High Spatial Variation Tropospheric Model for GPS-Data Simulation

A. Farah, T. Moore, C.J. Hill

Abstract:	Precise GPS simulated data requires accurate simulation of the two major sources of error in GPS measurements, namely the ionospheric and tropospheric delays. The ionospheric delay modelling has been handled in a previous work (Farah, 2002). In this paper the simulation of the tropospheric delay is discussed. The suggested model should be accurate in estimating the tropospheric delay as well as capable of simulating high spatial variations of the troposphere resulting in more realistic simulated GPS data. The Troposphere ranges from ground level to about 10 km in altitude. It contains approximately 75% of the total molecular or gaseous mass of the atmosphere and virtually all of the water vapour and aerosols. When considering GPS, the atmosphere is often conveniently divided into the ionosphere and the neutral atmosphere. Following this division each of the two layers is modelled separately because of their different characteristics. The propagation delay experienced in the neutral atmosphere is often termed the tropospheric delay, because 80 % of the delay can be attributed to the troposphere. The EGNOS guidelines recommend that a user apply a correction for tropospheric delay. The recommended model (the EGNOS model) provides an estimate of the zenith tropospheric delay which is dependent on empirical estimates of five meteorological parameters at the receiver - namely, pressure, temperature, water vapour pressure, temperature lapse rate and water vapour lapse rate. These estimates of the meteorological parameters are dependent on the receiver's height, latitude and day-of-year, and are interpolated from reference values for the yearly averages of the parameters and their associated seasonal variations, derived primarily from North American meteorological data. The EGNOS model is consistent with the recommendations of the ICAO SARPS and so also with the tropospheric model for the US WAAS program. In this paper, the EGNOS tropospheric correction model is considered as a possible tool for simulating the tropospheric delay in order to obtain more realistic simulated GPS data. Comparing the total tropospheric zenith delays from the EGNOS model with the CODE-tropospheric product has allowed the quality of the EGNOS model to be assessed. CODE, the Centre for Orbit Determination in Europe is one of the IGS-Analysis Centers (AC) and has produced zenithal tropospheric delay products for an increasing number of IGS-tracking stations since 1997 with a high degree of consistency with other IGS-AC's tropospheric products. Four IGS-tracking stations have been selected for this study. Data from four non-consecutive weeks in different seasons over a period of one year was tested to assess the seasonal variation of the weather conditions. It is shown that the EGNOS model agrees well with the CODE-estimations with a mean zenith delay difference of approximately 2 cm. The maximum zenith delay difference between the EGNOS model and the CODE-estimations was in the range of 5 cm to 16 cm, which agrees well with previous studies. A second study has investigated the behaviour of the EGNOS model with other established tropospheric models such as the Saastamoinen model, the Hopfield model, the Marini model and the Magnet model for three IGS-stations. It can be concluded from this study that the EGNOS model shows better agreement with the IGS estimations than the Magnet model and compares well with other models. The major shortcoming in the EGNOS model is its inability to simulate the variations in the troposphere over small regions. This shortcoming could be overcome by using the theory of Gaussian Random Fields, which has been previously used to model real life phenomena such as surface roughness (Chan, 1999).
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	347 - 354
Cite this article:	Farah, A., Moore, T., Hill, C.J., "High Spatial Variation Tropospheric Model for GPS-Data Simulation," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 347-354.
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