Gravity Recovery Using COSMIC GPS Data: Application of Orbital Perturbation Theory

The idea described in this report was initiated during the author's visit to the Ohio State University in the Summer of 1998, hosted by Prof. C.K. Shum.This research was partly supported by the National Science Council of ROC.COSMIC is a joint Taiwan-US mission to study atmosphere using GPS occultation. Its GPS data for precise orbit determination can be used for gravity recovery. In this report a kinematic approach was employed which assumes the positional data can be derived from the GPS data of COSMIC in the operational phase. Using the geometric relationship between the positional variations of orbit and the variations in the six Keplerian elements, improved formulae for the radial, along-track and cross-track perturbations were derived. Based on a comparison with true perturbations from numerical integrations, these formulae are more accurate than the commonly used order-zero formulae. The improved formulae were used to simulate gravity recovery using the COSMIC data. In one simulation with the OSU91A model to degree 50 as the a priori geopotential model, it is demonstrated that the EGM96 model can be improved up to degree 26 using one year of COSMIC data. A significant effort was devoted to the recovery of temporal gravity variation using COSMIC data. Sea level anomaly (SLA) was first generated using the Cycle 196 TOPEX/POSEIDON altimeter data. The steric anomaly due to thermal expansion was created using temperature data at 14 oceanic layers. The steric anomaly-corrected SLA was used to generate harmonic coefficients of temporal gravity variation. With a 3-cm noise at a one-minute sampling interval in the COSMIC data, the gravity variation cannot be perfectly reproduced, but the recovered field clearly shows the gravity signature due to mass movement in an El Niño. With a 0.1-cm noise, the temporal gravity variation up to harmonic degree 10 is almost exactly recovered and this prompts the need of a better processing technique and a sophisticated GPS receiver technology

Decadal Variation in Surface Characteristics over Xinjiang, Western China, from T/P Altimetry Backscatter Coefficients: Evidence of Climate Change

The backscatter coefficient, known as sigma0, is an important measurement of satellite radar altimetry and a key parameter for land altimetry because of its close relationship with the physical properties and geometric features of land coverage under global/regional climate change effects. Using the TOPEX/Poseidon GDR-M dataset from January 1993 to December 2004, we study the spatial and temporal distribution of sigma0 at bands Ku and C over Xinjiang, western China. The results show that the sigma0 is influenced by the water distribution over land and the time evolution of sigma0 has clear seasonal changes. River basins or deserts are classified over the spatial distribution based on different sigma0 values. For example, high sigma0 values are found in the Tarim River Basin and low values are found in the Taklimakan Desert. The periodic components of sigma0 time series are determined using the fast Fourier transformation method. The annual variation is the dominating cycle and the semi-annual variation is the secondary signal. The amplitudes of sigma0 time series at bands Ku and C are also given and most areas have quite low amplitudes except for the Tarim River Basin. Several areas including the Tarim River Basin, Tianshan Mountain and Taklimakan Desert are selected for sigma0 time series spacial analysis to discuss the reasons for variations in sigma0. The main factors are precipitation and vegetation growth, which are affected by the global/regional climate change. The correlation between the brightness temperature, which is related to the water-vapor content in the atmosphere measured by TMR at the 21 GHz channel and sigma0 at two bands, is analyzed

Vertical Displacement due to Ocean Tidal Loading Around Taiwan Based on GPS Observations

Ocean tidal loading (OTL) is an important factor in GPS positioning, especially along the vertical direction. OTL influences the precision of GPS positioning and produces height variations of up to 12 cm. In this study, daily GPS data obtained from 27 GPS tracking stations around Taiwan were collected, and four OTL models were applied to relative static GPS positions derived from these data. The GPS data were obtained from 8 - 14 August 2006 (summer) and 1 - 30 January 2007 (winter). The software Bernese 5.0 was utilized for data processing

A Preliminary Analysis of Lake Level and Water Storage Changes over Lakes Baikal and Balkhash from Satellite Altimetry and Gravimetry

Lakes Baikal and Balkhash are two of the world¡¦s major lakes affecting fresh water supplies in their catchments. Measurements from satellite altimetry (TOPEX/Poseidon, Jason-1 and -2), satellite gravimetry (GRACE) and a hydrological model (LDAS) are used to see the relationship between lake level change (LLC) and water storage change in these two lakes. At Lake Baikal, the average rate of LLC is negative for 1992 - 1998 and positive for 1998 - 2007, and the reversal of the LLC trend concurs with that of the temperature trend during the 1997 - 1998 El Nino. The rate of gravity change ranges from -0.5 to 0.5 ugal yr-1 with a low over the Tian Shan and a high over western Lake Baikal. Due to the climates over the two lakes, the phases of the annual gravity changes differ by up to 100 days. Using the rates of LLC and gravity changes, the ratios between the mass changes of the lake and its catchment over Lakes Baikal and Balkhash are estimated to 0.6 and 0.3, respectively. The result may help to establish water balance models over these two lakes

Preface to the Special Issue on “Geophysical and Climate Change Studies in Tibet, Xinjiang, and Siberia (TibXS) from Satellite Geodesy”

This special issue publishes papers on recent results in geophysical and climate change studies over Tibet, Xinjiang and Siberia (TibXS) based upon some of the key sensors used in satellite geodesy, including satellite gravimetric sensors (GRACE and GOCE), satellite altimeters (TOPEX, Jason-1 and -2, and ENVISAT), and Global Positioning System satellites. Results from ground- and airborne-based geodetic observations, notably those based on airborne gravimeter, superconducting gravimeter (SG) and seismometers are also included in the special issue. In all, 22 papers were submitted for this special issue; 17 papers were accepted

High-resolution gravity and geoid models in Tahiti obtained from new airborne and land gravity observations: data fusion by spectral combination

International audienceFor the first time, we carry out an airborne gravity survey and we collect new land gravity data over the islands of Tahiti and Moorea in French Polynesia located in the South Pacific Ocean. The new land gravity data are registered with GPS-derived coordinates, network-adjusted and outlier-edited, resulting in a mean standard error of 17 μGal. A crossover analysis of the airborne gravity data indicates a mean gravity accuracy of 1.7 mGal. New marine gravity around the two islands is derived from Geosat/GM, ERS-1/GM, Jason-1/GM, and Cryosat-2 altimeter data. A new 1-s digital topography model is constructed and is used to compute the topographic gravitational effects. To use EGM08 over Tahiti and Moorea, the optimal degree of spherical harmonic expansion is 1500. The fusion of the gravity datasets is made by the band-limited least-squares collocation, which best integrates datasets of different accuracies and spatial resolutions. The new high-resolution gravity and geoid grids are constructed on a 9-s grid. Assessments of the grids by measurements of ground gravity and geometric geoidal height result in RMS differences of 0.9 mGal and 0.4 cm, respectively. The geoid model allows 1-cm orthometric height determination by GPS and Lidar and yields a consistent height datum for Tahiti and Moorea. The new Bouguer anomalies show gravity highs and lows in the centers and land-sea zones of the two islands, allowing further studies of the density structure and volcanism in the region

An Analysis of Mechanical Constraints when Using Superconducting Gravimeters for Far-Field Pre-Seismic Anomaly Detection

Pre-seismic gravity anomalies from records obtained at a 1 Hz sampling rate from superconducting gravimeters (SG) around East Asia are analyzed. A comparison of gravity anomalies to the source parameters of associated earthquakes shows that the detection of pre-seismic gravity anomalies is constrained by several mechanical conditions of the seismic fault plane. The constraints of the far-field pre-seismic gravity amplitude perturbation were examined and the critical spatial relationship between the SG station and the epicenter precursory signal for detection was determined. The results show that: (1) the pre-seismic amplitude perturbation of gravity is inversely proportional to distance; (2) the transfer path from the epicenter to the SG station that crosses a tectonic boundary has a relatively low pre-seismic gravity anomaly amplitude; (3) the pre-seismic gravity perturbation amplitude is also affected by the attitude between the location of an SG station and the strike of the ruptured fault plane. The removal of typhoon effects and the selection of SG stations within a certain intersection angle to the strike of the fault plane are essential for obtaining reliable pre-seismic gravity anomaly results

Zonal and Meridional Ocean Currents at TOPEX/Poseidon and JASON-1 Crossovers around Taiwan: Error Analysis and Limitation

A crossover method for determining zonal and meridional ocean current components is examined using data at three crossovers of TOPEX/Poseidon and JASON-1 ground tracks over 2002 - 2006. To implement this method, a geoid model around Taiwan is constructed using surface and airborne gravity data. The modeled and observed geoidal heights at coastal benchmarks are consistent to 5 cm RMS with the means removed. The error and limitation of this method are discussed, concluding that, in order to obtain current velocities at a 10 cm s-1 accuracy and a 6-km resolution, the dynamic ocean topography (DOT) at a mm-level accuracy is needed, which is not possible to achieve today. By filtering DOT to a spatial scale of 100 km or coarser, a 10 cm s-1 accuracy of velocity may be obtained. One crossover (A) is situated south of Taiwan and near the Kuroshio, the second (B) is at the axis of the Kuroshio and the third is located in the northern Taiwan Strait. These three crossovers feature different ocean current patterns. At a spatial scale of 120 km, the agreement among the altimeter, the Princeton Ocean Model (POM), and the drifter-derived velocities is the best at B, followed by that at A, and then C. In fact, at C the altimeter-derived velocities contradict the POM-derived values, and the tide model error is to be blamed. Further improvement on geoid modeling is suggested

Bathymetry Estimation Using the Gravity-Geologic Method: An Investigation of Density Contrast Predicted by the Downward Continuation Method

The downward continuation (DWC) method was used to determine the density contrast between the seawater and the ocean bottom topographic mass to estimate accurate bathymetry using the gravity-geologic method (GGM) in two study areas, which are located south of Greenland (Test Area #1: 40 - 50¢XW and 50 - 60¢XN) and south of Alaska (Test Area #2: 140 - 150¢XW and 45 - 55¢XN). The data used in this study include altimetry-derived gravity anomalies, shipborne depths and gravity anomalies. Density contrasts of 1.47 and 1.30 g cm-3 were estimated by DWC for the two test areas. The considerations of predicted density contrasts can enhance the accuracy of 3 ~ 4 m for GGM