IAG SPECIAL COMMISSION VII
‘SATELLITE GRAVITY FIELD MISSIONS’
(IAG SECTION II)

Mid-term Report

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

K.H. Ilk¹ (chair), P. Visser² (co-chair), J. Kusche³ (scientific secretary)

¹ Institute of Theoretical Geodesy, University Bonn, Nussallee 17, D-53115 Bonn, Germany, Email: ilk@theor.geod.uni-bonn.de

² Delft Institute for Earth-Oriented Space Research, Kluyverweg 1, 2629 HS, Delft, The Netherlands, Email: Pieter.Visser@lr.tudelft.nl

³ Delft Institute for Earth-Oriented Space Research, Thijsseweg 11, 2629 JA, Delft, The Netherlands, Email: j.kusche@citg.tudelft.nl

Abstract

 This mid-term report describes the activities of the Special Commission VII “Satellite Gravity Field Missions” during the past one and a half years since the IAG General Assembly 1999 in Birmingham, UK. The tremendous progress in preparing and realizing satellite-borne gravity field missions in the past years made it necessary to redefine the objectives of SCVII slightly, especially in view of the dedicated tasks of various Special Study and Working Groups established in Birmingham. Satellite-borne gravity measurements can provide unprecedented views of the earth's gravity field and its changes with time. Together with complementary geophysical data, satellite gravity data represent a "new frontier" in studies of the system earth. It can be expected that the work of Special Commission VII 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 data.

1. Introduction and background

 SCVII is a continuation of a Special Commission that existed already the four years period before 1999 and had been established on the occasion of 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 title of SCVII is involved. 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 six years since 1995 showed a tremendous progress in preparing satellite-borne gravity missions: today we not only have an accepted satellite gravity gradiometry mission but, additionally, two satellite-to-satellite gravity missions already realized or immediately before realization. The multi-purpose high-low satellite-to-satellite mission, CHAMP, has been launched in July 2000. The low-low satellite-to-satellite mission, GRACE, is in the final preparation phase and will be launched end of 2001 and the satellite gravity gradiometry mission GOCE, at present in the scientific and industrial preparation phase, will be realized in 2005. 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 unprecedented accuracy, but also an eventual time dependency can be derived. Despite the fact that all three missions have the potential to measure the gravity field by sort of relative measurements between free falling sensors, they are not redundant. Indeed, the characteristics of high-low SST, low-low SST and SGG are rather complementary than competitive. SST is superior in the lower harmonics below degree and order 50 to 60. A mission like GRACE, therefore, is optimal for studying time-varying gravity effects at moderate wavelengths. The static part of the gravity field up to approximately degree 50 can be expected with high accuracy. A condition to detect temporal effects is a corresponding mission duration of several years. Satellite gradiometry is superior for obtaining high spatial resolution from a moderate mission period. Various studies showed that increase of measurement precision or decrease of altitude result in a clear gain of spatial resolution in case of SGG, while this effect is very moderate in case of SST. A SGG mission like GOCE is superior in the short wavelengths parts of the gravity field up to a spherical harmonics degree of 250. The results of a mission like GOCE start to be better than those of a low-low SST mission from degree 60 to 80 on. A high-low SST mission like CHAMP can provide an improvement in the knowledge of the gravity field of approximately one order of magnitude over present models for wavelengths between 400 to 2000km. The coming years, sometimes defined as the “Decade of Geopotential Research” will represent an enormous challenge for the geo-scientific community. This fact is reflected also in the activities of IAG, especially by Section II, which is dedicated to “Advanced Space Technology”, within its scope several Special Study and Working Groups have been established.

2. Objectives

 The Special Commission VII can act as a platform of discussion and information exchange related to these various missions. A discussion board should promote discussions related to various topics. National and international activities related to the gravity field missions are being distributed to the interested community. Links to the most important addresses related to these missions are given in the SCVII web page. There are three main problem areas; each of them consists of several sub topics. It is indicated whether specific problems are treated within a Special Study Group. Cooperation started between some of these groups and SCVII:

·         Analysis of the observation system:

·         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),

·         integrated sensor analysis - connections to SSG 2.162 "Precise orbits using multiple space techniques" (R. Scharroo),

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

·         Modeling and data analysis aspects:

·         comparison of analysis techniques (global and regional),

·         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),

·         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),

·         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),

·         downward continuation aspects,

·         Applications in geo sciences, oceanography, climate change studies and other interdisciplinary research topics in earth sciences:

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

·         inversion of the gravity field,

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

·         temporal variations of the gravity field and the cryosphere,

·         temporal variations of the gravity field and the hydrosphere.

Besides these topics the Special Commission should also 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. Any idea is welcome even when it sounds unrealistic at the present time. Examples are the mission proposals presented for discussion a couple of years ago: COLIBRI (Hummingbird), a multi low-low satellite-to-satellite tracking experiment or TIDES (Tidal Interferometric Detector in Space) which was considered to be based on laser doppler interferometry using ultra-stable lasers. Another example was GEOID, a mission based at that time on the University of Maryland’s Superconducting Gravity Gradiometer. While these ideas were still realistic because they are based on more or less available technology one could think also of completely new proposals as for example the measurement of the earth's gravity gradient with an atom interferometer-based gravity gradiometer, recently proposed by Snadden et al., published e.g. in the Physical Review Letters.

3. Members

SCVII has 56 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), Janusz Zielinski (Poland), Peiliang Xu (Japan).

4. Specific accomplishments

 From the various activities the members of SCVII were involved, we especially want to mention an initiative of SCVII in close cooperation with Pieter Visser and his SSG 2.193, related to the generation of a data set of simulated CHAMP, GRACE, GOCE and 24 GPS satellite orbits. The data set covers a time period of 30 days and includes the velocities, accelerations, and for GOCE the tensor components for specified gravity fields and reference frame specifications. For the beginning, the models are very simplified, e.g. there is no noise on the data. It is intended to provide more specific error models in the upcoming months. The data set should be used for investigations related to satellite borne gravity field missions, especially to compare

·         global and/or regional recovery techniques,

·         spherical harmonics (each parameter and degree variances) and gravity functionals in (geographic) blocks (center point and mean block values),

·         gravity functionals in (geographic) blocks (center point and mean block) values in the region specified in the data sheet.

The simulation material is available in packed form on two CD-roms. It can be received after demand or downloaded together with additional information material from the SCVII web page. 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):

 Besides this acitivity the bibliography of SGG and SST related references has been improved. Up to now the bibliography contains about 370 different references covering the last three decades. But this list is far from being complete. Even key papers may be missing and a lot of work has to be done to complete this bibliography.

Investigations have been performed and are still under way concerning the development of analysis techniques (global and regional) of SST and SGG observations as well as the downward continuation problem (see references in the SST/SGG bibliography within the last two years). This work has been performed within the frame of SCVII, but also outside of it.

 5. Conclusions and outlook

 The response to the offer to provide the simulation scenarios so far are very encouraging. The CD-roms have been sent to 15 members of various institutions and countries on demand and some may have downloaded the data set or part of it. We will extend our simulations to include error models for gravity gradients but also to the other mission scenarios. To come up with realistic error models we will establish a group of scientists that should discuss appropriate models in the coming weeks. Many groups around the world are working hard to develop 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. A large part of the groups that will be responsible for the data processing of these missions are already quite well prepared, but still need to extend and fine tune processing methods and software. In addition, many groups are working on new analysis methods and processing algorithms that are of different maturity level. For these activities, data sets of realistic simulated observations including error models will be of great help. Indeed, there will be only a couple of years and we are confronted with a huge number of data. There are various approaches for global and regional gravity recovery procedures, space-wise, time-wise, etc. some of them are using spherical harmonics, wavelets, covariance functions or any other space-localizing gravity field representations etc.. Another problem closely related to the recovery procedure are the topics "calibration" and "validation", but also data combination with terrestrial data or any already existing data set. 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 to be useful to use a unique data set.

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