COMMISSION VII:

“SATELLITE GRAVITY FIELD MISSIONS”

 

http://www.geod.uni-bonn.de/SC7.html

 

K.H. Ilk (chair), P. Visser (co-chair), J. Kusche (scientific secretary)

 

 

Abstract

 

This final report describes the activities of the Special Commission VII “Satellite Gravity Field Missions” during the past four years since the IAG General Assembly 1999 in Birmingham, UK. There was a tremendous progress in preparing and realizing satellite-borne gravity field missions in the past four years. In July 2000 the CHAMP mission had been launched and after a commissioning phase and an International Announcement of Opportunity the data of various product levels are accessible. The results achieved during the past years proofed the expectations of the scientific community. The first CHAMP progress meeting held in Potsdam in January 2002 documented the progress in gravity and magnetic field and in atmosphere research as well as in the techniques of orbit determination. In March 2002 the two GRACE satellites had been brought in nearly circular orbits. First results confirmed the expectations of the scientific community expected from satellite-borne gravity measurements and its value for unprecedented views of the earth's gravity field. The preparation of ESA’s satellite gradiometry mission GOCE is under way. Together with complementary geophysical data and precise measurements of the surface of the oceans and ice regions, satellite gravity data represent a "new frontier" in studies of the System Earth. Scientific programs to take full advantage of these new information are in preparation. It can be expected that the work of Special Commission VII with a slightly modified task can be more successful in the coming years than in the past. Indeed, the data available in the next future will attract many groups with different analysis concepts and with various ideas to investigate scientific and commercial applications of a very precise high resolution gravity field. This is the reason that SCVII has the chance to support the international exchange of ideas and to draw the greatest possible benefit out of these new innovative data.

 

Introduction and background

 

SC VII is a continuation of a Special Commission that existed already during the four years period before 1999 and had been established occasional the IAG General Assembly in Boulder, USA, in the year 1995. The task at that time was to create a platform that integrates all international activities related to gravity field determination by satellite gravity gradiometry and to prepare the conditions for a future mission. Around the year 1995 the common opinion was that only a dedicated satellite gravity gradiometry mission could provide a gravity field which meets the demands of the community. In the upcoming years since 1995 the situation changed in so far as the cheaper satellite-to-satellite tracking concept gained again more interest. This situation is also reflected in the change of the subject of SCVII. Before 1999 it was dedicated to the “Gravity Field Determination by Satellite Gravity Gradiometry” and in the follow-on period to the more general topics of “Satellite Gravity Field Missions”. The last eight years since 1995 showed a tremendous progress in preparing satellite-borne gravity missions: today we not only have an accepted satellite gravity gradiometry mission, GOCE, additionally, two satellite-to-satellite gravity missions, CHAMP and GRACE are already realized and the scientific data partly accessible to the interested community. The multi-purpose high-low satellite-to-satellite mission, CHAMP, has been launched in July 2000, the low-low satellite-to-satellite mission GRACE followed in March 2002 and the satellite gravity gradiometry mission GOCE, at present in the scientific and industrial preparation phase, will be realized in 2006. CHAMP, GRACE and GOCE have the potential to revolutionize the knowledge of the system earth. Not only the static part of the gravity field can be determined with unattained accuracy also an eventual time dependency can be derived. The coming years, sometimes defined as the “Decade of Geopotential Research” will represent an enormous challenge for the geo-scientific community. During the present decade additional satellite altimetry missions are already launched or will be realized. The satellite altimetry missions Jason-1 and ENVISAT are already in orbit since 2001 and 2002 respectively, and the cryosphere satellites ICEsat (in orbit since 2003) and CryoSat (envisaged for 2004) will supply the gravity field with important data of the geometry of the oceans and the ice regions. The combination of these data will open new vistas for the geo sciences.

 

Objectives

 

The Special Commission VII was envisaged to act as a platform of discussion and information exchange related to the various missions mentioned above. Three main problem areas had been defined; each of them consisted of several sub topics. It is indicated whether specific problems are treated within a Special Study Group.

 

 

(i)                   on the flight validation and calibration of satellite data of various mission types – connections to SSG 2.193 “Cal/val of new gravity mission instruments” (P. Visser, C. Jekeli) and the Working Group “Preparation of Standard Procedures for Global Gravity Field Validation” (Th. Gruber),

(ii)                 integrated sensor analysis - connections to SSG 2.162: Precise orbits using multiple space techniques,

(iii)                new sensors (laser interferometers, alternative gradiometers and accelerometers),

 

 

(i)                   comparison of analysis techniques (global and regional),

(ii)                 gravity field modeling aspects with view to the time dependency of the gravity field – connections to SSG 4.187: Wavelets in geodesy and geodynamics (W. Keller),

(iii)                combination of satellite data and (inhomogeneous) terrestrial data – connections to SSG 3.185: Merging data from dedicated satellite missions with other gravimetric data (N. Sneeuw),

(iv)                calibration of satellite derived data – connections to SSG 2.193 “Cal/val of new gravity mision instruments” (P. Visser, C. Jekeli) and the Working Group “Preparation of Standard Procedures for Global Gravity Field Validation” (Th. Gruber),

(v)                  downward continuation aspects,

 

 

(i)                   oceanographic aspects - connections to SSG 2.194: GPS water level measurements (C. Jekeli),

(ii)                 inversion of the gravity field,

(iii)                structure of atmosphere and ionosphere - connections to SSG 2.192: Spaceborne atmospheric GNS soundings (R. Hanssen),

(iv)                temporal variations of the gravity field and the cryosphere,

(v)                  temporal variations of the gravity field and the hydrosphere.

 

Besides these topics the Special Commission was created also to act as a brain pool for ideas of future developments in gravity field research. This encompasses not only applications to various fields of geo sciences but also developments of future satellite borne techniques to measure the gravity field.

 

Members

 

SC VII has 55 members and corresponding members, respectively, including the chair, co-chair and scientific secretary. The names of the members and corresponding members that expressed their interest in the work of SCVII are given in the following list.

 

Chair/co-chair/scientific secretary: Karl Heinz Ilk, Pieter Visser, Jürgen Kusche

 

Members/corresponding members: Dimitri Arabelos (Greece), Georges Balmino (France), Srinivas Bettadpur (USA), Johannes Bouman (The Netherlands), Ben F. Chao (USA), Jean Dickey (USA), René Forsberg (Denmark), Willi Freeden (Germany), Yoichi Fukuda (Japan), Martin van Gelderen (The Netherlands), Erik Grafarend (Germany), Richard S. Gross (UK), Thomas Gruber (Germany), Roger Haagmans (Norway), Bernhard Heck (Germany), Cheinway Hwang (Taiwan), Chris Jekeli (USA), Steve Kenyon (UK), Wolfgang Keller (Germany), Roland Klees (The Netherlands), Rolf König (Germany), Radboud Koop (The Netherlands), Ulrich Meyer (Germany), Federica Migliaccio (Italy), Jerry X. Mitrovica (USA), Philip Moore (UK), Jürgen Müller (Germany), Steve Nerem (USA), Helmut Oberndorfer (Germany), Erricos Pavlis (USA), Margarita Petrovskaya (Russia), Dan Roman (USA), Reiner Rummel (Germany), Fernando Sansò (Italy), E.J.O. Schrama (The Netherlands), Wolf-Dieter Schuh (Germany), Avri Selig (The Netherlands), Abdel Sellal (Algeria), Peter Schwintzer (Germany), C.K. Shum (USA), Martijn Smit (The Netherlands), Dru Smith (USA), Nico Sneeuw (Germany), Hans Sünkel (Austria), Byron Tapley (USA), Pierre Touboul (France), Christian C. Tscherning (Denmark), Illias Tziavos (Greece), John Wahr (USA), Michael Watkins (USA), Martin Vermeer (Finland), Peiliang Xu (Japan).

 

Specific accomplishments

 

A first step of the activities of SCVII was to establish a web site integrated into the IAG home page. National and international activities related to the gravity field missions hade been distributed to the interested community. Links to the most important addresses related to these missions were given. Concerning the list of objectives of SCVII only a view aspects could be worked off. The idea to establish a forum of discussion could not be realized. There was only a minor feedback from the members. Looking back we can say that this is understandable in so far as the groups involved in the preparation and execution of the gravity satellite missions have to work under competitive conditions and are to a certain amount independent from the input of a broader community. Therefore, it remained the service of SCVII for those who are not directly involved in a mission proposal or a mission project. This is the reason that one initiative of SCVII related to the generation of data sets of simulated CHAMP, GRACE, GOCE and 24 GPS satellite orbits turned out to be quite successful. The generated data sets cover a time period of 30 days and include the velocities, accelerations, and for GOCE the tensor components for specified gravity fields and reference frame specifications. As a first step, the mission scenarios were generated based on very simple models concerning the reference systems and the noise models. In 2001 SRON, the Netherland’s Space Agency, provided various error models for the tensor components of GOCE. Details were given in readme-files and additional information material on mathematical details were included. It took some iteration steps until the material was adapted to the needs of the users. The data sets were planned to be used for investigations related to satellite borne gravity field missions, especially to compare

 

 

In the starting phase the simulation material was available in packed form on two CD-roms. It could be received after demand or downloaded together with additional information material from the SCVII web page. Later a FTP-server was created, where the simulation material could be downloaded. The computation comparisons should be done for global and regional analysis techniques. As regional example an area with a rough gravity field in the South-East-Asian area has been selected (fig, geoid heights in m):

 

 

The response to the offer to provide the simulation scenarios was very encouraging. The CD-roms have been sent to 21 members of various institutions and countries on demand. Because of the fact that the simulation material could be downloaded from the SCVII FTP-server the detailed number of those who downloaded the material is not known. But it seems that many used the data for comparisons and simulation computations. Besides this activity the bibliography of SGG and SST related references had been moderately improved. Unfortunately, there was not much response to report all papers and articles to the SCVII chair to be included in the publication list. This is the reason that the bibliography is not up-to-date by far, especially concerning the huge number of papers published during the last years. Nevertheless, the bibliography gives an overview of references from the early days of SGG and SST activities. In 2001 a SC VII-meeting had been held occasional the IAG General Assembly in Budapest.

 

Conclusions and outlook

 

The realization of the missions CHAMP and GRACE and the fact that the real data are (partly) available made the simulation material of these missions unnecessary, at least for most of the applications. Nowadays, there are many groups around the world involved in the development of software for analyzing satellite born gravity field observations, as satellite gravity gradiometry observations, satellite-to-satellite tracking data, either in the high-low or in the low-low mode. To provide a simple platform for any scientist or for groups of scientists of the international community with the task to check and to improve his/their own developments or to compare the effectiveness of their procedures to the procedures of others it seems still to be useful to use a unique data set. For comparisons the simulated mission scenarios might be useful even nowadays where real data are already available. It would be very useful to compare the various approaches for global and regional gravity recovery procedures, space-wise, time-wise, based on spherical harmonics, wavelets, covariance functions or any other space-localizing gravity field representation. More sophisticated simulation scenarios would be helpful together with a selected set of real data. Another problem closely related to the recovery procedure are the topics "calibration" and "validation", but also data combination with terrestrial data or alternative data sets. These topics should be important points for a task list of a follow-on SC VII.