Application of Global High-resolution Dem in the Oco Mission

The Orbiting Carbon Observatory (OCO), scheduled for launch in December 2008, will make the first space-based measurements of atmospheric CO2 with the precision, spatial and temporal resolution needed to characterize CO2 sources and sinks on regional spatial scales and seasonal to interannual time scales. Variations in surface topography have a direct impact on the retrieval of the columnaveraged CO2 dry air mole fraction, XCO2, since elevation changes of ~25 meters are equivalent to the 1 ppm XCO2 measurement precision sought by OCO. The OCO Level 1B Geolocation process uses a global high-resolution digital elevation model (DEM) dataset to geo-register OCO soundings on the Earth surface. Topographic characteristics (slope, roughness, etc) influence the quality of the Xco2 retrieval. This paper describes the challenge of applying a high-resolution, high vertical precision, and globally distributed DEM in an automated ground geolocation processing. In this processing, an OCO sounding is first geo-registered to the Earth geoid surface, then to a relatively coarse topography represented by the 30 arc-second EOS/AM1 DEM, and finally to a fine topography at the 3 arc-second. The primary source of the high-resolution DEM dataset is the Shuttle Radar Topography Mission (SRTM) DEM, supplemented by other ancillary DEM data sources. An automated production software system has been built within the OCO ground data processing system to generate the geolocation data, and tested with simulated OCO data and the high-resolution DEM data.