A Modified Shao-C Tropospheric Delay Correction Model for Ethiopia
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Date
2013-06
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Addis Ababa University
Abstract
The troposphere a ects electromagnetic signal propagation causing signal path bending
and the alteration of the electromagnetic wave velocity. Tropospheric delay can introduce
a considerable error in satellite positioning if it is not properly estimated. The average
GPS signal delay in the zenith direction can vary 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), zenith wet delay (ZWD) and precipitable
water(PW) over Ethiopia, are analyzed using ECMWF (European Centre for Medium-
Range Weather Forecast) ERA-interim 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 of over Great Rift Valley regions of
Eastern Africa which are managed by UNAVCO (University NAVstar COnsortium).
A modi ed version of tropospheric delay correction model, SHAO-C used in China,
is assessed for its performance over Ethiopia. ZTD is modeled directly by a cosine
function together with an initial value and an amplitude at a reference height in each
grid, and the variation of ZTD along altitude, latitude and longitude is tted with a
second order polynomials. The coe cients of the modi ed SHAO-C are generated using
the ECMWF ERA-Interim data at 0.75x0.75 degree latitude-longitude grid, featuring
regional characteristics in order to facilitate a wide range of navigation and other surveying
applications. The altitude is obtained from high resolution Digital Elevation Model
(DEM). The overall average bias ranges between -4.3 cm to -1 cm for ZTD, -3.42 cm
to -0.584 cm for ZWD and -1.12 cm to -0.34 cm for PW, and the RMSE is less than 4.5
cm. The results are assessed for ful lling the requirements of most GNSS navigation or
positioning applications in terms of the tropospheric delay correction
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A Modified Shao-C Tropospheric Delay