![]() ![]() Their installations are often restricted to a specific structure close to the river or sea such as a stilling well, a mast or a bridge, where they may be exposed to vandalism or theft. The commercial versions of these sensors may be costly, ranging from few hundreds (e.g., pressure gauge) to a few thousands of U.S. Measurements of sea surface and river water level using ground-based sensors conventionally rely on contact methods, such as traditional float and stilling well gauges (Noye, 1974) and bubbler pressure gauges (Pugh, 1972), or proximal sensing gauges, such as acoustic (Boon & Brubaker, 2008 Gill & Mero, 1990), radar (Costa et al., 2006 Woodworth & Smith, 2003) and camera (Chandler et al., 2002 Eltner et al., 2018 Kim et al., 2011) sensors. However, the coarse temporal resolution of satellite altimetry missions such as SWOT and the requirement for monitoring smaller rivers and tributaries underline the significance of in situ monitoring sites. The SWOT interferometric swath will pass over a given location two or three times every 21-day orbital cycle (Tuozzolo et al., 2019). SWOT will provide global maps of water surface elevation, slope and inundated areas for rivers wider than 100 m (Biancamaria et al., 2016). The upcoming NASA's Surface Water Ocean Topography (SWOT) satellite mission will collect high-accuracy measurements of inland surface water elevation (10 cm error for 1 km 2 areas) at unprecedented scales (10â70 m resolution) using Ka-band interferometric synthetic aperture radar. While satellite remote sensing techniques have been utilized to monitor oceanic and land surface water with unprecedented global coverage, their measurements are associated with moderate uncertainties and temporal resolution (>7-day return) (Escudier et al., 2017 Jarihani et al., 2013). In particular, stream flow monitoring gauges have been declining sharply since the mid 1980s due to high maintenance cost, funding shortfalls and (geo-) political constraints (Hannah et al., 2011 Reid et al., 2019 Ruhi et al., 2016, 2018). Ground-based measurements are still scarce in many regions. One of the challenges for hydrologists and environmental scientists is the need to obtain and sustain in situ water level measurements for calibrating and improving forecast models, validating satellite and airborne data products, and developing early-warning flood systems. These instruments are financially ( ![]() However, scientific-grade or geodetic GNSS instruments are expensive, which is a limiting factor for their prompt and more widespread deployment as a dedicated environmental sensor. Although reflectometry is not the primary application of Global Positioning System (GPS) and similar Global Navigation Satellite Systems (GNSS), fast-growing GNSS tracking networks has led to the emergence of GNSS interferometric reflectometry technique for monitoring surface changes such as water level. ![]()
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