Terrestrial Reference Frame Realization Research Articles

Optimizing realization of the terrestrial reference frame on a regional basis: A case study using the crustal movement observation network of China

It is critical in the study of crustal deformation to reduce TRF-related system errors and enhance the accuracy of GPS coordinate time series. To reduce system errors associated with the coordinate time series, which are related to the terrestrial reference frame (TRF) realization, we develop a recursive TRF realization strategy in regional GPS data processing. We have processed the whole set of CMONOC (Crustal Movement Observation Network of China) GPS data by using the Bernese 5.2 software and employed the controlled datum removal (CDR) filter to deal with the rank defect problem in the daily coordinate normal equations (NEQs). On the recursive TRF realization, we sequentially perform time series modeling with integral trajectory models, TRF realization with all continuous stations treated as “pseudo” fiducial stations, and frame alignment to the ITRF2014 with the 6-parameters Helmert transformation, in an iterative mode. We obtain the final coordinate time series through 3 times of iterations. Compared to the results derived from the conventional TRF realization strategy with the average root mean squares (RMS) being 1.98 mm, 2.62 mm and 5.39 mm for the east, north and up components, respectively, the average RMS scatters earn significant reduction up to 30%, 43% and 16% in the first loop, with their quantities being 1.41 mm, 1.51 mm and 4.57 mm for the east, north and up components, respectively, and negligible changes in the following 2 loops. In contrast to previous studies, our strategy is feasible in the processing of regional geodetic networks and concentrated on the TRF-related system errors without any pre-assumption and spatial limitation. In essence, the recursive strategy enhances the weights for the CMONOC stations and inevitably weakens the weights for stations exterior to the CMONOC to accommodate the misfits between the referential and estimated daily solutions in the least squares adjustment, thus resulting in the slightly increase of RMS scatters for the coordinate time series of globally distributed stations. For the CMONOC stations, the north component of coordinates time series has a maximum RMS reduction, revealing the identical precision for both horizontal components, thus indicating that our strategy remedies the frame defects stemming from the extremely uneven distribution of the fiducial network, and retrieves the “real” precision of GPS observations. The insignificant RMS reduction on the vertical component may be attributed to the insufficiency in the time series modeling. Our recursive TRF realization strategy can benefit the velocity estimation for campaign stations.

100 kHz satellite laser ranging demonstration at Matera Laser Ranging Observatory

The new challenges related to the monitoring of Earth’s shape and motion have led the global geodetic observing system to set more stringent requirements on the precision and stability of the terrestrial reference frame (TRF). The achievement of this ambitious goal depends on the improvement of space geodesy techniques, satellite laser ranging (SLR) in particular, being the main instrument for TRF realization. In this work, we study the potential of very high repetition rate SLR by performing a data acquisition campaign with an Ekspla “Atlantic 60” 100 kHz repetition rate laser at the Matera Laser Ranging Observatory (MLRO). This system constitutes an increase of two orders of magnitude in repetition rate with respect to the current SLR stations, while maintaining a good single-shot timing performance. The system has been active for 4 consecutive nights, consistently tracking several low Earth orbit satellites as well as LAGEOS 1 and 2. The results have shown a single-shot time jitter close to other stations, but with unprecedented statistics for approx 10 ps single-shot precision. The analysis of the residuals of LAGEOS satellites allowed us to identify multiple peaks, due to the retroflection from different corner cubes. This opens up the possibility of attitude determination of retroreflector arrays, as well as a new method for spin rate measurement.

Improved Global Nonlinear Surface Mass Variation Estimates From Geodetic Displacements and Reconciliation With GRACE Data

AbstractSpatiotemporal coverage gaps and the nature of distant signature sources in Global Navigation Satellite System (GNSS) and The Gravity Recovery and Climate Experiment (GRACE) data require global inversions of data combinations for cross validation and complete and continuous monitoring of surface water mass variations. For spectral completeness, accurate data‐based degree‐1 surface mass variation coefficient or equivalent geocenter motion estimates are also desired but very difficult to obtain. To improve the reliability of such estimates, we carry out a new effort to combine different space geodetic technique data in a Kalman filter and time series approach to Terrestrial Reference Frame realization using an advanced formulation. With origin at the center of mass of the Earth system, the new parameterization now includes site displacements in the state vector explicitly for realistic assessment of their covariance matrices. More robust geocenter motion results are achieved when the displacement information is combined further with GRACE gravity data in a unified inversion. Previously, various systematic effects in GNSS data resulted in rather large discrepancies in global surface mass change estimates when compared with those from GRACE gravity data. Here, we show that significantly improved GNSS data after reprocessing and the refined Estimating the Circulation and Climate of the Ocean (ECCO) bottom pressure model reveal a global surface mass variation pattern that has been largely reconciled with that from GRACE data.

Impact of network constraining on the terrestrial reference frame realization based on SLR observations to LAGEOS

The Satellite Laser Ranging (SLR) network struggles with some major limitations including an inhomogeneous global station distribution and uneven performance of SLR sites. The International Laser Ranging Service (ILRS) prepares the time-variable list of the most well-performing stations denoted as ‘core sites’ and recommends using them for the terrestrial reference frame (TRF) datum realization in SLR processing. Here, we check how different approaches of the TRF datum realization using minimum constraint conditions (MCs) and the selection of datum-defining stations affect the estimated SLR station coordinates, the terrestrial scale, Earth rotation parameters (ERPs), and geocenter coordinates (GCC). The analyses are based on the processing of the SLR observations to LAGEOS-1/-2 collected between 2010 and 2018. We show that it is essential to reject outlying stations from the reference frame realization to maintain a high quality of SLR-based products. We test station selection criteria based on the Helmert transformation of the network w.r.t. the a priori SLRF2014 coordinates to reject misbehaving stations from the list of datum-defining stations. The 25 mm threshold is optimal to eliminate the epoch-wise temporal deviations and to provide a proper number of datum-defining stations. According to the station selection algorithm, we found that some of the stations that are not included in the list of ILRS core sites could be taken into account as potential core stations in the TRF datum realization. When using a robust station selection for the datum definition, we can improve the station coordinate repeatability by 8%, 4%, and 6%, for the North, East and Up components, respectively. The global distribution of datum-defining stations is also crucial for the estimation of ERPs and GCC. When excluding just two core stations from the SLR network, the amplitude of the annual signal in the GCC estimates is changed by up to 2.2 mm, and the noise of the estimated pole coordinates is substantially increased.

Network Effects and Handling of the Geocenter Motion in Multi‐GNSS Processing

AbstractBoth, the network configuration and the way of terrestrial reference frame (TRF) realization, affect the global geodetic products delivered from the Global Navigation Satellite Systems (GNSS) data processing. The purpose of this study is to analyze the differences in GNSS products, such as station coordinates, Earth rotation parameters, geocenter coordinates (GCC), and satellite orbits delivered from the double‐difference multi‐GNSS (GPS, GLONASS, and Galileo) processing, which may arise from (1) using a homogeneous and inhomogeneous network of multi‐GNSS stations, (2) different approaches to the TRF realization using minimum constraint conditions, and (3) different approaches to handling of GCC in GNSS global processing. The questionable quality of GCC delivered from the global GNSS solutions is described with a special attention to network effects and system‐specific parameters. We found that Galileo can provide GCC, whose quality corresponds to the GPS series. Moreover, the GCC from Galileo is of a better quality than those based on GLONASS data, despite the same number of nominal orbital planes and a much lower number of active satellites. When the No‐Net‐Translation constraint is not applied on the GNSS network, the station coordinate repeatability is worsened by about 70%, 55%, and 25% for the north, east, and up components, respectively, compared to the solution when applying No‐Net‐Translation and when having the network origin consistent with the international TRF. We thus infer that the No‐Net‐Translation condition is mandatory in global GNSS solutions.

Impact of terrestrial reference frame realizations on altimetry satellite orbit quality and global and regional sea level trends: a switch from ITRF2008 to ITRF2014

Abstract. A terrestrial reference frame (TRF) is a basis for precise orbit determination of Earth-orbiting satellites, since it defines positions and velocities of stations, the tracking data of which are used to derive satellite positions. In this paper, we investigate the impact of the International Terrestrial Reference Frame realization ITRF2014, as compared to its predecessor ITRF2008, on the quality of orbits, namely, on root-mean-square (rms) fits of observations and orbital arc overlaps of three altimetry satellites (TOPEX/Poseidon, Jason-1, and Jason-2) in the time interval from August 1992 to April 2015 and on altimetry products computed using these orbits, such as single-satellite altimeter crossover differences, radial and geographically correlated mean sea surface height (SSH) errors and regional and global mean sea level trends. The satellite orbits are computed using satellite laser ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) observations of a global network of stations. We have found that using ITRF2014 generally improves the orbit quality as compared to using ITRF2008. Thus, the mean values of the rms fits of SLR observations decreased (improved) by 2.4 % and 8.8 % for Jason-1 and Jason-2, respectively, but are almost not impacted for TOPEX/Poseidon when using ITRF2014 instead of ITRF2008. The internal orbit consistency in the radial direction (as derived from arc overlaps) is reduced (improved) by 6.6 %, 2.3 %, and 5.9 % for TOPEX/Poseidon, Jason-1, and Jason-2, respectively. Single-satellite altimetry crossover analyses indicate reduction (improvement) in the absolute mean crossover differences by 0.2 mm (8.1 %) for TOPEX, 0.4 mm (17.7 %) for Jason-1, and 0.6 mm (30.9 %) for Jason-2 with ITRF2014 instead of ITRF2008. The major improvement of the mean values of the rms of crossover differences (0.13 mm; 0.3 %) has been found for Jason-2. Multi-mission crossover analysis shows slight improvements in the standard deviations of radial errors: 0.1 %, 0.2 %, and 1.6 % for TOPEX, Jason-1, and Jason-2, respectively. The standard deviations of geographically correlated mean SSH errors improved by 1.1 % for Jason-1 and 5.4 % for Jason-2 and degraded by 1.3 % for TOPEX. The change from ITRF2008 to ITRF2014 orbits only has minor effects on the estimation of regional and global sea level trends over the 22-year time series from 1993 to 2015. However, on interannual timescales (3–8 years) large-scale coherent trend patterns are observed that seem to be connected to drifts between the origins of the tracking station networks. This leads to the changes in interannual global mean sea level of up to 0.06 mm yr−1 for TOPEX, 0.05 mm yr−1 for Jason-1, and up to 0.12 mm yr−1 for Jason-2, i.e., up to 4 % of the corresponding sea level signal based on altimetry for timescales of 3 to 8 years. The respective changes in the regional sea level trend on these timescales are up to 0.4 mm yr−1 in the time span from April 1993 to July 2008 and up to 1.0 mm yr−1 in the time span from July 2008 to April 2015.

KALREF—A Kalman filter and time series approach to the International Terrestrial Reference Frame realization

AbstractThe current International Terrestrial Reference Frame is based on a piecewise linear site motion model and realized by reference epoch coordinates and velocities for a global set of stations. Although linear motions due to tectonic plates and glacial isostatic adjustment dominate geodetic signals, at today's millimeter precisions, nonlinear motions due to earthquakes, volcanic activities, ice mass losses, sea level rise, hydrological changes, and other processes become significant. Monitoring these (sometimes rapid) changes desires consistent and precise realization of the terrestrial reference frame (TRF) quasi‐instantaneously. Here, we use a Kalman filter and smoother approach to combine time series from four space geodetic techniques to realize an experimental TRF through weekly time series of geocentric coordinates. In addition to secular, periodic, and stochastic components for station coordinates, the Kalman filter state variables also include daily Earth orientation parameters and transformation parameters from input data frames to the combined TRF. Local tie measurements among colocated stations are used at their known or nominal epochs of observation, with comotion constraints applied to almost all colocated stations. The filter/smoother approach unifies different geodetic time series in a single geocentric frame. Fragmented and multitechnique tracking records at colocation sites are bridged together to form longer and coherent motion time series. While the time series approach to TRF reflects the reality of a changing Earth more closely than the linear approximation model, the filter/smoother is computationally powerful and flexible to facilitate incorporation of other data types and more advanced characterization of stochastic behavior of geodetic time series.

Topographically induced height errors in predicted atmospheric loading effects

Atmospheric pressure variations are known to induce vertical displacements of the Earth's surface with magnitudes large enough to be detected by geodetic observations. Estimates of these loading effects are derived using global reanalysis fields of surface pressure as input. The input surface pressure has a minimum spatial sampling, which does not capture true surface pressure variations due to high topographic variability in some regions. In this paper, we investigate the effect that unmodeled topographic variability has on surface pressure estimates and subsequent estimates of vertical surface displacements. We find that the estimated height changes from the topographic surface pressure can be significant (2–4 mm) for sites in regions of high topographic variability. When we compare the estimated height changes to Global Positioning System residuals from the 2005 International Terrestrial Reference Frame Realization, we find that the heights derived from the topographic surface pressure, versus those from the normal surface pressure, perform better at reducing the scatter on the height coordinate time series.

Tropospheric delay gradients from numerical weather prediction models: effects on GPS estimated parameters

Several numerical weather prediction (NWP) models provide information on the 3D state of the neutral atmosphere which has enabled GNSS researchers to have improved a priori information of the delay induced in the GNSS signals. However, the quality of weather models on the one hand and computational difficulties on the other, are motivations to develop an algorithm based partly on NWP models, while still estimating the remaining residual delay through GNSS processing strategies. An algorithm has been developed to estimate horizontal delay gradients from Meteorological Service of Canada NWP models. The GNSS software “Bernese” has also been modified to handle these gradients, as well as zenith delay and mapping functions based on NWP models in phase and code observation equations. Month-long precise point positioning results show strong correlation between north–south hydrostatic gradients and latitude differences, with significant but less strong correlation with the height and zenith total delay parameters. The longitude components were not sensitive to the implementation of gradients. High precision GNSS applications such as long term geodynamics studies, realization of terrestrial reference frames and climatology and consequential interpretations may be affected by ignoring the asymmetry of the neutral atmosphere. In addition to estimating the gradients, implementing a priori information on gradients in the processing software may have an impact on estimated results and consequential interpretations.

Terrestrial reference frame effects on global sea level rise determination from TOPEX/Poseidon altimetric data

Abstract Over the past years, satellite altimetry has produced several significant improvements in our scientific understanding of the oceans. However, several results related to global or regional sea level changes still too often rely on the assumption that orbit errors coming from station coordinates adoption can be neglected in the total error budget. The goal of this paper is to specifically study this general assumption and to assess its limitation. In a first step, in the case of the TOPEX/Poseidon satellite, we first characterized orbital errors coming from the adoption of a specific Terrestrial Reference Frame using a Monte-Carlo-based simulation method using DORIS data. In a second step, we analyzed the effect of these systematic orbital errors on the mean sea level derived from altimeter data. From these results, we derived linear transfer functions that can be used for several purposes like error budget estimation in altimetry or local tie specifications for the implementation of new tracking stations. These simulations show that the main source of errors comes from current imprecision in the Z -axis realization of the frame. A 10 mm error in the TZ realization can create a −1.2 mm of systematic errors in the derived mean sea level due to the North–South asymmetric distribution of the oceans all over the world. Significant sea level rise could erroneously be attributed to a possible warming of the biosphere while they just come from systematic errors in the Terrestrial Reference Frame used to generate the satellite operational orbits. Finally, we assessed the accuracy of present Terrestrial Reference Frame realizations and derived a realistic error budget for this specific source of error. For the ITRF97 realization, a current precision of 3.0 mm in sea level and 0.37 mm/yr in sea level rise was obtained. These precisions should gradually improve with future Terrestrial Reference Frame realizations.

Parameter sensitivity of TOPIX orbit and derived mean sea level to DORIS stations coordinates

When determining precise orbits from altimetric satellites, the choice of the terrestrial reference frame is a key issue but still an open scientific topic. The terrestrial reference frame realization choice in the precise orbit determination processing will usually lead to systematic effects coming from the adoption of translations, scale factor and rotations but also others effects more difficult to assess, coming from a possible erroneous choice of a station coordinate. The goal of this paper is to try to characterize the effect of an erroneous individual station coordinate in vertical and horizontal components on the satellite orbit determination (characterization of the error, order of magnitude, consequences assessment of the on derived oceanographic products). Simulations have been realized at Institut Géographique National using the GIPSY/OASIS software from actual TOPEX DORIS data. These studies show that the stations whose latitude are close to the orbit inclination have the largest effect on the orbit error. These simulations will also help us to better estimate the required accuracy of a new tracking station. Additionally the study has been extended to try to predict the future uncertainty in the mean sea level determination in the 2000–2010 period derived for the present accuracy of the ITRF97 coordinates and velocities. Present uncertainties in the ITRF97 realization will create systematic errors in the TOPEX derived mean sea level at the 1 mm level in 2010.