An observation operator for ground-based GPS slant delays

An observation operator for the Global Positioning System (GPS) Slant Total Delay (STD) observations is developed. The operator is implemented in the High Resolution Limited Area Model (HIRLAM) for data assimilation of tropospheric humidity information contained in the geodetic estimates of microwave signal phase delays. The observation operator integrates refractivity along the signal path between the satellite and the receiver to obtain STD. The signal path is solved using the geometrical path, as given by the geometrical azimuth and zenith angles of the satellite as viewed at the receiver, as the starting point. The tropospheric effect on the signal path is accounted for. A preliminary validation of the operator has been performed against observational data. Model counterparts are compared with the geodetic estimates of Zenith Total Delay (ZTD) and STD. The bias and standard deviation between modelled and observed STD are −1.0 and 1.7 cm, respectively, at zenith, and −7.2 and 8.6 cm, respectively, at zenith angle 80◦.