Bias and Uncertainty in the Time, Position, and Speed Data of Consumer-Grade GPS Devices

The goal of this study is to quantify the accuracy (bias) and precision (uncertainty) of the time, position, and speed data acquired by a range of consumer-grade devices (4 bike computers, 5 watches, 1 application on 3 smart phones, and a camera) that access Global Positioning System (GPS) satellite signals. We acquired data at each device’s maximum sampling rate (typically 1 Hz) during 207 minutes (twelve sessions of ~17 min) over 61.6 km of road cycling. The time and position data from these devices were compared to real-time kinematic (RTK) data acquired using a differential GPS system, and speed data from these devices were compared to a high-resolution wheel speed sensor synchronized to the RTK data in order to statistically estimate the bias and 95th percentile confidence intervals of the uncertainty of the devices’ data. Overall, we found the position and speed data from the devices generally lagged the reference by 4 s or less, although the lags between the speed and position data within a device were less (0.0 to 1.2 s) and more precise. We found small position biases (0.1 to 1.4 m), although the major axis of the 95th percentile confidence ellipses of the position uncertainties ranged from ±3.4 to ±7.2 m across the devices. The speed biases were also small (-0.6 to 0.0 m/s) and had 95th percentile confidence intervals that were between 0.35 and 1.04 m/s wide. These findings help establish the accuracy and uncertainty across a range of consumer-grade GPS-enabled devices and to probabilistically interpret these data for collision reconstruction purposes.