Assessment of the SHAO-C Tropospheric Delay Correction Model Over Ethiopia

English Abstract:

Tropospheric signal delay can introduce a considerable error in satellite positioning if it is not properly modeled. In the neutral atmosphere, the average GPS signal delay in the zenith direction can varies from 1.8 m to 2.5 m depending on meteorological conditions and site location. In this work, the spatial and temporal variations of the zenith tropospheric delay (ZTD) over Ethiopia, are analyzed using ECMWF (European Centre for Medium-Range Weather Forecast) pressure-level atmospheric data and compared with ZTD time series for 5-year period from 2007-2011 measured at several GPS stations from the Crustal Movement Observation Network over Great Rift Valley regions of Eastern Africa managed by UNAVCO. A new tropospheric delay correction model, SHAO-C used in China, is evaluated for its performance over Ethiopia. The ZTD along altitude, latitude and longitude is fitted with a second order polynomials at a reference height, and the mean ZTD is modeled directly by a harmonic function together with an initial value and an amplitude in each grid. The coefficients of this modified model are generated using the ECMWF ERA-Interim data at a resolution of 0.75X0.75 degree latitude-longitude grid. The altitude is obtained from high resolution digital elevation model (DEM). The agreement between GPS and model ZTD is found to be very good which reflected in overall average bias between -4.3 cm to -1 cm, and RMSE less than 4.5 cm. The model results are within the requirements of most GNSS navigation or positioning applications in terms of the tropospheric delay correction.

Index Terms— ZTD, SHAO-C, Troposphere Delay Correction, Navigation, Positioning