IC SG6

JSG 0.6: Applicability of current GRACE solution strategies to the next generation of inter-satellite range observations

Chairs: M. Weigelt (Germany), A. Jäggi (Switzerland)
Affiliation: Comm. 2

Problem statement

The GRACE-mission (Tapley et al., 2004b) proved to be one of the most important satellite missions in recent times as it enabled the recovery of the static gravity field with unprecedented accuracy and, for the first time, the determination of temporal variations on a monthly (and shorter) basis. The key instrument is the K-band ranging system which continuously measures the changes of the distance between the two GRACE satellites with an accuracy of a few micrometer. Thanks to the success of this mission, proposals have been made for the development of a GRACE-follow-on mission and a next-generation GRACE satellite system, respectively. Apart from options for a multi-satellite mission, the major improvement will be the replacement of the microwave based K-band ranging system by laser interferometry (Bender et al., 2003). The expected improvement in the accuracy is in the range of a factor 10 to 1000. Two types of solution strategies exist for the determination of gravity field quantities from kinematic observations (range, range-rate and range-acceleration). The first type is based on numerical integration. The most common ones are the classical integration of the variational equations (Reigber, 1989; Tapley et al., 2004a), the Celestial Mechanics Approach (Beutler et al., 2010) or the short-arc method (Mayer-Gürr, 2006). The second type of solution strategies tries to make use of in-situ (pseudo)-observa-tions. The most typical ones are the energy balance approach (Jekeli, 1998; Han, 2003), the relative accelera-tion approach (Liu, 2008) or the line-of-sight gradiometry approach (Keller and Sharifi, 2005). From a theoretical point of view all approaches are in one way or the other based on Newton's equation of motion and thus all of them should be applicable to the next generation of satellite missions as well. Practically, problems arise due to the necessity of approximations and linearizations, the accumulation of errors, the combination of highly-precise with less precise quantities, e.g. K-band with GPS, and the incorporation of auxiliary measure-ments, e.g. accelerometer data. These problems are often circumvented by introducing reference orbits, reducing the solution strategies to residual quantities, and by frequently solving for initial conditions and/or additional empirical or stochastic parameter. In the context of the next generation of low-low satellite-to-satellite tracking systems, the question is whether these methods are still sufficient to fully exploit the potential of the improved range observations.

Objectives

Observations are related to gravity field quantities by means of geometry, kinematics and dynamics. The gravity field is then represented by global or local base functions. The focus of this study group is primarily on the use of spherical harmonics as base function with different approaches to relate the observations to the gravity field. However, since local methods also proofed to yield high-quality solutions, this group will be affiliated with the pro-posed study group on the "Methodology of Regional Gravity Field Modelling" by M. Schmidt and Ch. Gerlach in order to investigate the interplay with regional model-ling. The usage of other global base functions is also wel-come. The objectives of the study group are therefore to:

• investigate each solution strategy, identify approxima-tions and linearizations and test them for their permissibility to the next generation of inter-satellite range obser-vations,
• identify limitations or the necessity for additional and/or more accurate measurements,
• quantify the sensitivity to error sources, e.g. in tidal or non-gravitational force modelling,
• investigate the interaction with global and local modelling,
• extend the applicability to planetary satellite mission, e.g. GRAIL,
• establish a platform for the discussion and in-depth understanding of each approach and provide documentation.

It will not be the objective of this study group to identify the “best” approach as from a theoretical point of view all approaches are able to yield a solution as long as the neces-sary observations with sufficient accuracy have been made and approximations and linearization errors remain below the proposed accuracy of the new range observation. Fur-ther, solutions need validation which is done best with different and independent solution strategies in order to identify possible systematic effects.

Methodology and Output

The investigation will be based on an in-depth analysis of the theoretical foundations of each approach in combina-tion with a simulation study with step-wise increasing realism. The preparation of the simulated data set and each approach will be assigned separate work packages with subtasks, which include the above mentioned objectives. Each member is supposed to assign himself to at least one work package and contribute by adding to the discussion of the principles of each approach, supplying simulated data sets, carry out numerical investigations or develop solutions to specific problems. The primary output is the result of the collaborative investigation of the different approaches aiming at the identification of possible challenges and the development of solutions ensuring their applicability to the next generation of inter-satellite range observations. These findings are supposed to be well documented in journal paper, possibly in a special issue of Journal of Geodesy or similar by the end of 2014. A workshop is envisaged in the vicinity of the Hotine-Marussi symposium in 2013.

Members

Matthias Weigelt (Germany), chair
Adrian Jäggi (Switzerland), chair
Markus Antoni (Germany)
Oliver Baur (Austria)
Richard Biancale (France)
Sean Bruinsma (France)
Christoph Dahle (Germany)
Christian Gerlach (Germany)
Thomas Gruber (Germany)
Shin-Chan Han (USA)
Hassan Hashemi Farahani (The Netherlands)
Wolfgang Keller (Germany)
Jean-Michel Lemoine (France)
Anno Löcher (Germany)
Torsten Mayer-Gürr (Austria)
Philip Moore (UK)
Himanshu Save (USA)
Mohammad Sharifi (Iran)
Natthachet Tangdamrongsub (Taiwan)
Pieter Visser (The Netherlands)

Corresponding members

Christian Gruber (Germany)
Majid Naeimi (Germany)
Jean-Claude Raimondo (Germany)
Michael Schmidt (Germany)