minimaldetectable bias (MDB) will become larger; (2) the variations of the bias-to-noiseratio (BNR) are involved, and (3) all these results coincide with those obtained bythe first-order approximation method

ITRF2008 plate motion model

International audienceThe ITRF2008 velocity field is demonstrated to be of higher quality and more precise than past ITRF solutions. We estimated an absolute tectonic plate motion model made up of 14 major plates, using velocities of 206 sites of high geodetic quality (far from plate boundaries, deformation zones and Glacial Isostatic Adjustment (GIA) regions), derived from and consistent with ITRF2008. The precision of the estimated model is evaluated to be at the level of 0.3 mm/a WRMS. No GIA corrections were applied to site velocities prior to estimating plate rotation poles, as our selected sites are outside the Fennoscandia regions where the GIA models we tested are performing reasonably well, and far from GIA areas where the models would degrade the fit (Antarctica and North America). Our selected velocity field has small origin rate bias components following the three axis (X, Y, Z), respectively 0.41 ± 0.54, 0.22 ± 0.64 and 0.41 ± 0.60 (95 per cent confidence limits). Comparing our model to NNR-NUVEL-1A and the newly available NNR-MORVEL56, we found better agreement with NNR-MORVEL56 than with NNR-NUVEL-1A for all plates, except for Australia where we observe an average residual rotation rate of 4 mm/a. Using our selection of sites, we found large global X-rotation rates between the two models (0.016°/Ma) and between our model and NNR-MORVEL56 of 0.023°/Ma, equivalent to 2.5 mm/a at the Earth surface

Joint segmentation of multiple GPS coordinate serie

International audienceFor the first time, a joint general segmentation procedure of multiple GPS coordinate series is proposed fornearby stations. It allows simultaneously estimating station-specific trends, seasonal signals and a common groundmotion signal between series from adjacent stations. An extension of the model and the estimation procedure, which isan iterative procedure, proposed following Picard et al. (2011) and Bertin et al. (2014) is considered in order to takeinto account for the specificities of the GPS data. The tested approach has been shown to be efficient in providingmeaningful offsets and is found to be a relevant method for avoiding segmenting the true physical signal.Pour la première fois, une procédure de segmentation multiple de séries de coordonnées est proposée pourdes stations GPS géographiquement proches. Elle permet d’estimer simultanément des vitesses de déplacements etdes signaux saisonniers spécifiques à chaque série tout en déterminant un signal de déplacement commun à toutesles stations. Une extension du modèle proposé par Picard et al. (2011) et Bertin et al. (2014) est considérée afin deprendre en compte les différentes caractéristiques liées aux données GPS ainsi que la procédure d’estimation, procédureitérative. Les résultats obtenus sur quatre ensembles de séries réelles GPS sont très pertinents d’autant plus que laméthode permet de ne pas segmenter le signal physique en identifiant des ruptures liées au mouvement réel du so

Anomalous harmonics in the spectra of GPS position estimates

Prior studies of the power spectra of GPS position time series have found pervasive seasonal signals against a power-law background of flicker noise plus white noise. Dong et al. (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, Penna and Stewart (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, semiannual, and annual periods are expected to be most seriously affected. We have examined spectra for the 167 sites of the International GNSS (Global Navigation Satellite Systems) Service (IGS) network having more than 200 weekly measurements during 1996.0–2006.0. The non-linear residuals of the weekly IGS solutions that were included in ITRF2005, the latest version of the International Terrestrial Reference Frame(ITRF), have been used. To improve the detection of common-mode signals, the normalized spectra of all sites have been stacked, then boxcar smoothed for each local north (N), east (E), and height (H) component. The stacked, smoothed spectra are very similar for all three components. Peaks are evident at harmonics of about 1 cycle per year (cpy) up to at least 6 cpy, but the peaks are not all at strictly 1.0 cpy intervals. Based on the 6th harmonic of the N spectrum, which is among the sharpest and largest, and assuming a linear overtone model, then a common fundamental of 1.040 ± 0.008 cpy can explain all peaks well, together with the expected annual and semiannual signals. 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 satellite laser ranging (SLR) and very long baseline interferometry (VLBI) sites, which are both an order of magnitude less numerous and dominated by white noise. There is weak evidence for a few isolated peaks near 1 cpy harmonics in the spectra of geophysical loadings, but these are much noisier than for GPS positions. Alternative explanations related to the GPS technique are suggested by the close coincidence of the period of the 1.040 cpy frequency, about 351.2 days, to the ‘‘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 satellite orbits. Mechanisms could involve orbit modeling defects or aliasing of site-dependent positioning biases modulated by the varying satellite geometry