GRACE Science Team Meeting 2008

A.2 - Making The Case for GRACE-Follow On

(Time Variable Gravity, Low-Earth Orbiters, and Bridging Gaps)
Srinivas Bettadpur

(Science Rationale for GRACE Follow-on: A GIA Perspective)
Erik Ivins

(GRAF - A GRACE Follow-On Mission Feasibility Study)
Frank Flechtner

(Simulation Study of a Follow-On Gravity Mission to GRACE)
Bryant Loomis

(Alternative Mission Architectures for a Gravity Recovery Satellite Mission)
David Wiese

(Higher Accuracy Goals for Future GRACE-Type Missions)
Peter L. Bender

(Alias Reduction in a Dual-Pair GRACE Follow-On)
Ki-Weon Seo

(Accelerometers for the GOCE Mission: performance status)
Bernard Foulon

Title: Time Variable Gravity, Low-Earth Orbiters, and Bridging Gaps
Session: A.2 - Making The Case for GRACE-Follow On
First Author: Srinivas Bettadpur
Presenter: Srinivas Bettadpur
Co-Authors: J. Ries, H. Save

Abstract: The ground-breaking quality and applications of GRACE-derived mass-flux estimates has renewed interest in extending these time-series to the past, and into potential breaks between GRACE-like missions in the future. While it is clear that conventional satellite geodetic techniques cannot match the quality and resolution of GRACE data, we can obtain a much longer time-history if we can obtain at least the lowest resolution features. In this paper, we review the role of terrestrial and GPS-based tracking to a single orbiter in filling data gaps and in extending time series of mass flux from satellite gravity estimates.

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Title: Science Rationale for GRACE Follow-on: A GIA Perspective
Session: A.2 - Making The Case for GRACE-Follow On
First Author: Erik Ivins
Presenter: Erik Ivins

Abstract: A GRACE follow mission has great potential to add data for constraining models of glacial apostasy in both currently glaciated, and non-glaciated regions. The case for a follow-on mission is especially strong in regards to the North American continent. Here the long-wavelength sensitivity of the mission mapping design, repeat orbital mapping intervals, polar orbit, etc., are all quite well suited to solving a number of major questions about the size, aerial extent, and deglaciation history of the Laurentide ice sheet. Some examples include, determination of the origin of the melt-water rate spikes in seal-level rise that occurred between 9.5 and 8.2 kilo-years before present, and the demise of the Quebec/Labrador sector at times just prior to the final outburst flood from Lake Agasiiz-Ojibway. Examples of the sensitivity are computed.

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Title: GRAF - A GRACE Follow-On Mission Feasibility Study
Session: A.2 - Making The Case for GRACE-Follow On
First Author: Frank Flechtner
Presenter: Frank Flechtner
Co-Authors: K.H. Neumayer, B. Doll, J. Munder, Ch. Reigber, J.C. Raimondo (2)

Abstract: After more than 6 years of very successful operation in orbit, the US-German GRACE mission has demonstrated in a very impressive way its outstanding capability to monitor mass motions in the Earth system with unprecedented accuracy and temporal resolution. These results have stimulated many novel research activities in hydrology, oceanography, glaciology, geophysics and geodesy, which also indicate that long term monitoring of such mass motions, possibly with improved spatial and temporal resolution is a must for further understanding of various phenomena.

GRACE had been designed for 5 years lifetime, but due do the robust design and some margin on S/C consumables, GRACE can operate likely until 2012 thus about 10 years. Considering this, GFZ Potsdam has recently launched a short study with STI as industrial partner, holding a wealth of GRACE technical experience, to investigate the feasibility/boundaries of a follow on mission taking into account system, cost, programmatic and schedule aspects.

An additional goal of the study is to work out some improvement in terms of temporal and spatial resolution, based on lessons learned from GRACE and based on further developed state of the art technology. These results will form the basis for further discussions with potential national and international partners in 2009. The presentation will focus on the main targets of the study.

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Title: Simulation Study of a Follow-On Gravity Mission to GRACE
Session: A.2 - Making The Case for GRACE-Follow On
First Author: Bryant Loomis
Presenter: Bryant Loomis
Co-Authors: R. Steven Nerem, Scott Luthcke, Dave Rowlands

Abstract: The expected performance of a follow-on gravity mission to GRACE (GFO) is investigated with a full numerical simulation procedure. Two different GFO configurations are considered in the study. First, the GFO is assumed to be equipped with an interferometric laser ranging system, a drag-free system, and a reduced orbital altitude. The second configuration is equipped with the laser ranging system only and is flown in the same orbit as GRACE. GRACE employs a microwave satellite-to-satellite ranging device with an accuracy of better than 1 micron/s while it is expect that a laser ranging device would measure range-rate to an accuracy of ~1 nm/s or better. The inclusion of a drag-free system reduces the errors associated with the GRACE on-board accelerometers and a reduction in the orbital altitude increases the sensitivity of the orbiting satellites to the smaller spatial features in the Earth's gravitational field. It is expected that these design changes for a gravity recovery mission similar to GRACE should improve the spatial resolution to which the Earth's gravity field can be recovered, thus benefiting many areas of Earth systems research. A method for local time variable gravity recovery through mass concentration blocks (mascons) is used. All major sources of error are considered and include the satellite-to-satellite ranging instrument noise, satellite positioning error, temporal aliasing, and errors introduced by imperfections in the tidal, oceanographic, and atmospheric models.

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Title: Alternative Mission Architectures for a Gravity Recovery Satellite Mission
Session: A.2 - Making The Case for GRACE-Follow On
First Author: David Wiese
Presenter: David Wiese
Co-Authors: W. M. Folkner; R. S. Nerem

Abstract: This paper examines four candidate mission architectures for a future gravity recovery satellite mission to assess their potential in measuring the gravity field more accurately than GRACE. All satellites were assumed to have an improved measurement system, with an inter-satellite laser ranging instrument and a drag-free system for removal of non-gravitational accelerations. Four formations were studied: a two-satellite collinear pair similar to GRACE; a four-satellite architecture with two collinear pairs; a two-satellite cartwheel formation; and a four-satellite cartwheel formation. The ability of each architecture to recover the gravity field was evaluated using numerical simulations performed with JPL's GIPSY-OASIS software package. Thirty days of data were used to estimate gravity fields complete to degree and order 60. Evaluations were done for 250 and 400 km nominal orbit altitudes. The sensitivity of the recovered gravity field to under-sampled effects was assessed using simulated errors in atmospheric/ocean dealiasing (AOD) models. Results showed the gravity field errors associated with the four-satellite cartwheel formation were approximately one order of magnitude lower than the collinear satellite pair when only measurement system errors were included. When short-period AOD model errors were introduced, the gravity field errors for each formation were approximately the same. The cartwheel formations eliminated most of the longitudinal striping seen in the gravity field errors. A covariance analysis showed the error spectrum of the cartwheel formations to be lower and more isotropic than that of the collinear formations. Preliminary results assessing the ability of the so-called “Bender Formation will also be presented. This formation consists of two collinear satellite pairs: one in a five-day repeating polar orbit and the other in a 23-day repeating, 63 degree inclined orbit.

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Title: Higher Accuracy Goals for Future GRACE-Type Missions
Session: A.2 - Making The Case for GRACE-Follow On
First Author: Peter L. Bender
Presenter: Peter L. Bender
Co-Authors: D.N.Wiese; R.S.Nerem; J.M.Wahr; E.J.O.Schrama; P.N.A.M.Visser

Abstract: A number of authors have made use of a long-track analysis for GRACE-type data in different geophysical studies. With this approach, it appears possible to limit the serious effects of non-local gravity variations on the results. By the effects of non-local gravity variations, we mean particularly variations in the satellite-to-satellite range due to gravity variations before the present one-revolution arc of data and at distant locations. Such effects can be limited to quite long wavelength errors in the range along the projection of the orbit onto an equipotential surface near the satellite altitude.

If the measurement accuracy is improved by using laser interferometry in future missions, the shortest wavelengths of interest will benefit most strongly, since they are highly attenuated at satellite altitude. And these short wavelengths should not be disturbed much by the non-local gravitational variations. However, part of the burden will be on the data users for downward continuing the results to the Earth's surface by geophysical modeling of the effects being studied and by appropriate spatial and temporal filtering.

The limitations due to infrequent sampling over a given region of interest will still be present, but they should not be made considerably worse by what has happened at other locations. If atmospheric mass corrections over a given region are available with roughly 1 cm of equivalent water height uncertainty, then considerably improved satellite measurement accuracy probably can be made good use of. However, this needs to be verified by simulations.

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Title: Alias Reduction in a Dual-Pair GRACE Follow-On
Session: A.2 - Making The Case for GRACE-Follow On
First Author: Ki-Weon Seo
Presenter: Ki-Weon Seo
Co-Authors: Clark R. Wilson

Abstract: GRACE observations of time variations in Earth gravity are limited in part by alias errors due to under-sampling of residual ocean tides, and variations in atmospheric and ocean mass distribution that remain after numerical models are used to remove these effects. Although the quality of tide, atmosphere, and ocean models continues to improve over time, alias errors will probably continue to limit future gravity satellite missions. In this study we show that a GRACE follow-on with one additional satellite pair, at about 90 degrees separation in longitude, is highly effective in reducing alias errors. In addition, we examine the similar alias reduction based on range-rate measurements along ICESat orbit, which is an example of low altitude repeated orbit, and its combination with GRACE orbit. Simulations from two pairs of satellite coverage show that an alias error around spherical harmonic (SH) order 15 due to resonance is diminished significantly, and aliased semi-diurnal tides at low SH degree and order are also suppressed. These results indicate that the improved spatial coverage provided by two pairs of satellites should be a major goal of future gravity satellite missions.

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Title: Accelerometers for the GOCE Mission: performance status
Session: A.2 - Making The Case for GRACE-Follow On
First Author: Bernard Foulon
Presenter: Bernard Foulon
Co-Authors: Onera's GOCE Team

Abstract: Developed by ONERA under contract with ThalesAleniaSpace France as Prime Contractor of the Gradiometer, the six accelerometers of the ESA GOCE mission have been submitted to a dedicated test plan in order to guarantee a level of noise acceleration as low as 2.0 10-12 ms-2 Hz-1/2 as required by the mission scientific performance.

The accelerometers are based on a principle similar to the ones flying on board the twin GRACE satellites but with some technological evolution to improve their resolution by 2 orders of magnitude.

Their contribution to the mission is double by providing the Satellite with the linear accelerations as input to the continuous drag compensation system and with the scientific data measurements to be on-ground processed.

The presentation will shortly describe the accelerometer together with the test plan philosophy and provide a summary of the results of the dedicated on-ground tests including free fall tests in the Bremen drop tower.

Such instrument can also contribute to improve the performance of some GRACE-follow-on mission by measuring more accurately the non gravitational forces acting on the satellites as well as sensor for drag compensation system.

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