Low-Cost Handheld Global Positioning System for Measuring Surf-Zone Currents

Low-cost, handheld, L1 (1575.42 MHz) global positioning systems (GPSs) provide scientists with the opportunity to acquire position and velocity estimates at reduced expense (order of [O]$100), size (similar to cell phone), weight (O[70 g]), and engineering time. Two different low-cost, handheld GPS units and four different position-correcting configurations are evaluated here to deter-mine their practicality in measuring surf-zone currents. Three of the simpler configurations result in relative position and velocity errors of O(2 in) and O(0.5 in s(-1)) for stationary tests. Surf-zone position and velocity signal-to-noise spectral ratios for the three configurations suggest that only motions <0.01 Hz can be confidently estimated for these surf-zone systems. For the fourth configuration, a GPS handheld unit that internally records GPS carrier phase is postprocessed using more sophisticated software for position corrections to obtain absolute position and velocity estimates. Simple modifications are required to improve the position accuracy by reducing patch antenna signal multipathing. For this configuration, the absolute position error for dynamic surveys was similar to 0.40 in, and the velocity error on land relative to a survey-grade GPS system was 0.01 m s(-1). The handheld GPS was attached to a surf-zone drifter and evaluated in the field. The flow field of a rip-current system was obtained with 24 surf-zone drifters. The drifters tracked simultaneous dye releases well, verifying that the observations are valid Lagrangian estimates. Owing to the low cost and small size of the handheld GPS, a large number of drifter systems can be deployed for absolute position tracking and velocity estimates of surf-zone currents.