POSSIBLE MISSION ARCHITECTURES FOR A GRACE FOLLOW-ON MISSION INCLUDING A STUDY ON UPGRADED INSTRUMENTATION SUITES, AND MULTIPLE SATELLITE PAIRS IN MODERATELY-INCLINED ORBITS

Loomis, B. D. (1), Wiese, D. N. (1), Nerem, R. S. (1), Bender, P. L. (2), Visser, P.N.A.M. (3)
(1) Colorado Center for Astrodynamics Research, Dept. of Aerospace Engineering Sciences, Univ. of Colorado, Boulder, CO 80309, USA
(2) JILA, Univ. of Colorado and Nat. Inst. of Standards & Technology, Boulder, CO 80309, USA
(3) Delft Institute of Earth Observation and Space Systems (DEOS) Delft University of Technology Kluyverweg 1, 2629 HS, Delft, The Netherlands

The Gravity Recovery and Climate Experiment (GRACE) has been providing monthly estimates of the Earth’s time variable gravity field since its launch in March 2002. The GRACE gravity estimates are used to study temporal mass variations on global and regional scales, which are largely caused by a redistribution of water mass in the Earth system. The accuracy of the GRACE gravity fields are primarily limited by the satellite-to-satellite range-rate measurement noise, accelerometer errors, and temporal aliasing caused by un-modeled high-frequency variations in the gravity signal. Full numerical simulations are performed for a GRACE Follow-On mission (GFO) to determine if a future satellite gravity recovery mission with improved technologies will provide better estimates of time-variable gravity, thus benefiting many area of Earth systems research.

Several GFO configurations are considered for this study. The first case considered is a two-satellite collinear pair similar to GRACE. The best-case two-satellite mission is equipped with an interferometric laser ranging system and a drag-free system in a lower altitude orbit. The laser ranging system improves the satellite-to-satellite range-rate measurement accuracy to ~1 nm/s as compared to ~1 micron/s for GRACE K-band microwave ranging, and the drag-free system more accurately removes the non-conservative forces acting on the satellites than the GRACE on-board accelerometers. Two “hybrid” missions are also considered. One hybrid is the GRACE design where the K-band ranging is replaced by the laser; and the other is a drag-free, low altitude scenario with K-band ranging. A comparison of simulated gravity estimates is an important design tool in selecting the most important technologies to be considered for a future mission. A method for local time variable gravity recovery through mass concentration blocks (mascons) is used to form simulated gravity estimates for Greenland and the Amazon region for three GFO configurations and GRACE.

Simulation results show that only modest improvement is realized for even the best-case two-satellite mission due to temporal aliasing errors. These errors are caused by high-frequency variations in the hydrology signal and imperfections in the atmospheric, oceanographic, and tidal models which are used to remove unwanted signal. Thus, it is recommended that future research efforts be directed towards reducing temporal aliasing errors.

There are three ways in which temporal aliasing errors can be reduced: 1) improving the atmospheric, oceanographic, and tidal models; 2) co-estimating of certain parameters, such as tidal coefficients; and 3) increasing the sampling frequency of the mission. This study primarily focuses on reducing temporal aliasing errors through the latter by studying architectures with multiple satellite pairs. Several architectures are considered, each consisting of two collinear pairs of GRACE satellites: one in a 5-day repeating polar orbit, and the other in a moderate inclined orbit (~65 degrees) with either a 10-day, 15-day, or 23-day repeating groundtrack. The altitude of each collinear pair is around 300 km, and is assumed to fly drag-free and be equipped with a laser interferometer. Global spherical harmonic solutions are made out to degree and order 150 for the respective duration of the repeating groundtrack (10-day, 15-day, or 23-day).

When compared with a single polar pair of collinear satellites in a complementary repeat orbit (10-day, 15-day, or 23-day), the multiple satellite pairs show a substantial reduction in the level of error in recovering a hydrological signal, with the 15-day repeating groundtrack the favored configuration. Recovering ice mass loss from Greenland and Antarctica shows very little improvement due to less dense coverage in the polar regions. Furthermore, the lower inclined pair of satellites leads to a reduction in the longitudinal striping in the gravity solution due to added east-west sensitivity in the observable.

Finally, an alternate processing methodology is explored which estimates daily low degree and order gravity fields in an effort to reduce the temporal aliasing errors through 2). These low degree and order gravity fields are used to correct the final multi-day solution. Results show improvement at higher degrees and reduced longitudinal striping in the gravity solutions. The reduced striping is due to a reduction in the aliasing-induced striations seen in the gravity solutions. These estimated daily solutions could, in theory, be used to improve the de-aliasing products; however, this concept is not explored here.