An Independant Evaluation of the Accuracy of Handheld GPS Receivers
By Elizabeth M. Nel and Michael E. Jackson

The use of the Global Positioning System is rapidly becoming widespread as users in both industry and science realize its capacity for providing absolute and relative coordinates referenced to a common geodetic datum. Many users do not need the precision and accuracy provided by geodetic receivers measuring both carrier phase and code data (e.g. a part in 106 - 108 of baseline length), but rely instead on handheld receivers which record only the Coarse/Acquisition (C/A) code data. According to the manufacturers, handheld systems yield accuracies of approximately 15 to 30 meters CEP (circular error probable, or median error defined as the radius of a horizontal circle containing 50 percent of the data points) in the autonomous mode (using the receiver without a base station) and 2 to 5 meters CEP using a differential correction to a base station. Prospective users often experience difficulty in comparing the relative accuracies and costs of different brands of receivers, especially since the accuracy depends not only on receiver hardware but also on local environmental conditions, the duration of the measurement period, whether or not selective availability (S/A) was activated, and whether or not the data were differentially corrected and averaged. Assuming similar data storing capacities and equal numbers of receiver channels, accuracy and price are usually directly related and different brands are generally comparably priced. One can expect to pay approximately $3,600 for a standalone three-channel receiver and approximately $10,000 for the differential processing capability provided by two six-channel receivers. Receivers without internal storage and designed solely for navigation purposes are substantially less expensive. The purpose of this article is to provide an independent verification of the accuracy of handheld GPS receiver data collected under the manufacturer-recommended optimal conditions. We used both handheld and geodetic receivers to compare high-precision surveyed positions, differentially corrected then averaged positions, uncorrected averaged positions collected in the autonomous mode, and single uncorrected positions collected in the autonomous mode. We examine both the horizontal and vertical precisions based on handheld receiver repeatabilities and accuracy compared to positions determined using the geodetic-quality receivers.

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