DefinePK

Abstract

A hand-held Garmin eTrex Venture Global Positioning System (GPS) receiver operated in autonomous mode was used to investigate the accuracy of autonomous GPS in measuring ground speed. The accuracy of GPS-derived ground speed was tested by collecting and storing in a laptop computer, every 10 sec, ground speed data from the receiver at eight different ground speeds of a vehicle. The targeted ground speeds involved were 5, 10, 15, 20, 25, 30, 40 and 50 km h-1. The vehicle`s speedometer was used as an initial indicator of the ground speed. A pulse transmitter was utilized to detect the frequency, for every 10 sec, of a magnet fixed on one of the vehicle`s wheels and passing by the transmitter every full rotation of the wheel. Data from the pulse transmitter was acquired and stored in a data logger. Given that the wheel`s circumference was known, frequencies were used to calculate the ground speeds which were used as a reference. By the use of matching times of GPS and data logger data records, GPS-derived ground speeds were compared against the reference speed measurements. For nearly 600 data points comprising the data set for the eight test ground speeds, it was found that the error in GPS-derived ground speeds was, on the average, 1.27 km h-1 (less than 7%.) However, high fluctuations in vehicle`s ground speed when, for example, turning greatly impacted this error. An error of -80.16% was produced due to a drop of vehicle`s ground speed from 18.65 to 11.19 km h-1. Results of steady state ground speed analysis revealed that the average error was less than 1 km h-1 (less than 5.3%), except for the 15 km h-1 data set where the average error reached 1.72 km h-1 (9.92%.)