Accuracy of non-differential GPS for the determination of speed over ground
Introduction
Accurate determination of an individual's speed is fundamental to many studies of human and animal locomotion. Speed is the rate of change of position and its determination requires measurements of distance and time components, which can be achieved directly or indirectly. Most commonly used methods for determining an individual's speed require direct measurement of both distance and time. Chronometry over a known distance using a simple stopwatch (Sharp, 1997) or by more accurate means such as light gates is limited to use under controlled conditions, on a pre-defined track. In addition, chronometry only determines average speed over the course; fluctuations of speed or route taken are ignored. Alexander used the time taken to pass a defined landmark on video film of free-running ungulates to calculate speed, but recognised the limited accuracy of the method due to the frame rate of the camera (Alexander et al., 1977) and parallax effects. High-speed video motion analysis and differentiation with respect to time of the position of a fixed marker can provide speed data many times per second, however, such systems are expensive, only effective within a limited volume and usually rely on infrared light, which limits their application outdoors. Indirect methods of predicting speed include foot-mounted pedometers (Saris and Binkhorst, 1977) or measurement of stance time via accelerometers (Weyand et al., 2001). Laser speed guns, which rely on the principle of Doppler shift, are commonly employed for the determination of vehicular speed, the velocity of ballistics and the speed of movement of small mammals (Marsden and King, 1979). However, these techniques are limited to single point, instantaneous measurements. Integration of body-mounted accelerometer signals is possible but error handling during integration is difficult (Perrin et al., 2000; Herren et al., 1999).
An increasingly popular method of determining an individual's position is the Global Positioning System (GPS). GPS was originally developed as a military tool. It comprises a network of ground-station controlled satellites, which emit low power radio signals containing atomic clock time data. Transit-time delays in these time signals are used by the ground-based GPS receiver to triangulate position. In order to limit the potential accuracy of the system, small random errors were introduced into the satellite clock signals by the US government (termed selective availability, or SA). This spurred the development of several approaches to enhance the accuracy of GPS. Differential GPS (dGPS) compares the known position of a fixed receiver with that determined by satellite triangulation. The difference is then used to correct the transit time of individual satellite signals either in real time via a radio link or, less commonly, in a collected data set during subsequent analysis. It is currently not clear, however, whether the improved positional accuracies of dGPS are mirrored by enhancements in the accuracy of speed determination, since GPS speed determination does not rely solely on differentiation of position data over time but also depends on Doppler shift of the carrier wave.
Further increases in positional accuracy can be achieved by determining the phase difference in the carrier wave signal from a satellite as seen by two neighbouring receivers (carrier wave differentiation). Sub-centimetre accuracies have been reported for this system (Leick, 1995) and it has been employed in studies for the measurement of both trunk position (Terrier et al (2000), Terrier et al (2001)) and speed (Larsson and Henriksson-Larsén, 2001). Schutz and Herren (2000) report accuracies with a standard deviation of 0.03 m s−1 for running with this system. The equipment is however both costly and bulky since units weigh 2 kg or more (Leica System 500, Leica Geosystems, Heerburg, Switzerland) and are therefore of limited potential for many studies of field locomotion. Further, the discontinuation of SA in May 2000 has meant that the accuracy of standard, non-differential GPS systems is improved for position and possibly for speed determination. The development of satellite-based differential systems such as Wide Angle Augmentation System (WAAS) and European Geostationary Navigation Overlay Service (EGNOS), which transmit correction data via satellite rather than land-based radio beacons, may mean that, in future, units the size of current non-differential units but with the accuracy of basic differential GPS will be available.
Improvements in technology, including reduced time-to-fix (TTF), as well as miniaturisation of GPS receivers for implementation in automobiles and mobile phones (7 g OEM modules are now available), has stimulated interest in GPS for applications in animal tracking (Steiner et al., 2000; von Hünerbein et al., 2000). Battery requirements are still a constraint, however, due to the high power consumption of GPS receivers. Low power duty cycling can be undertaken for studies in animal tracking; however, continuous data are often required for applications involving speed measurement. The positional accuracy of GPS systems since SA removal has been determined (Adrados et al., 2002) but validation of non-differential GPS for velocity determination has not been undertaken. Manufacturers quote accuracies in the region of 0.1–0.2 m s−1, with the specific algorithm employed being the variable which most influences accuracy between manufacturers. However, due to commercial confidentiality further information on how the system calculates speed and the limitations of the system are not forthcoming.
The accuracy of the Global Positioning System is influenced by several variables. The number of satellites available to the receiver is clearly important and a theoretical minimum of four satellites is required to obtain a 3D position fix. In addition, the geometrical arrangement of the satellites relative to each other and the receiver affects the quality of the triangulation for position. This is quantified in a measurement known as dilution of precision (DOP), which is inversely proportional to the volume of a cone delineated by the position of the satellites and the receiver. An ideal DOP of 1, i.e. the greatest predicted accuracy of triangulation, will be seen when one satellite is directly overhead and the remainder are equally spaced around the horizon. In contrast, higher DOP values will be seen if the satellites are tightly clustered overhead and the maximum value of 50 means that the fix is unreliable. Clearly, the orientation of the satellites and the identity of the satellites used changes over time and thus experimental conditions cannot be wholly standardised. The response of the GPS system to changing satellite availability is of interest for potential application of the system in conditions of less than ideal sky-view.
This study was designed to test the hypothesis that non-differential GPS is an accurate and reliable method for the determination of speed over ground.
Section snippets
Materials and methods
The speed of a cyclist was determined simultaneously by GPS and by a custom designed bicycle speedometer during a series of trials under varying conditions. These data were used to determine GPS accuracy during cycling at constant speed around a running track, on curves of two different radii, on a straight road and during rapid acceleration/deceleration.
A road-racing bicycle was instrumented with a Hall-effect proximity switch (RS Components Ltd., part no. 307–466, Northamptonshire, UK), which
Experiment 1
A total of 5060 GPS speed values were recorded during the track study. The actual speeds achieved by the cyclist ranged from 2.1 to 10.8 m s−1. The cyclist consistently failed to achieve the highest target speed of 35 km h−1, dropping as low as 25% below it.
The speed determined by the GPS receiver was within 0.2 m s−1 of the true speed measured for 45% of the values (Fig. 2) with a further 19% lying within 0.4 m s−1. A negative error (i.e. GPS underestimation of speed) of greater than 1.0 m s−1 was seen
Discussion
The hypothesis of this study was that GPS is an accurate and reliable method for the determination of speed over ground. The results show that GPS is generally accurate for speed determination under all conditions where a position fix is obtained although some erroneous values are generated.
The accuracy of the wheel speedometer is critical to the results of the study. Wheel diameter was determined from the running track experiments using the lap distance and the total number of revolutions over
Conclusion
The GPS is accurate for the determination of speed over ground (about 10 times more accurate than a car odometer) when moving at relatively constant speed in straight lines and is competent at determining speed on curved paths, although some overshoot does occur during transitions. Absolute error increases slightly at higher speeds but in percentage terms is less. In addition, when the system is tested under conditions of sudden changes in speed some inadequacies become evident. The system
Acknowledgements
We thank the Horserace Betting Levy Board for funding THW and the BBSRC for contributing to the work carried out here.
References (16)
- et al.
Global Positioning System (GPS) location accuracy improvement due to selective availability removal
Comptes Rendus Biologies
(2002) - et al.
The use of Doppler shift radar to monitor physiological and drug induced activity patterns in the rat
Pharmacology Biochemistry and Behaviour
(1979) - et al.
A GPS logger and software for analysis of homing in pigeons and small mammals
Physiology and Behaviour
(2000) - et al.
High-precision satellite positioning system as a new tool to study the biomechanics of human locomotion
Journal of Biomechanics
(2000) - et al.
Fast locomotion of some African ungulates
Journal of Zoology, London
(1977) - et al.
The prediction of speed and incline in outdoor running in humans using accelerometry
Medicine and Science in Sports and Exercise
(1999) - Kalman, R.E., 1960. A new approach to linear filtering and prediction problems. Transaction of the American Society of...
- et al.
The use of dGPS and simultaneous metabolic measurements during orienteering
Medicine and Science in Sports and Exercise
(2001)
Cited by (166)
Intercomparison of surface velocimetry techniques for drone-based marine current characterization
2024, Estuarine, Coastal and Shelf ScienceDetection of AIS messages falsifications and spoofing by checking messages compliance with TDMA protocol
2023, Digital Signal Processing: A Review JournalStride frequency derived from GPS speed fluctuations in galloping horses
2022, Journal of BiomechanicsEstimation of energy expenditure in adults with accelerometry and heart rate
2022, Science and SportsDrone-based large-scale particle image velocimetry applied to tidal stream energy resource assessment
2022, Renewable EnergyCitation Excerpt :Latitude and longitude were converted to UTM easting and northings, and then change in position calculated and converted to speed using the recorded timesteps. The accuracy of position or speed estimates was not assessed for these GPS units: the units typically have absolute positional accuracy of 3–6 m, however relative positional accuracy over the short-term is much higher [87]; previous studies using similar units have suggested that the majority of speed errors are within ±0.2 m s−1 [88–90] and a mean error of 0.01 m s−1 has been reported [90]. Presence of drifters in the field of view equates to artificial seeding which might have been expected to skew results; however comparison between videos with and without drifters showed no difference [91].
The inter-device reliability of global navigation satellite systems during team sport movement across multiple days
2022, Journal of Science and Medicine in SportCitation Excerpt :External factors (e.g. cloud cover, temperature, and location) in which data are collected generally change between sessions, which may influence the satellite number and horizontal dilution of precision (HDOP) of the devices. Satellite number is the number of satellites the GNSS receiver is interacting with whilst calculating the position of the GNSS unit, and therefore a greater number of satellites are beneficial.13 HDOP is the geographical positioning of satellites relative to each other and the receiver.