Analysis of GNSS baseline solutions in Ghana

Accuracy is highly desired in all geodetic and mapping projects. Global Navigation Satellite Systems (GNSS) has many positioning capabilities which can produce the desired accuracies needed for establishing Ground Control Points (GCPs). However accuracy of positioning results from GNSS survey routines are influenced by session duration and baseline length. This study is targeted at determining optimum observation times for specific baseline lengths for GCP fixing and densification in Ghana. The study used four ground stations in Accra, Kumasi, Assin-Fosu and Sunyani. Each station was occupied for a total of four hours on three different days and the data were later split into various time segments to obtain different data sets. Using the Kumasi station as base, each of the baselines was processed three times for each of 3-day 4-hour observation sessions and the mean results accepted as the ‘true’ position values. Repeatability tests were carried out on the computed baselines and the ratios ranged between 1:1,141,100 and 1:4,918,000. All comparisons were based on the true position values. Root Mean Square Error (RMSE) and accuracies were computed from the differences. A plot of the RMSE against time showed that accuracy of positions continued to improve but after 50 minutes of observation there was no significant improvement in the accuracy of measured baselines. This study therefore set 50 minutes as the optimum duration for GNSS baselines up to 195km when using geodetic grade GNSS receivers in differential mode to establish GCPs.