NODC Standard Product: US Navy Geosat altimeter Crossover Differences (XDRs) for the Geodetic Mission (8 disc set) (NODC Accession 0054498)NOAA/NODC

This accession contains a copy of the NODC eight CD-ROMs product set of the Geosat altimeter Crossover Differences data Records (XDRs) for altimeter data obtained between March 30, 1985 and September 30, 1986, the period known as Geosat's Geodetic Mission (GM). These data are in binary format.The U.S. Navy Geosat altimeter mission lasted for nearly 5 years, from April 1985 to January 1990. Geophysical data records (GDRs) from the final 3 years, when the satellite was performing the exact repeat mission (ERM), were produced by NOAA's National Ocean Service and have been widely distributed. In contrast, GDRs from Geosat's first 18 months (April 1985 to September 1986), known as the geodetic mission, are classified because of military applications related to knowledge of the marine geoid. Only the altimetric heights are considered secret, however, and unclassified subsets of these data have been released. Another unclassified data type is the crossover difference. A crossover is defined as the intersection of the satellite ground track with itself. At this location, the two crossing passes (one ascending and one descending) provide independent sea level measurements at the same place but at different times. The difference of these two sea level measurements (a crossover difference) is not considered classified because the geopotential component of sea level is the same at the intersection of the two tracks and thus cancels. Crossover differences contain information about uncertainties in the satellite ephemeris and therefore enable correction of radial orbit error. The Geosat altimeter crossover difference records (XDRs) contained on these CD-ROMs are documented in a NOAA handbook (Cheney et al. 1991b); see reference listing in the supplementary information field below. Although only the 18-month GM data are classified, XDRs have been constructed for the first 2.5 years of the Geosat mission, e.g., data from the GM plus the first year of the ERM. This was done to minimize the effect of the 5-week data gap (October 1 to November 8, 1986) between the two missions when the altimeter was off during orbit maneuvers. By computing crossovers for the entire 2.5-year period, continuous sea level time series can be derived.These discs may be ordered through the NODC Online Store at http://ols.nndc.noaa.gov/plolstore/plsql/olstore.prodspecific?prodnum=W00013-CDR-S0001. http://datadiscoverystudio.org/geoportal/rest/metadata/item/585198661aff4577a10339d18de5ce5c/html