IAG SPECIAL STUDY GROUP 2.193

GRAVITY FIELD MISSIONS: CALIBRATION AND VALIDATION

(IAG SECTION II)

Report 2000

http://www.deos.tudelft.nl/~pieter/IAG.SSG/index.shtml

 P.N.A.M. Visser¹ (chair) and C. Jekeli² (co-chair)

¹ Delft Institute for Earth-Oriented Space Research, Kluyverweg 1, 2629 HS, Delft, The Netherlands, Email: Pieter.Visser@lr.tudelft.nl

² Civil & Environmental Engineering and Geodetic Science, 470 Hitchcock Hall, 2070 Neil Avenue, Columbus, OH 43210, Email: jekeli@osu.edu

Abstract

This report describes the activities and the achievements of the IAG Special Study Group (SSG) 2.193 "Gravity field missions: calibration and validation" in its founding year 2000. The objective of the study group is to define, evaluate and characterize possibilities and methods for calibration and validation (Cal/Val) of observations taken by gravity field missions and the derived gravity field model products. These activities have become very relevant with the launch of the German CHAMP satellite in July 2000 and the upcoming launch of GRACE in the fall of 2001, and GOCE in 2005. These missions make use of new generations of GPS receivers and new technologies like ultra-sensitive accelerometers, a gradiometer and drag-free control system. Many questions in the field of calibrating the several types of observations are open and need to be addressed. In addition, new concepts for validation derived gravity field products have to considered, designed and developed, and old concepts reviewed due to the unprecedented demands in accuracy and both spatial and temporal resolution. In 2000, several studies have been carried out to assess and review many Cal/Val possibilities. In addition, several institutes and organizations are preparing for ground and airborne campaigns to support Cal/Val activities. Finally, a joint effort with the Special Commission "SC7: Gravity Field Determination by Satellites" has resulted in a first set of simulated observations made available to the scientific community. This data set can be used for testing purposes.

1. Introduction and background

The recent launch of the German CHAMP satellite in July 2000 can be seen as the starting point of what can be defined as the "Decade of Geopotential Research", with two more missions to come: the U.S./German/French GRACE in 2001 and the European Space Agency (ESA) GOCE in 2005. These satellites make use of advanced technologies that will enable global gravity field mapping with unprecedented accuracy and resolution in space and time.

 CHAMP is currently providing observations that will enable the production of the first consistent long-wavelength mean gravity field model employing a geodetic-quality GPS receiver for high-low Satellite-to-Satellite Tracking (SST) in combination with an ultra-sensitive accelerometer measuring non-conservative forces. GRACE aims at monitoring long- to medium-wavelength gravity field variations, but will also enable the mapping of the mean global gravity field with a resolution significantly surpassing existing models. To this aim GRACE will consist of two low-flyers enabling high-accuracy low-low SST tracking in combination with GPS high-low SST and accelerometers, that are even more sensitive than the one on board of CHAMP. Finally, GOCE will be the first satellite equipped with a space-borne gravity gradiometer (SGG) together with a high-quality GPS/GLONASS receiver and a drag-free control system. GOCE aims at recovering the global mean gravity field with unprecedented resolution.

The observations that will be produced by the satellites need to be properly calibrated and reduced to gravity field model products that need to be validated. The calibration entails the conversion of the raw instrument measurements into engineering units within known limits of accuracy and precision, for example cm and mm/s for the SST measurements, m/s2 for the accelerometer and Eötvös units (E or 10-9 m/s2/m) for the differential accelerometer measurements by the gradiometer. The validation concerns the conversion of these engineering quantities into geophysical units with sufficient accuracy, for example cm for geoid undulations, mgal for gravity anomalies and E for the gravity gradients.

The satellites will provide new types of observations never used before in gravity field modeling and the spectral (error) characteristics of these are not well understood. Several calibration methods need to be reviewed, assessed, enhanced and/or designed in order to retrieve these characteristics and guarantee a proper calibration. Due to the required accuracy and resolution, the same can be said about validation methods for the gravity field products.

 A number of different categories of possibilities for Cal/Val are considered and subject of the activities of this SSG :

1.                          pre-flight calibration of the science instruments;

2.                          on-the-fly calibration and validation;

3.                          use of ground truth data;

4.                          comparison with existing state-of-the-art gravity field models;

5.                          intercomparison between gravity field products from different missions, but also based on different instruments                                  within one mission.

2. SSG 2.193 - members

SSG 2.193 has 20 regular members, including the chair and co-chair and 18 corresponding members. The SGG started its activities in January 2000. The names of the members of the SGG and countries are given in the following list:

Chair/co-chair:

Pieter Visser (The Netherlands)/Chris Jekeli (USA)

Members:

Miguel Aguirre (The Netherlands), Dimitri Arabelos (Greece), Srinivas Bettadpur (USA),

Richard Coleman (Australia), Rene Forsberg (Denmark), Cheng Huang (China),

Cheinway Hwang (Taiwan), Karl-Heinz Ilk (Germany), Steve Kenyon (USA),

Gerard Kruizinga (USA), Jürgen Müller (Germany), Felix Perosanz (France),

Tadahiro Sato (Japan), Martijn Smit (The Netherlands), Dru Smith (USA),

Hans Suenkel (TU-Graz/Austria), Peter Schwintzer (Germany), John Wahr (USA)

Corresponding members:

Alberto Anselmi (Italy), Georges Balmino (France), Stefano Cesare (Italy),

Reinhard Dietrich (Germany), Yoichi Fukuda (Japan), Johnny Johannessen (Norway),

Helmut Oberndorfer (Germany), Christian LeProvost (France), John Manning (Australia),

Reiner Rummel (Germany), Jens Schroeter (Germany), Avri Selig (The Netherlands),

C.K. Shum (USA), Christian Tscherning (Denmark), Pierre Touboul (France),

Phil Woodworth (Great-Britain), Changyin Zhao (USA), Yaozhong Zhu (China)

Most of the members have been involved very actively in the (partly) final preparation stages for CHAMP and GRACE and preparatory stages of GOCE leading to activities relevant but not under coordination of this SSG. Many of the members met informally during several international conferences and symposia in 2000, e.g. the EGS, AGU and IAG GGG2000 meetings. Many of them also contributed to national and international studies and research programs that include preparations for Cal/Val activities, and also acted as consultants to instrument manufacturers and industry in general.

Due to changes in positions and obligations, it may become necessary to revise the membership lists of this SSG.

3. Specific accomplishments

Some of the members formed part of a study team that investigated and assessed a number of Cal/Val methods and possibilities summarized in [Albertella et al., 2000; Visser et al., 2000]. Moreover, several preparatory activities took place by members in preparation of gravity field modeling from future satellite missions that might be beneficial to Cal/Val [Kusche et al., 2000; Oberndorfer et al., 2000; SID, 2000]. These activities included closed-loop simulations and full-scale modeling of satellite (sub)systems required to understand instrument and satellite behavior, helping in establishing realistic error models and procedures for Cal/Val. Also a study was conducted to assess gravity field modeling possibilities from arrays of GPS-carrying low-flying satellites [Hwang, 2000].

The launch of CHAMP and the final preparatory phase for GRACE triggered many efforts and activities related to observation data processing and quality control in general and Cal/Val of both observations and gravity field model products in general. Cal/Val possibilities and capabilities were demonstrated based on both supporting satellite observations, e.g. SLR observations in case of CHAMP, and ground support data [AGU, 2000].

Several members participated in both national and international cooperations for setting up ground and airborne (gravimetry) campaigns to support Cal/Val of the future gravity missions GRACE and GOCE. Certain members also cooperate with members of the related SSG 2.162 "Precise Orbits Using Multiple Space Techniques", chaired by Remko Scharroo, and the Working Group " Preparation of Standard Procedures for Global Gravity Field Validation", chaired by Th. Gruber, partly in the framework of Science Working Teams (e.g. JASON-1) and Precise Orbit Determination teams (e.g. ENVISAT). Finally, this SSG contributed to the generation of a data set of simulated CHAMP, GRACE and GOCE observations that was made available to the scientific community. This activity was coordinated by Special Commission SC7, chaired by Karl-Heinz Ilk.

4. Conclusions and outlook

Many activities have been undertaken by colleagues, members of this SSG in particular, related to Cal/Val of gravity field mission observations and gravity field model products. Most of the efforts were carried out in the framework of ongoing programs related to specific satellite missions, namely CHAMP, GRACE and GOCE, and not specifically in the framework of this SSG. It is foreseen that these activities will be extended and intensified in the years to come. It is the aim of this SSG to serve in the future as a means for exchanging and distributing information relevant to these efforts, and trigger cooperation between different colleagues at both national and international level. In light of this, it is intended to promote and continue cooperation with SC7, SSG 2.162 and the above mentioned Working Group.

5. References

AGU (2000), 2000 Fall Meeting, American Geophysical Union, San Fransisco, December 15-19, 2000, Published as a Supplement to EOS, Transactions, AGU, Vol. 81. No. 48, pp. F307-F311, November 28

Albertella, A., F. Migliaccio, F. Sanso, and C.C. Tscherning (2000), From Eötvös to mGal, WP4: Scientific Data Production Quality Assessment using Local Space-wise Preprocessing, Contribution to Final Report, ESA/ESTEC Contract No. 13392/98/NL/GD, 313-329, SID/DEOS, April 2000.

Hwang, C. (2000) Gravity recovery using COSMIC GPS data: application of orbital perturbation theory, Journal of Geodesy, in press

Kusche, J., K.H. Ilk, and S. Rudolph (2000) Impact of terrestrial data on future satellite gravity field solutions, Towards an Integrated Global Geodetic Observing System (IGGOS), R. Rummel, H. Drewes, W. Bosch, and H. Hornik (Eds.), IAG Symposium 120, pp. 189-192, Springer.

Oberndorfer, H., J. Müller, R. Dorobantu, C. Gerlach, R. Rummel, N. Sneeuw, R. Koop, P. Visser, P. Hoyng, A. Selig, and M. Smit (2000) Simulation of the GOCE Gravity Field Mission, Towards an Integrated Global Geodetic Observing System (IGGOS), R. Rummel, H. Drewes, W. Bosch, and H. Hornik (Eds.), IAG Symposium 120, pp. 201-204, Springer.

SID (2000), GOCE End-to-end Closed Loop Simulation, Final Report, ESTEC Contract No. 12735/98/NL/GD, SRON/IAPG/DEOS, January 2000.

Visser, P.N.A.M., R. Koop, and R. Klees (2000), From Eötvös to mGal, WP4: Scientific data production quality assessment, Contribution to Final Report, ESA/ESTEC Contract No. 13392/98/NL/GD, 157-176, SID/DEOS, April 2000.  

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