4:00pm - 4:15pm
Towards the best GOCE Gravity Gradients
1RHEA for ESA - European Space Agency, The Netherlands; 2ESA - European Space Agency, The Netherlands
The GOCE gravity gradients show perturbations that correlate with the geomagnetic field. In particular the cross-track gravity gradient is significantly perturbed in the regions around the geomagnetic poles. Recently, it was found that the perturbing effect is due to an unmodelled quadratic factor that occurs in the conversion from the electrode control voltages to accelerations, which caused the highly dynamic acceleration signal from cross-track drag to map onto the cross-track gravity gradient. Fortunately, it is possible to model and completely remove the perturbing effect, arriving at a more accurate, “clean” cross-track gravity gradient, which is demonstrated in this presentation.
Also the other gravity gradients show perturbations that correlate with the magnetic field. Those perturbations are much smaller than the perturbation of the cross-track gravity gradient, but appear systematic and are therefore important to remove as well. We will show in this presentation to which extent these perturbation can be removed from the gravity gradients by re-calibrating the accelerometer data and assess the impact on the gravity field retrieval from GOCE data.
4:15pm - 4:30pm
GOCE Southern Polar Gap Now Closed – Results of the ESA Antarctica PolarGap Project
1DTU Space, Denmark; 2British Antarctic Survey; 3Norwegian Polar Institute
The GOCE gravity field mission 2009-13 left polar gaps beyond latitudes of 83.6 degrees due to the orbit inclination. The Antarctic polar gap, essentially void of terrestrial gravity field data, was covered with airborne gravity data in the 2015/16 ESA “Polar Gap” project. With the Arctic gap covered by several airborne gravity campaigns in the last decade, the complete global mapping of the earths gravity field at GOCE resolution (~80 km) has thus now been achieved, a “holy grail” for geodesy for more than a century.
The Polar Gap project used a Twin-Otter aircraft, operating from remote field camps and the Amundsen-Scott South Pole station. The aircraft was equipped with a Lacoste and Romberg gravimeter supplemented with a high-grade iMAR IMU unit, providing reliable airborne gravity measurements at 2 mGal accuracy in spite of the rough conditions. The survey was tied to absolute gravity coastal sites at sub-mGal accuracy to secure bias-free airborne gravity data. The PolarGap data has been augmented with other data, especially sparse NASA IceBridge gravity tracks and AGAP IPY data, for computation of gravity gradients at GOCE altitudes as well. Comparisons of all data confirms the high quality of the new gravity data, to be eventually included in future global gravity field models.
In addition to the gravity sensors, magnetometers, ice penetrating radar, scanning lidar and the ESA 13 GHz ASIRAS radar was also flown, providing supplementary data for both SWARM and CryoSat in the polar gap, as well as general geophysical data for understanding subglacial topography and geology. Major new features detected from the geophysical data includes a deep subglacial valley system between the Pole and the Filchner-Ronne ice shelf region, as well as extended mountain systems under the ice, consistent with observed ice flow patterns from spaceborne radar velocity measurements.
4:30pm - 4:45pm
The Status and Current Contributions of the GRACE Mission
1University of Texas at Austin, United States of America; 2GFZ German Research Centre for Geosciences, Germany; 3NASA Jet Propulsion Laboratories, United States of America
The twin satellites of the Gravity Recovery and Climate Experiment (GRACE) were launched on March 17, 2002 and have operated for nearly 15 years. The mission objectives are to sense the spatial and temporal variations of the Earth’s mass through its effects on the gravity field at the GRACE satellite altitude. The major cause of the time varying mass is water motion and the GRACE mission has provided a continuous decade long measurement sequences which characterizes the seasonal cycle of mass transport between the oceans, land, cryosphere and atmosphere; its inter-annual variability; and the climate driven secular, or long period, mass transport signals. In 2012, a complete reanalysis of the mission data, referred to as the RL05 data release, was initiated. The monthly solutions from this effort were released in mid-2013 with the mean fields following in subsequent years. The mission is entering the final phases of operations with mission end expected to occur before July 1, 2017. The current mission operations strategy emphasizes extending the mission lifetime to minimize the break measurements before the GRACE Follow-On Mission is launched. This presentation will review the mission status and the projections for mission lifetime, describe the issues that influence the operations philosophy, discuss the approaches to bridge the gap between GRACE and GRACE FO and discuss the content of science data products during this transition period.
4:45pm - 5:00pm
GRACE High-Frequency Temporal Gravity Solution on Hydrology Applications
1The Ohio State University, United States of America; 2Institute of Geodesy & Geophysics, Chinese Academy of Sciences, Wuhan, China; 3School of Earth & Ocean Science, Cardiff Univ., Cardiff, UK; 4Institute of Geodesy, Istanbul Technical University, Turkey; 5Dept. of Geomatics, National Cheng Kung Univ., Taiwan; 6University of Bonn, Institute of Geodesy and Geoinformation, Bonn, Germany; 7Institute of Applied Mathematics, AMSS, CAS, China; 8Technische Universität München, Deutsches Geodätisches Forschungsinstitut, Germany; 9Sternberg Astronomical Institute, MSU, Russia; 10Bowling Green State University
It is estimated that by 2025 five billion people worldwide will live in water-stressed countries. Moreover, problems of water balance and availability will likely be aggravated by anthropogenic climate change, population shifts, and land-use changes. Extreme hydrology and weather hazards also significantly affect social well-being and economics. In this study, we will use the improved formulation for the energy balance approach (EBA) to estimate the high-frequency temporal gravity field using data from the GRACE twin satellites, which will be applied to surface and ground water storage estimation over several aquifers and river basins. Specifically, we will: (1) apply the GRACE high-frequency temporal gravity field on groundwater estimation; (2) evaluate the impact of different hydrology models on separating groundwater signal; (3) identify and quantify extreme hydrology and weather hazards using high-frequency GRACE solutions; (4) evaluate different input atmosphere models on the output high-frequency gravity inversion and the impact on the extreme hydrology and weather hazards analysis.
5:00pm - 5:15pm
Swarm as an Observing Platform for Large Surface Mass Transport Processes
1Center for Space Reseach, University of Texas at Austin; 2Astronomical Institute, University of Bern; 3Astronomical Institute, Czech Academy of Sciences; 4GFZ German Research Centre for Geosciences; 5Delft University of Technology; 6Institute of Geodesy, Graz University of Technology; 7ESRIN, ESA
The Swarm satellite mission provides important information regarding the temporal changes of Earth’s gravity field. Several European institutes routinely process Swarm GPS data to produce kinematic orbits, which forms the basis for the estimation of monthly gravity fields. Each institute follows a different gravity field estimation approach and all together they provide complementary advantages. As a result, the combined gravity field model is superior to any individual contribution, improving the accuracy of the measurement of mass transport processes. These models are an integral part of the European Gravity Service for Improved Emergency Management project (http://egsiem.eu), thus providing independent input, in additional to dedicated geodetic data.
We illustrate the agreement of the Swarm models with the much more accurate GRACE solutions, at 1666km wavelength and over most of the Swarm mission lifetime. We additionally highlight large surface mass transport processes represented by the Swarm GPS data.
5:15pm - 5:30pm
HlSST And SLR - Bridging the Gap Between GRACE and GRACE-Follow On
1Leibniz University Hanover, Germany; 2Astronomical Institute, University Bern, Switzerland; 3Institute of Geodesy and Geoinformatics, Wroclaw University of Environmental and Life Sciences, Poland; 4GeoForschungsZentrum Potsdam, Germany
GRACE is undoubtedly one of the most important sources to observe mass transport on global scales. Numerous applications have shown the validity and impact of using its data. Within the EGSIEM project GRACE gravity field solutions from various processing centers are processed and combined to further increase the spatial and temporal resolution. However, it is expected that GRACE will not continue to observe mass variations from space till is successor GRACE-Follow On will be operational. Thus there is a need for an intermediate technique that will bridge the gap between the two missions and will allow 1) for a continued and uninterupted time series of mass observations and 2) to compare, crossvalidate and link the two time series. Here we will focus on the combination of high-low satellite-to-satellite tracking (hlSST) of low-Earth orbiting satellites by GNSS in combination with SLR. SLR is known to provide highest quality time-variable gravity for the very low degrees (2-5). HlSST provides a higher spatial resolution but at a lower precision in the very low degrees. Thus it seems natural to combine these two techniques and their benefit has already been demonstrated in the past. Here we make use of the lessons learned within the EGSIEM project and focus on various aspects of combination such as the optimal strategy and relative weighting schemes. We discuss also the achievable spatial and temporal resolutions of different satellite scenarios, such e.g. using Swarm satellites in combination with Sentinel and/or single GRACE satellites.