DETERMINATION OF OPTIMUM HEIGHT AND MASK ANGLE OF GLOBAL POSITIONING SYSTEM (GPS) RECEIVER FOR POINT POSITIONING

Abstract

Differential and Geodetic Global Position System (DGPS) receivers are capable of surveying measurement with geodetic positioning accuracy. They are designed to be mounted on a tripod usually with a tribrach and set up for observations with assigned mask angle and height of antenna above the survey marks. Assignation of inaccurate mask angle and height of antenna, during observations, is undoubtedly the most prevalent and frequent biases in GPS set up during control surveying which usually leads to less accurate geodetic positioning. Other errors in GPS observations which have been adequately addressed by many researchers are due to ionospheric, stratospheric, tropospheric, and multipath delays among others. Therefore, in this research, an attempt has been made to reduce the arbitrary assignation of GPS mask angle and antenna angle to the barest minimum in order to optimize the accuracy of GPS geodetic positioning. That is, optimal antenna height and mask angle of GPS antenna were determined for geodetic point positioning to be adopted for all GPS observations. The objectives were the determination of the positional coordinates of survey mark at different GPS antenna heights by varying the height upward at 1cm interval with constant mask angle until the optimum height was achieved and determination of the coordinates of survey mark at different mask angle by varying the mask angle upward at 10 interval with the constant optimum height until the optimum mask angle was achieved at the rover station. The survey mark used was a known point (control point with existing coordinates which were authenticated with so many research both by staff and students) within the Federal University of Technology, Akure using Tersus GPS equipment receiver in static mode of observation. Seventy-six (76) coordinate observations were carried out for the determination of optimum antennal height while twenty-three (23) coordinates were observed for the determination of optimum mask angle. The observed field data was post processed using Tersus Geo office processor software and the results obtained were analysed statistically. The values of antenna heights and mask angles were plotted against their respective level of errors. It was observed that 1.76m is the antenna height with the least error level while 200 is the mask angle with the least level of error. Statistical analysis using hypothesis test carried out on the results showed that there is a significant difference in the level of accuracy between the use of 15˚ mask angle as recommended by the manufacturer of the GPS and the of 20˚ mask angle obtained from this research at 95% confidence level. Therefore, it can be concluded that, in a fairly level terrain, 20˚ and 1.76m have proved to be the optimal mask angle and antenna height respectively for all GPS observations during point positioning. Hence, it is recommended that the results obtained in this research should be adopted for GPS observations for better geodetic point positioning.