A Climatic Based Asymmetric Mapping Function Using a Dual Radiosonde Raytracing Approach

Reza Ghoddousi-Fard and Peter Dare

Abstract:	The delay induced by the neutral part of the atmosphere on GPS signals still remains one of the most important accuracy limiting factors in high precision positioning applications. Estimation of the residual neutral atmospheric delay (after considering an a priori model) is common practice. It is widely accepted to model the neutral atmospheric delay in the zenith direction and multiply the zenith values by a known mapping function. During the last few decades a large number of mapping functions have been developed. Most of these mapping functions are based on a symmetric atmosphere assumption. This assumption seems to be unrealistic especially when dealing with GPS signals received from low elevation angle satellites. Low elevation signals first reach the neutral atmosphere with a horizontal distance of hundreds of kilometres from the location of the receiver, where the atmospheric condition is likely to be different than at the location of the GPS receiver. Furthermore, a small change in azimuth for a low elevation satellite creates a much larger change in the location of where the GPS signal enters the neutral atmosphere compared to the same change in azimuth from a high elevation satellite. Previous studies suggested the effect of azimuthal variations in the atmosphere should be considered in high precision GPS positioning applications. The effect of azimuthal variations on high accuracy positioning and zenith delay estimation might be crucial where the use of low elevation data is important and the amount of the wet delay is smaller than other regions (hence, more accurate estimation of the delay is required to achieve the same relative accuracy); both might be the case in high latitudes. In this paper a new mapping function has been developed based on a semi-3D raytracing of radiosonde data spanning one year in Canada and the northern US. All radiosondes in a fixed radius around each one of the radiosondes in the mentioned region were considered to perform dual raytracing (i.e., using two radiosondes launched at the same time). The result of this semi-3D raytracing which has dependency on azimuth (resulting from each pair of radiosondes) was compared with the raytracing results from the one central radiosonde (normal raytracing with azimuthal symmetric assumption). The differenced values over one year of data contain variations of slant delays as a function of azimuth and location. A statistical approach was employed to model these variations by means of fitting functions to the data series resulting from different low elevation angles. The primary results show a clear systematic slant hydrostatic delay difference in the North-South direction over all months whereas no such clear trend can be seen in East- West direction. This is in agreement with other studies using low resolution numerical weather models. Due to a lower effective height of the atmosphere impacting on the wet delay, and sparcity of the radiosonde stations, this approach might not be suitable for studying asymmetry in the wet delay. However, due to the fact that the hydrostatic part of the delay is about 10 orders of magnitude larger than the wet part, a more accurate hydrostatic mapping function is of more concern than a wet one. Further research to validate GPS tropospheric gradient estimation using the dual raytracing approach at collocated GPS and radiosonde stations is ongoing. The effect of the new mapping function on positioning and zenith delay estimation might be seen in the results of long term processing of GPS data.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	2870 - 2879
Cite this article:	Ghoddousi-Fard, Reza, Dare, Peter, "A Climatic Based Asymmetric Mapping Function Using a Dual Radiosonde Raytracing Approach," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2870-2879.
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