Design and implementation of algorithms for Earth troposphere calibration from dual-frequency GPS measurements

Earth troposphere calibration plays an important role in the orbit determination process for interplanetary spacecraft, during cruise phases as well as planetary insertion, maneuvers and planetary tours. Different techniques can be employed for troposphere calibration, each yielding different levels of achievable precision. At present, the ESA Deep Space Tracking Network makes use of a simple system based on meteorological measurements at its stations, thus allowing, with an accurate theoretical model, to obtain a very precise estimation of the static “dry” component of the troposphere. The unstable component depending on the water vapor content cannot be estimated through meteo data so that the whole troposphere calibration process has an uncertainty in the order of tens of centimeters. A precise method for Earth troposphere calibration is based on a combination of surface meteorological measurements and GPS measurements, capable to estimate also the average water vapor content in the observed atmosphere. This paper describes the activity carried out in the Radio Science lab of the University of Bologna, for the development of a data analysis software based on GPS observables, capable to obtain the wet component of the path delay due to the Earth troposphere. The proposed algorithm is based on the double differences techniques and the preliminary results obtained using GPS data freely available, in combination to those acquired by a GPS receiver installed in our lab are shown and discussed.