A ray‐tracing technique for improving Satellite Laser Ranging atmospheric delay corrections, including the effects of horizontal refractivity gradients

In this paper we study the effects of horizontal refractivity gradients on the propagation of optical signals for a globally distributed set of Satellite Laser Ranging (SLR) stations. Using two‐dimensional ray tracing and globally distributed satellite data from NASA’s Atmospheric Infrared Sounder (AIRS), as well atmospheric fields from the National Center for Environmental Prediction (NCEP), we calculate the effects of horizontal gradients on a global set of SLR stations using 2 years of data during 2004 and 2005. We investigate in detail how seasonal and diurnal changes, latitudinal dependence, topography, and oceans affect the gradients at specific locations, as well as the application of these gradients on a set of SLR data from the LAGEOS 1 and 2 geodetic satellites. For the year 2004, AIRS ray‐tracing results showed maximum north‐south (NS) gradient delays of 50 mm at 10° elevation angle at Herstmonceux, UK and east‐west (EW) gradient delays of 35 mm at Yarragadee, Australia. Mean NS and EW gradients had delays not exceeding 5 mm for both AIRS and NCEP analysis at all stations. The standard deviations of AIRS gradients ranged from 6 to 12 mm, while NCEP gradients ranged from 3 to 9 mm, depending on location and time of year. The effects of applying AIRS and NCEP total ray‐tracing results, including horizontal gradient contributions, to a set of global SLR geodetic data resulted in reduction of the solution residuals by up to 43% in variance percent difference. This is a highly significant contribution for the SLR technique’s effort to reach an accuracy at the 1‐mm level this decade.