Future Mission Design Options for Spatio-Temporal Geopotential Recovery
Sampling the Earth from single-satellite missions or single-orbit formations is necessarily limited by the mandatory balance between spatial and temporal resolution. A short repeat period leads to sparse ground-track spacing. Conversely, dense satellite c
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Future Mission Design Options for Spatio-Temporal Geopotential Recovery T. Reubelt, N. Sneeuw, and M.A. Sharifi
Abstract Sampling the Earth from single-satellite missions or single-orbit formations is necessarily limited by the mandatory balance between spatial and temporal resolution. A short repeat period leads to sparse ground-track spacing. Conversely, dense satellite coverage can only be attained at the cost of time resolution. For future gravity field missions, geoscience communities are pushing for ever higher resolution than GRACE, both in time and space. A logical consequence would be multi-satellite and/or multi-groundtrack configurations. We investigate the basic parameters that determine space-time resolution. Under the assumption of a repeat orbit two basic rules for sampling the Earth from space are provided. The familiar Nyquist rule of thumb links the number of revolutions in a repeat period to the maximum spherical harmonic degree. A second sampling rule, expressing the balance between spatial and temporal resolution, is coined the Heisenberg rule. Simulations demonstrate how future mission concepts might benefit from multi-satellite/multigroundtrack configurations.
degrees. The mission concept consists of a twin satellite leader-follower formation with an intersatellite distance of about 220 km, flying on a near-polar orbit. The K-band sensor, which provides range-rate with an accuracy of a few μm/s, allows for monthly timevariable geopotential recovery with a spatial resolution of about 1,000 km. The monthly GRACE solutions clearly show the continental scale hydrological (H) cycle (Tapley et al., 2004). Long-term GRACE time series also reveal trends in deglaciation in Antarctica or Greenland (Chen et al., 2006) as well as sea level rise. Despite the huge progress in time-variable gravity research, the GRACE concept suffers from inherent deficiencies that lead to an erroneous North-South striping pattern (Schrama et al., 2007). Figure 22.1 attributes the error pattern to three sources:
With the GRACE mission, which was launched in spring 2002, for the first time global time-variable gravity fields could be recovered beyond the lowest
i. The range-rate-observable in a leader-follower formation itself, which is an anisotropic functional, combined with the error structure due to NorthSouth groundtracks and with a limited sensor accuracy. ii. Aliasing due to temporal and spatial undersampling of the time-variable signals, combined with a time-variable groundtrack. iii. Errors and aliasing due to limited dealiasing products used to correct the GRACE-measurements for high-frequency signals of the atmosphere and ocean and of the ocean tides.
T. Reubelt () Institute of Geodesy, Universität Stuttgart, Stuttgart D-70174, Germany e-mail: [email protected]
The mitigation of these problems is the main task when designing a future mission for improved timevariable gravity determination (Sneeuw et al., 2004). Problem (i) is addressed on the one hand by a future laser-instrument, which can measu
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