This paper introduces the first ESA Core Earth Explorer mission, GOCE. in the context of ESA’s Living Planet programme. GOCE will measure highly accurate, high spatial resolution differential accelerations in three dimensions along a well characterised orbit: the mission is planned for launch in early 2006. The mission objectives are to obtain gravity gradient data such that new global and regional models of the static Earth’s gravity field and of the geoid can be deduced at length scales down to 100 km. These products will have broad application in the fields of geodesy, oceanography, solid-earth physics and glaciology.

GOCE: ESA’s First Earth Explorer Core Mission

Gravity field and steady-state ocean circulation mission

The first ESA (European Space Agency) Earth explorer core mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) was launched on 17 March 2009 into a sun-synchronous dusk–dawn orbit with an exceptionally low initial altitude of about 280 km The onboard 12-channel dual-frequency GPS (Global Positioning System) receiver delivers 1 Hz data, which provides the basis for precise orbit determination (POD) for such a very low orbiting satellite As part of the European GOCE Gravity Consortium the Astronomical Institute of the University of Bern and the Department of Earth Observation and Space Systems are responsible for the orbit determination of the GOCE satellite within the GOCE High-level Processing Facility Both quick-look (rapid) and very precise orbit solutions are produced with typical latencies of 1 day and 2 weeks, respectively This article summarizes the special characteristics of the GOCE GPS data, presents POD results for about 2 months of data, and shows that both latency and accuracy requirements are met Satellite Laser Ranging validation shows that an accuracy of 4 and 7 cm is achieved for the reduced-dynamic and kinematic Rapid Science Orbit solutions, respectively The validation of the reduced-dynamic and kinematic Precise Science Orbit solutions is at a level of about 2 cm

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GPS-derived orbits for the GOCE satellite

The Earth’s non-spherical mass distribution and atmospheric drag cause the strongest perturbations on very low-Earth orbiting satellites (LEOs). Models of gravitational and non-gravitational accelerations are utilized in dynamic precise orbit determination (POD) with GPS data, but it is also possible to derive LEO positions based on GPS precise point positioning without dynamical information. We use the reduced-dynamic technique for LEO POD, which combines the geometric strength of the GPS observations with the force models, and investigate the performance of different pseudo-stochastic orbit parametrizations, such as instantaneous velocity changes (pulses), piecewise constant accelerations, and continuous piecewise linear accelerations. The estimation of such empirical orbit parameters in a standard least-squares adjustment process of GPS observations, together with other relevant parameters, strives for the highest precision in the computation of LEO trajectories. We used the procedures for the CHAMP satellite and found that the orbits may be validated by means of independent SLR measurements at the level of 3.2 cm RMS. Validations with independent accelerometer data revealed correlations at the level of 95% in the along-track direction. As expected, the empirical parameters compensate to a certain extent for deficiencies in the dynamic models. We analyzed the capability of pseudo-stochastic parameters for deriving information about the mismodeled part of the force field and found evidence that the resulting orbits may be used to recover force field parameters, if the number of pseudo-stochastic parameters is large enough. Results based on simulations showed a significantly better performance of acceleration-based orbits for gravity field recovery than for pulse-based orbits, with a quality comparable to a direct estimation if unconstrained accelerations are set up every 30 s.

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Pseudo-Stochastic Orbit Modeling Techniques for Low-Earth Orbiters

Current knowledge of the Earth's gravity field and its geoid, as derived from various observing techniques and sources, is incomplete Within a reasonable time, substantial improvement will come by exploiting new approaches based on spaceborne gravity observation Among these, the European Space Agency (ESA) Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission concept has been conceived and designed taking into account multi-disciplinary research objectives in solid Earth physics, oceanography and geodesy Based on the unique capability of a gravity gradiometer combined with satellite-to-satellite high-low tracking techniques, an accurate and detailed global model of the Earth's gravity field and its corresponding geoid will be recovered The importance of this is demonstrated by a series of realistic simulation experiments In particular, the quantitative impact of the new and accurate gravity field and geoid is examined in studies of tectonic composition and motion, Glaciological Isostatic Adjustment, ocean mesoscale variability, water mass transport, and unification of height systems Improved knowledge in each of these fields will also ensure the accumulation of new understanding of past and present sea-level changes

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The European Gravity Field and Steady-State Ocean Circulation Explorer Satellite Mission: Its Impact on Geophysics

[1] In this paper we present results of a global gravity field recovery using half a year of CHAMP data. We use the energy integral of the motion of a satellite to transform satellite velocities into values of gravitational potential. The feasibility of this approach has already been demonstrated by several groups, using CHAMP reduced-dynamic orbits. We show, that the potential recovered from this kind of orbits depends on the a priori gravity field used for orbit determination. Thus, it cannot be excluded that errors present in the prior field propagate into the new CHAMP gravity model. It is the intention of this paper to avoid this dependency through the use of kinematic orbits, which are free from prior information. The derived potential model, TUM-1S, is validated by comparison to ground data and by satellite orbit residuals. It is shown to be comparable in quality to other state-of-the-art gravity field models.

A CHAMP-only gravity field model from kinematic orbits using the energy integral

During the past years the European Space Agency (ESA) has worked on the definition of a comprehensive Earth observation satellite programme for the era after ENVISAT. This “ESA Living Planet” (1998) programme has recently been published. It distinguishes three types of missions: the research oriented medium scale explorer missions, the smaller opportunity missions, and the monitoring type Earth watch missions. In preparation of a pre-selection of the first explorer missions a meeting of the European Earth science community had been organized at Granada/Spain in May 1996. The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) was one of nine candidates that were presented there. Following a recommendation of the Earth Science Advisory Committee of ESA four of the nine were selected for a phase-A study. They were: the gravity field and steady-state ocean circulation mission, an atmospheric dynamics mission, an Earth radiation experiment, and a land surface processes and interaction mission.

Satellite Gravity Gradiometry with GOCE

GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) is one of the four selected ESA Earth Explorer Missions. The main objective of GOCE is the determination of the Earth’s gravity field with high spatial resolution and with high homogeneous accuracy. For this purpose, two observation concepts will be realised. Satellite-to-Satellite Tracking (SST) in high-low mode will be used for the orbit determination and for the retrieval of the long-wavelength part of the gravity field. Satellite Gravity Gradiometry (SGG) will be employed for the derivation of the medium/short-wavelength parts of the gravity field..

H. Oberndorfer

Papers

A CHAMP-only gravity field model from kinematic orbits using the energy integral

Satellite Gravity Gradiometry with GOCE

Simulation of the GOCE gravity field mission

Simulation of the GOCE gravity field mission

GOCE closed-loop simulation