Abstract
The tropospheric delay still remains a limiting factor to the accuracy of space based positioning techniques. The estimation of station positioning, especially height component, which is particularly important for more applications is susceptible to errors in modeling the tropospheric delay due to correlations between the station positioning and residual troposphere delay parameters. As the demand on positioning accuracy and precision has increased, it has begun a necessary of relaying on large external data sets, rather than relatively simple models for treating the tropospheric delay. This method has been possible by advances made in numerical weather models which provide accurate representations of global atmospheric conditions and by advances in computing speed which allow us to perform a large number of computations over a short period of time. The purpose of this work is to develop a new model for estimating the tropospheric delay and then assess the benefits of applying this model at various geographic atmospheric conditions of Egypt. By comparing new model with some common models such as Saastamoinen model, Hopfield model, Niell-MF, Black & Eisner-MF, UNB3 model and Vienna-MF, the results show that, new model for estimation tropospheric delay has an acceptable level of accuracy in describing the dry tropospheric delay in Egypt as it agrees closely with the numerical integration based model. The mean accuracy of this new model has been assessed to be about 9.64 mm with rms 11 mm at an elevation angle of 30° and for an elevation angle of 5°, the mean accuracy is about 83.23 mm with rms 96.42 mm for atmospheric conditions of Egypt.
Highlights
GPS pseudorange and carrier-phase measurements are affected by several random and systematic errors
The performances of new equation were determined by comparing their results with those obtained from highly accurate numerical integration model at three stations of Egypt at January, July and average data of year
At 70° zenith angle (20° elevation angle) the mean difference between new model and NIM is about 27.89 mm with rms of 31.73 mm, at 80° zenith angle it is about –281.37 mm with rms of 283.01 mm, at 83° zenith angle it is about –801.75 mm with rms 803.12 mm and Zenith angle 0° 5° 10° 15° 20° 25° 30° 35° 40° 45° 50°
Summary
GPS pseudorange and carrier-phase measurements are affected by several random and systematic errors. These errors are originated from satellites, receivers and signal propagation through the atmosphere. The GNSS signal propagating through the bottom part of the atmosphere is refracted and bended; size of the effects is directly correlated with the troposphere density variations (Thayer 1974). Tropospheric delay depends on the temperature, humidity and pressure. It varies with the height of receiver setup point and the type of propagation media below signal path. Signals from satellites at low elevation angles travel a longer path through the troposphere than those at higher elevation angles. The effect is a delay that reaches 2.0–2.5 m in the zenith direction (satellite directly overhead) and increases approximately with the co-secant of the elevation angle, yielding about a 20–28 m delay at a 5° elevation angle, about 9.30 m for a 15° elevation angle (Brunner et al 1993)
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