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Abstract:
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Precision orbit determination (POD ) plays a vital role in the success of space -borne laser altimetry missions , such as ICESat (Ice , Cloud , and Land Elevation Satellite ) . Although current ICESat POD processing standards are achieving remarkable accuracy , new time -varying geopotential models derived from the GRACE (Gravity Recovery And Climate Experiment ) mission were investigated as candidates to improve POD performance for the planned ICESat -2 mission . The objective of this research is to examine the effect of these time -varying geopotential models - - which include models of non -tidal atmospheric and ocean variability , seasonal variability caused by water mass motion , and secular variations caused by present -day ice -melt and glacial isostatic adjustment - - on ICESat POD . The quality of the POD solutions produced with the new geopotential models was quantified by examining the usual orbit quality tests - - DDHL (double -differenced high -low ) and SLR (satellite laser ranging ) observation residuals and orbit overlaps . Although the solutions produced in every test case indicated consistency and high accuracy of 1 -2 cm , these metrics were rather insensitive to the small changes in the POD solutions induced by the new geopotential models , and were incapable of identifying any statistically significant improvements in the POD . However , examination of geographically correlated radial orbit perturbations showed that the radial orbit differences exhibited significant variability on the order of several millimeters , and were coherent with the temporal variability of the models implemented . Since radial orbit errors directly relate to the scientific quantities of interest in the ICESat mission - - the altimetry measurements and derived ice -sheet surface elevations - - this result is of obvious importance . The most notable effects included an annual radial orbit variation of up to 4 mm over the Amazon region induced by implementing the GRACE Annual model , and a secular variation of radial orbit differences over Greenland when the GRACE Trend model was applied . The effect of radial orbit error on ice -sheet altimetry was quantified by examining the mean geographically correlated radial orbit differences . Since the ice sheet elevation rates computed by ICESat scientists are on the order of tens of centimeters per year , it was concluded that , although the radial orbit perturbations are readily observable , with magnitudes on the order of a few millimeters they are too small to have a significant impact on the altimetry science . However , depending on the scientific objectives and radial orbit accuracy requirements set for ICESat -2 , these effects may be important , and the use of time -varying geopotential models in ICESat -2 POD may be beneficial . |