Improvement of Current Refraction Modeling in Satellite Laser Ranging (SLR) by Ray Tracing through Meteorological Data

The accuracy of current modern space-based geodetic systems such as Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), the Global Positioning System (GPS), and satellite altimetry all suffer from limitations in the modeling of atmospheric refraction corrections. The current modeling of atmospheric refraction in the analysis of SLR data comprises the determination of the atmospheric delay in the zenith direction and subsequent projection to a given elevation angle, using a mapping function (MF). Recently a new zenith delay (ZD) model of sub-millimeter accuracy [Mendes and Pavlis, 2004] and a new MF of sub-centimeter accuracy [Mendes et al., 2002] were developed, applicable to the wavelengths used in modern SLR instrumentation. We have already assessed and validated the new ZD model and MF’s using 2-d ray tracing and globally distributed data from the Atmospheric Infrared Sounder (AIRS), the European Center for Medium Weather Forecasting (ECMWF) and the National Center for Environmental Prediction (NCEP). However, the models still remain far from the required sub-millimeter accuracy goal for future SLR analysis standards as set forth by the International Laser Ranging Service (ILRS) based on the requirements place on SLR by the Global Geodetic Surveying System (GGOS) [Pearlman et al., 2005]. To further improve atmospheric delay modeling, we need to look at the application of ray tracing and horizontal refractivity gradients on SLR data collected at the core SLR sites around the globe. We have found horizontal gradient delays of up to 5 cm at an elevation angle of 10° at certain times of year and SLR site locations. The effects of applying ray tracing results, including horizontal gradients to a set of global SLR geodetic data resulted in reduction of the observation residuals by up to 45% in variance, and 3 mm in RMS. This is a highly significant contribution for the SLR technique's effort to reach an accuracy at the 1-mm level this decade.