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Journal of Biomechanics

Volume 38, Issue 8, August 2005, Pages 1717-1722
Journal of Biomechanics

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Accuracy of WAAS-enabled GPS for the determination of position and speed over ground

https://doi.org/10.1016/j.jbiomech.2004.07.028Get rights and content

Abstract

The Global Positioning System (GPS) offers many advantages over conventional methods for the determination of subject speed during biomechanical studies. Recent advances in GPS technology, in particular the implementation of the Wide-Angle Augmentation System and European Geostationary Navigation Overlay Service (WAAS/EGNOS), mean that small, highly portable units are available offering the potential of superior accuracy in the determination of both position and speed. This study set out to examine the accuracy of a WAAS-enabled GPS unit for the determination of position and speed. Comparison with the new and published data showed significant enhancements in both position and speed accuracy over a non-WAAS system. Position data collected during straight line cycling showed significantly lower sample-to-sample variation (mean absolute deviation from straight line 0.11 vs. 0.78 m) and greater repeatability from trial to trial (mean absolute deviation from actual path 0.37 vs. 4.8 m) for the WAAS-enabled unit compared to the non-WAAS unit. The speed determined by the WAAS-enabled GPS receiver during cycling in a straight line was within 0.2 ms−1 of the actual speed measured for 57% of the values with 82% lying within 0.4 m s−1, however, the data tended towards underestimation of speed during circle cycling, with 65% of values within 0.2 m s−1 and 87% within 0.4 m s−1 of the actual value. Local dGPS and dual frequency techniques are more accurate still, however, traditional differential GPS (dGPS), employing FM radio transmission of correction data to a separate receiver, now offers no advantage over WAAS and appears redundant.

Introduction

The Global Positioning System (GPS) has been shown to be an accurate method to determine the position of a subject during biological and biomechanical studies (Schutz and Herren, 2000; Perrin et al., 2000; Schutz and Chambaz, 1997; Terrier et al., 2000, Terrier et al., 2001). Simple, cheap and lightweight non-differential GPS units have been used for applications in animal tracking (von Hünerbein et al., 2000) and the accuracy of these systems for the determination of speed has recently been tested (Witte and Wilson, in press). Differential GPS (dGPS) enhances the positional accuracy of GPS receivers. dGPS is based upon the calculation and transmission of error correction signals for individual satellite data from ground-based radio beacons at known locations. dGPS offers positional accuracies down to metre accuracy, and, with the addition of a local beacon and carrier-wave phase differentiation, centimetre accuracy. The enormous potential of these systems for studies of locomotion has recently been investigated (Terrier et al., 2001; Terrier and Schutz, 2003; Larsson, 2003), however, they may be unsuitable for many studies due to cost and physical size.

In order to improve position determination accuracy for aircraft guidance during low-visibility approaches to airfields, the FAA has recently developed the Wide-Angle Augmentation System (WAAS), a satellite-based dGPS. A system employing identical protocols has also been implemented in Europe (European Geostationary Navigation Overlay Service, EGNOS). These systems employ the same fundamental principle as dGPS, using ground-based receivers to provide error correction data for individual satellite data; however, the implementation of the concept should offer superior accuracy. dGPS provides single-point error correction data based upon the errors experienced at a single reference station. These are then applied to the mobile receiver, which may be up to several hundred kilometres away. In contrast, WAAS employs so-called ‘wide area master stations’, which collect error data from multiple reference stations via terrestrial communications. The system then derives a location-specific grid of error correction data, which are transmitted in the form of a WAAS correction message to an additional Geostationary Earth Orbit satellite. The data are re-transmitted in the form of pseudo-range code, which can be received via the same antenna as the standard GPS signal. Whilst not increasing the physical size or hardware complexity of the receiver this system therefore offers improved positional accuracy, possibly beyond the level of dGPS. Small WAAS-enabled GPS units are coming onto the market and the total mass of the module used in the current study, including antenna, is 32 g. Enhanced positional accuracy does not necessarily translate directly into improved speed accuracy, as GPS speed determination does not rely solely on differentiation of position (Witte and Wilson, in press). Modules also use Doppler shift on the carrier wave of the satellite data and complex Kalman filtering algorithms to determine speed. Thus, it is unclear whether the increased positional accuracy of WAAS-enabled GPS units is reflected in similar improved accuracies for the determination of speed.

This study set out to test the hypothesis that WAAS/EGNOS GPS is significantly more accurate for the determination of position and speed than standard non-dGPS.

Section snippets

Materials and methods

Two experiments were performed:

  • (1)

    In order to determine the sample-to-sample variability and the repeatability of GPS position data a cyclist was asked to perform a series of trials along a straight, level road equipped with both a WAAS-enabled GPS module and a non-WAAS GPS module.

  • (2)

    Speed accuracy was determined by direct comparison of a WAAS/EGNOS-enabled GPS module to a custom-built bicycle speedometer, during cycling in a straight line and on curves of two different radii.

Experiment 1

A total of 25 trials were collected for the WAAS-enabled unit and 18 for the non-WAAS unit (Fig 1a,b). This equated to 1303 and 805 total position fixes for the WAAS and non-WAAS units, respectively. Linear regression analysis on each individual trial yielded a median (25th percentile, 75th percentile) absolute offset of 0.11 m (0.05 m, 0.20 m) and 0.78 m (0.31 m, 1.8 m) for WAAS and non-WAAS, respectively. Linear regression analysis over all trials yielded a median absolute offset of 0.37 m (0.17 m,

Discussion

This study set out to test the hypothesis that WAAS/EGNOS-enabled GPS offers enhanced accuracy for the determination of the position and speed over ground of a subject during field locomotion.

The mean number of satellites seen by the WAAS-enabled receiver used in this study was considerably higher than that seen by Witte and Wilson (in press). The chipset used in this study is the same as the one used previously, although it was WAAS-enabled. Therefore, the increased number of satellites seen

Acknowledgements

We thank the Horserace Betting Levy Board for funding THW and the BBSRC for contributing to the work described here. Glen Lichtwark, Jo Watson and Kate Smith are acknowledged for assistance with data collection.

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Also at Structure and Motion Laboratory, University College London, Royal National Orthopaedic Hospital, Brockley Hill, Stanmore, Middlesex HA7 4LP, UK.

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