HR: 1330h
AN: G23B-04    [Abstracts]
TI: Degradation of Grace Monthly Geopotentials in 2004 Explained
AU: Wagner, C A
EM: Carl.Wagner@noaa.gov
AF: NOAA Laboratory for Satellite Altimetry, 1335 East-West Highway, Silver Spring, MD 20910-322 United States
AU: * McAdoo, D C
EM: Dave.McAdoo@noaa.gov
AF: NOAA Laboratory for Satellite Altimetry, 1335 East-West Highway, Silver Spring, MD 20910-322 United States
AU: Klokocnik, J
EM: jklokocn@sunkl.asu.cas.cz
AF: Czech Academy of Sciences, Ondrejov Observatory, Prague, CZ 251 65 Czech Republic
AU: Kostelecky, J
AF: Czech Technical Univ. of Prague, CZ 250 66, Zdilby, CZ 250 66 Czech Republic
AB: Through the first 8 months of 2004 the Grace ground tracks have declined in density with the approach of the 61-revs/4-day orbit resonance in September. At the same time the precision of the monthly Geopotentials has also declined significantly. When an NxN band limited Geopotential is recovered from data on a near circular resonant orbit of R-revs /D-days, the limit N must be < 2R for each significant Geopotential frequency to have unique representation in the repeating information waves every D days. In or near "ideal" resonances (N<2R) the ratio of Geopotential harmonics to repeating wave phases is on average ~ 4/1. When N > 2R (as in Grace recoveries of 120x120 fields in or near September 2004) each wave is represented by about 4 (~120/61) frequencies and the ratio of wave phases to harmonics is close to 1/1, a compression of information which seriously degrades the resulting solution. We present a theory for general recoveries of band-limited fields from resonant orbits where R < 2N following the work of Colombo in the 1980s on the "ideal" block diagonal case (R > 2N). The theory applied to Grace low-low intersatellite range-rate data in its R/D = 61/4 orbit shows: (i), the need for a priori information to condition all Geopotential harmonics in a 120x120 solution and (ii), even with conditioning a degradation of degree-precision of between 1 and 3 orders of magnitude over an "ideal" recovery. Clearly, hydrologic time variations from Grace tracking near such "deficient resonances" will be severely compromised.
DE: 1214 Geopotential theory and determination
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2005 Joint Assembly