CARRIER PHASE SURVEYING WITH GARMIN HANDHELD GPS RECEIVERS

Abstract

GRINGO (GPS RINEX Generator) is a program which has been developed at the Institute of Engineering Surveying and Space Geodesy (IESSG) at the University of Nottingham, to record the pseudorange and carrier phase data from 12-channel Garmin handheld GPS receivers, in standard RINEX (Receiver INdependent Exchange) format. It offers owners of these receivers the possibility of post-processing to an accuracy of approximately 5 m (with pseudoranges) or a few cm (with carrier phase), without having to invest in separate DGPS receiving equipment or expensive survey-grade receivers. They retain the benefits of an inexpensive receiver with a user-friendly interface and powerful navigation features, but gain the possibility of improved accuracy if needed.A number of experiments have been carried out to assess the accuracy of positioning with Garmin raw measurement data.Experiments over a 3 km baseline with various session lengths have shown that float solutions with the Garmin carrier phase can achieve better than 20 cm in 15 minutes, and approximately 5 cm with 30 minutes. Ambiguity fixed solutions can achieve approximately 1 cm accuracy in 30 minutes. A zero baseline test, with two garmin receivers attached to a single static antenna, has shown that under ideal conditions, even sub-millimetre accuracies are possible in as little as 10 minutes.Over a longer baseline (>200 km), the effects of the atmosphere on the measured baseline become significant. A comparison of one such baseline against simultaneous measurements with geodetic receivers has confirmed that the atmosphere affects the Garmin receivers in the same way as it does the geodetic receivers.This paper will present the results of these and other experiments.