Abstract
Abstract. We have developed a ground-based Global Navigation Satellite System Reflectometry (GNSS-R) technique for monitoring water levels with a comparable precision to standard tide gauges (e.g. pressure transducers) but at a fraction of the cost and using commercial products that are straightforward to assemble. As opposed to using geodetic-standard antennas that have been used in previous GNSS-R literature, we use multiple co-located low-cost antennas to retrieve water levels via inverse modelling of signal-to-noise ratio data. The low-cost antennas are advantageous over geodetic-standard antennas not only because they are much less expensive (even when using multiple antennas in the same location) but also because they can be used for GNSS-R analysis over a greater range of satellite elevation angles. We validate our technique using arrays of four antennas at three test sites with variable tidal forcing and co-located operational tide gauges. The root mean square error between the GNSS-R and tide gauge measurements ranges from 0.69–1.16 cm when using all four antennas at each site. We find that using four antennas instead of a single antenna improves the precision by 30 %–50 % and preliminary analysis suggests that four appears to be the optimum number of co-located antennas. In order to obtain precise measurements, we find that it is important for the antennas to track GPS, GLONASS and Galileo satellites over a wide range of azimuth angles (at least 140∘) and elevation angles (at least 30∘). We also provide software for analysing low-cost GNSS data and obtaining GNSS-R water level measurements.
Highlights
Precise water level measurements are needed for monitoring the global oceans, lakes and rivers, all of which are vulnerable to anthropogenic climate change (Goudie, 2006; Adrian et al, 2009; Slangen et al, 2016)
There is a good agreement between the Global Navigation Satellite System Reflectometry (GNSS-R) and tide gauge measurements at all sites; for the 29 d period at Trois-Rivières the root mean square error (RMSE) is 1.02 cm, for the 17 d period at Sainte-Anne-de-Bellevue the RMSE is 0.69 cm, and for the 2 d period at Piermont the RMSE is 1.16 cm
By comparing it with nearby operational tide gauges, we found an upper limit on the precision of water level measurements of 0.69–1.16 cm at all sites, whereas previous studies using geodetic-standard antennas have found an RMSE of 2–50 cm
Summary
Precise water level measurements are needed for monitoring the global oceans, lakes and rivers, all of which are vulnerable to anthropogenic climate change (Goudie, 2006; Adrian et al, 2009; Slangen et al, 2016). The risk posed by extreme water level events is especially concerning given that current projections of globally averaged sea level rise by 2100 due to mass changes from the Antarctic ice sheet alone vary from 0 to 1.7 m (Pattyn and Morlighem, 2020), whilst projections of mass loss from Greenland’s largest outlet glaciers for the same period may be underestimated (Khan et al, 2020). Continuous records of sea level changes in the polar regions that could be used to constrain the response of ice sheets to ongoing climate change remain critically sparse (Baumann et al, 2020)
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