Abstract

A low cost (US$30) consumer grade GPS receiver with a sideways mounted antenna has been applied to measure tidal water levels at a meso-tidal coastal site using an interferometric reflectometry approach. The proof of concept system was installed approximately 16m above mean sea level in close proximity to a conventional bubbler tide gauge that provided validation data. The received Signal to Noise Ratios (SNR) for the satellites in view were recorded for several months during two successive years and the observed frequencies of the interferometric oscillations used to calculate the difference in elevation between the receiver and the water surface. Comparisons with concurrent and historic in situ tide gauge data at the site initially helped to identify a calibration issue with the in situ gauge. The GPS based measurements were shown to be in excellent agreement with the corrected in situ gauge, exhibiting a Root Mean Square difference of 5.7cm over a tidal range exceeding 3m at spring tides and a daily averaged RMS of 1.7cm. The SNR data from the low cost GPS receivers are shown to provide significantly higher quality data for this purpose compared with high end geodetic grade receivers at similar sites. This low cost, widely available technology has the potential to be applied globally for monitoring water levels in a wide variety of circumstances and applications that would otherwise be cost or situation prohibitive. It could also be applied as an independent cross check and quality control measure for conventional water level gauges.

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