Assessment of performance of the SHAO-C tropospheric delay correction model over low latitude and complex topography

Tropospheric signal delay can introduce a considerable error in satellite positioning if it is not properly modeled. In this work, the spatial and temporal variations of the zenith tropospheric delay (ZTD) over low latitude and complex topography of Ethiopia and Eritrea from the Greater Horn of Africa (GHA) region, are analyzed using ECMWF (European Center for Medium-Range Weather Forecast) pressure-level atmospheric data and compared with ZTD over 5-year period from 2007-2011 measured at several GPS stations of UNAVCO in the region. A new tropospheric delay correction model, SHAO-C used in China, is evaluated for its performance over the region with most GPS observational stations located in 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 model are generated using the ERA-Interim data at moderate resolution. 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 is reflected in overall average bias between -4.3 to -1.0 cm, and RMSE less than 4.5 cm. The results are within the requirements of most GNSS navigation or positioning applications in terms of the tropospheric delay correction.