Test Campaign of an Earth Troposphere Calibration System Based on Dual-Frequency GPS Measurements

The path delay introduced by the Earth troposphere is one of the main error sources for deep space tracking observables. In order to remove all the error sources from these observables, a precise Earth troposphere calibration system has to be developed in order to estimate this delay. ESA’s Deep Space Antenna are currently equipped with meteorological stations, capable of measuring the atmospheric parameters and, thanks to mathematical models, to reconstruct the dry component of the Earth troposphere, leaving a residual, uncalibrated wet component which represents about 10% of the total tropospheric delay. In order to avoid this, the short time-scale variations of the wet component of the Earth troposphere must be calibrated at ESA deep space antennas by means of water vapour radiometers or GPS measurements. ESA ground stations must be upgraded to media calibration systems at least capable to calibrate the total (dry and wet) zenith path delay at centimetre level, to allow reducing S/C navigation uncertainties. The natural choice is the use of the high performance GPS receivers already installed at all ESA ground station complexes for station location purposes. At the Radio Science laboratory of the University of Bologna a GPS-based software has been developed and tested and the results compared with those obtained by other techniques.