Evaluation of Five Tropospheric Delay Models on Global Navigation Satellite System Measurements in Southern Nigeria

Abstract

Throughout Nigeria, the structure and facilities needed for the operation of a Global Navigation Satellite System (GNSS) based Continuously Operating Reference Stations (CORS) has been set up at different locations in the country generally known as NIGerian Reference GNSS NETwork (NIGNET) for surveying and mapping. Different researchers have conducted investigations into the effect of the troposphere over the NIGNET. This study aims at comparing analytically the effect of five different a priori tropospheric models on GNSS signals in Southern Nigeria with a view to obtaining the best-fit model. The objectives include evaluation of the global tropospheric models in the baseline and position domain; and determining the best model for southern Nigeria. Observational data used were obtained from Office of Surveyor General of Nigerian Mapping Agency (OSGoF). GPS data were obtained from October 2010, to April 2011. Six processing strategies were employed these include; application of no model, application of five global tropospheric delay models (Black, Davis et al, Hopfield, Neil and Saastamoinen) models using Trimble Total Control software version 2.73. Each of the strategies went through free and constrained adjustments and the results were compared. The five models investigated show no significance difference in their performance; better improvements in the position domain were achieved by the application of the Niell model compared to the rest of the models. The Niell model produced a better mitigation of the tropospheric delay, with an average percentage improvement of 67.1%; while Davis et al, the modified Hopfield and Saastaminen models have 70%, 71.1% and 71.7% percentage improvement respectively. The result also indicates that, the Neill model gave the best result and a better improvement in the entire network with the lowest mean average zenith tropospheric delay (ZTD) of 2.535m and least average RMSE of 0.67m. The specific objective of this study is to determine the best tropospheric delay for the study area and to recommend to practicing Surveyor on the model to be used. The research shows that, the Neil model gives the best result when compared with other model. Hence, it is recommended when processing GNSS observations for tropospheric delay to obtain a more accurate result.