Saturday, 13 January 2007
Assessing the Precision of GPS Radio Occultation
There have been previous theoretical, experimental, and comparison studies to determine the precision of Global Positioning System (GPS) radio occultation (RO) but the current stage of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites presented a unique opportunity to determine the precision using collocated soundings. The collocated soundings from COSMIC Data Analysis and Archive Center (CCDAC) were constrained on latitude bands as well as local time and scintillation index and the standard deviation of the soundings were calculated for a specific height grid. This study showed the consistency of the tropopause in different regions and established that the occultations are not affected by the tropopause which was concluded by viewing the high precision of a parameter, PPMT, which stands for ″precision parameter for middle troposphere″ by region. Larger PPMT (e.g., implying lower precision) values were observed for the southern hemisphere for GPS RO soundings that were separated by 200 and 300 km that are related to significant refractivity variations due to active weather systems on both mesoscale and synoptic scale. The precision of the refractivity determined in this study of collocated GPS RO less than 1 km apart is 0.02%, which for temperature is approximately 0.05ºC. The precision of a typical radiosonde system is on the order of 0.5°C or higher, therefore, the GPS RO is one order magnitude more precise than the radiosonde. With such a precision and spatial coverage, GPS RO is the best technique for climate analysis as well as weather prediction.