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Wiese, David N ¹ ; Nerem, Steve ²

¹ Colorado Center for Astrodynamics Research
² Colorado Center for Astrodynamics Research

Since its launch in 2002, the Gravity Recovery and Climate Experiment (GRACE) mission has been providing measurements of the time-varying Earth gravity field. Scientists have used this data to measure terrestrial water storage in river basins, ice mass loss from Greenland, Antarctica, and glaciers, post glacial rebound signals, as well as other important geophysical processes. The GRACE mission architecture includes two satellites in near-circular, near-polar orbits separated in the along-track direction by approximately 220 km (e.g. collinear). A microwave ranging instrument measures changes in the distance between the spacecraft, while accelerometers on each spacecraft are used to measure changes in distance due to non-gravitational forces. This research focuses on quantifying the performance of alternative mission architectures for a follow-on mission to GRACE in hopes of improving temporal and/or spatial resolution. Previous research compared the traditional two-satellite collinear pair to a two-satellite and four-satellite cartwheel formation. Results showed that the cartwheel formation, which provides radial and along-track measurements due to the natural dynamics of the formation, offers improved sensitivity to the mass distribution of the Earth, and thus, higher spatial resolution in the derived gravity field. However, when short-period errors due to the atmospheric/ocean dealiasing (AOD) models are considered, the cartwheel formation, while reducing the longitudinal striping, does not offer higher spatial resolution in the derived gravity field. One way to reduce temporal aliasing errors, and thus increase the spatial resolution of the derived gravity field, is to have better temporal resolution, which can be accomplished through multiple satellite pairs. Current and future research is focused on performing an exhaustive study of possible mission architectures, including constellations, and optimizing a four-satellite architecture in hopes of reducing temporal aliasing errors. Preliminary work has been performed on constellations consisting of both 16 and 32 collinear satellite pairs in polar orbits, as well as a four-satellite architecture consisting of one polar orbiting collinear pair in a 5-day repeat period coupled with a lower inclined 63o collinear pair in a 23-day repeat period. An alternate processing strategy has been implemented in which daily estimates of the gravity field (to low degree and order) are made in hopes of reducing temporal aliasing errors. Results show the four-satellite architecture, while estimating daily 20x20 gravity fields, offers an order of magnitude improvement in recovering a hydrology signal over a traditional two-satellite collinear pair. It even outperforms the constellations considered thus far, albeit having a 23-day temporal resolution versus a 6-hour temporal resolution. All simulations are performed using NASA Goddard Spaceflight Center's GEODYN software package.