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

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.

本数据集包含美国国家海洋数据中心（National Oceanographic and Data Center，NODC）出品的8张光盘套装副本，对应1985年3月30日至1986年9月30日期间获取的盖赛特（Geosat）测高仪交叉差异数据记录（Crossover Differences Data Records，XDRs），该时段即为盖赛特的大地测量任务（Geodetic Mission，GM）。所有数据均采用二进制格式存储。

美国海军盖赛特测高仪任务总时长近5年，始于1985年4月，终于1990年1月。任务最后3年，卫星执行精确重复任务（Exact Repeat Mission，ERM）期间获取的地球物理数据记录（Geophysical Data Records，GDRs）由美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration，NOAA）国家海洋服务中心制作并广泛分发。与之相对，任务前18个月（1985年4月至1986年9月，即大地测量任务阶段）的GDRs因涉及海洋大地水准面相关军事应用而处于保密状态。不过仅测高高度数据被列为机密，该数据集的非保密子集已对外发布。另一类非保密数据类型为交叉差异数据：卫星地面轨迹自身的交点即为交叉点，在此位置，两条交叉轨迹（一条升轨、一条降轨）可在同一地点、不同时刻获取独立的海平面测量值。这两个海平面测量值的差值（即交叉差异数据）无需保密，因为海平面的重力势分量在两条轨迹的交点处完全一致并相互抵消。交叉差异数据包含卫星星历的不确定性信息，因此可用于校正径向轨道误差。

本批光盘所载的盖赛特测高仪交叉差异数据记录（XDRs）已在NOAA手册（Cheney等，1991b）中进行说明，详见下文补充信息字段中的参考文献列表。尽管仅18个月的大地测量任务阶段数据处于保密状态，但研究人员已基于盖赛特任务前2.5年的数据构建了XDRs，例如结合大地测量任务阶段数据与精确重复任务阶段第一年的数据。此举旨在缓解两次任务间长达5周的数据中断（1986年10月1日至11月8日，期间测高仪因轨道机动处于关停状态）带来的影响。通过对完整2.5年周期内的交叉点进行计算，可获得连续的海平面时间序列。

可通过NODC在线商店访问以下链接订购此光盘：http://ols.nndc.noaa.gov/plolstore/plsql/olstore.prodspecific?prodnum=W00013-CDR-S0001.