Degradation of GNSS Accuracy by Multipath and Tree Canopy Distortions in a School Environment: A Recent Study

Accurate position determination with GNSS technology requires unimpeded view of the sky. It has however been noticed that control stations established with GNSS technique for surveying/research projects no longer yield the desired accuracy after some years. This could be as a result of environmental features (buildings, fences, trees, etc) sprouting around them due to infrastructural development and urbanization. With increased use of GNSS in urban and tree canopy environment setting, there is a need to understand the accuracies achievable in such landscapes. Most previous GNSS evaluations have been performed under "clear sky" conditions, where views to satellites are unobstructed. This study investigates the extent of degradation of accuracy at fifteen stations located within a school environment. The study was done by comparing GNSS fixes of those stations with their corresponding positions determined in a total station (electronic tacheometer) survey. The latter was used as study control because it is not affected by the environmental features and factors monitored. The study showed that some of the stations are no longer suitable for the GNSS technique (GDOP is too high (171.6)); while for others, their GNSS-derived positions differed from those of total station by as much as 5.7m. All the controls were therefore reclassified in accordance with national and international accuracy standards. We also recommend that once in a while such checks and reclassification should be done for existing controls.