First harvest of research based on the final GOCE gravity model

This image, based on the final GOCE gravity model, charts current velocities in the Gulf Stream in meters per second. -  TUM IAPG
This image, based on the final GOCE gravity model, charts current velocities in the Gulf Stream in meters per second. – TUM IAPG

Just four months after the final data package from the GOCE satellite mission was delivered, researchers are laying out a rich harvest of scientific results, with the promise of more to come. A mission of the European Space Agency (ESA), the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) provided the most accurate measurements yet of Earth’s gravitational field. The GOCE Gravity Consortium, coordinated by the Technische Universität München (TUM), produced all of the mission’s data products including the fifth and final GOCE gravity model. On this basis, studies in geophysics, geology, ocean circulation, climate change, and civil engineering are sharpening the picture of our dynamic planet – as can be seen in the program of the 5th International GOCE User Workshop, taking place Nov. 25-28 in Paris.

The GOCE satellite made 27,000 orbits between its launch in March 2009 and re-entry in November 2013, measuring tiny variations in the gravitational field that correspond to uneven distributions of mass in Earth’s oceans, continents, and deep interior. Some 800 million observations went into the computation of the final model, which is composed of more than 75,000 parameters representing the global gravitational field with a spatial resolution of around 70 kilometers. The precision of the model improved over time, as each release incorporated more data. Centimeter accuracy has now been achieved for variations of the geoid – a gravity-derived figure of Earth’s surface that serves as a global reference for sea level and heights – in a model based solely on GOCE data.

The fifth and last data release benefited from two special phases of observation. After its first three years of operation, the satellite’s orbit was lowered from 255 to 225 kilometers, increasing the sensitivity of gravity measurements to reveal even more detailed structures of the gravity field. And through most of the satellite’s final plunge through the atmosphere, some instruments continued to report measurements that have sparked intense interest far beyond the “gravity community” – for example, among researchers concerned with aerospace engineering, atmospheric sciences, and space debris.

Moving on: new science, future missions


Through the lens of Earth’s gravitational field, scientists can image our planet in a way that is complementary to approaches that rely on light, magnetism, or seismic waves. They can determine the speed of ocean currents from space, monitor rising sea level and melting ice sheets, uncover hidden features of continental geology, even peer into the convection machine that drives plate tectonics. Topics like these dominate the more than 100 talks scheduled for the 5th GOCE User Workshop, with technical talks on measurements and models playing a smaller role. “I see this as a sign of success, that the emphasis has shifted decisively to the user community,” says Prof. Roland Pail, director of the Institute for Astronomical and Physical Geodesy at TUM.

This shift can be seen as well among the topics covered by TUM researchers, such as estimates of the elastic thickness of the continents from GOCE gravity models, mass trends in Antarctica from global gravity fields, and a scientific roadmap toward worldwide unification of height systems. For his part Pail – who was responsible for delivery of the data products – chose to speak about consolidating science requirements for a next-generation gravity field mission.


TUM has organized a public symposium on “Seeing Earth in the ‘light’ of gravity” for the 2015 Annual Meeting of the American Association for the Advancement of Science in San Jose, California. This session, featuring speakers from Australia, Canada, Denmark, France, Germany and Italy, takes place on Feb. 14, 2015. (See http://meetings.aaas.org/.)

This research was supported in part by the European Space Agency.

Publication:


“EGM_TIM_RL05: An Independent Geoid with Centimeter Accuracy Purely Based on the GOCE Mission,” Jan Martin Brockmann, Norbert Zehentner, Eduard Höck, Roland Pail, Ina Loth, Torsten Mayer-Gürr, and Wolf-Dieter Shuh. Geophysical Research Letters 2014, doi:10.1002/2014GL061904.

ESA makes first GOCE dataset available

The first products based on GOCE satellite data are now available online through ESA’s Earth observation user services tools. ESA launched the satellite in March 2009 on a mission to map Earth’s gravity with unprecedented accuracy and spatial resolution.

The final gravity map and model of the geoid based on GOCE data will provide users with well-defined products that will be instrumental in advancing science and applications in a broad range of disciplines.
However, there are a number of steps that have to be taken in order to turn the raw data into suitable products for users.

Raw data are downlinked from GOCE to the ground stations in Kiruna, northern Sweden, and on Svalbard, Norway. They are immediately forwarded to the Flight Operations Segment at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany, which then links them through to the Payload Data Ground Segment at ESA’s European Space Research Institute (ESRIN) in Frascati, Italy. Here, through a process of calibration and validation, the data undergo an important transformation from telemetry to ‘level-1b’ data products.

Level-1b products are the time series of converted, calibrated and validated measurements taken by GOCE. They consist mainly of the gravity gradients in the instrument reference system and the orbit data (satellite-to-satellite tracking observations, positions and velocity) in an Earth-fixed coordinate system.

In addition, satellite data such as the attitude of the spacecraft and other housekeeping data complete the level-1b data.

These level-1b data, covering the period 1-30 November 2009, are available free of charge to scientific and non-commercial users, and much more will come in the following weeks and months.

“After an intensive calibration and validation phase, we are all very happy to see the science community at large getting involved in the exploitation of the data,” ESA GOCE Mission Manager Rune Floberhagen said.

“GOCE is proving to be a remarkable Earth science mission and we are more than confident that these data will not only change the way we see the gravity field of our planet but also the way the gravity field information is used in many branches of the geosciences.”

Subsequently, these level-1b data will be processed to level-2 through the High-level Processing Facility (HPF). Under ESA’s control, 10 European universities and research facilities that have complementary expertise in gravity and geodesy-related science fields, have joined together and will be operating the HPF throughout GOCE’s lifetime.

The first gravity field model (level-2 data) will be released at ESA’s Living Planet Symposium in Bergen, Norway, from 28 June to 2 July.

“The Living Planet Symposium will in many ways be a coming-out event for GOCE. We will spend two full days on the findings of the mission so far, and we certainly expect the release of our first gravity model to mark the beginning of a long and successful series of gravity field models based on GOCE’s novel measurement techniques,” Floberhagen said.

The final gravity map and model of the geoid will provide users with well-defined data products that will be instrumental in advancing science and applications in a broad range of disciplines, ranging from geodesy, geophysics and surveying to oceanography and sea-level research.