# Development of a Multi-Sensor GNSS-IoT System for Precise Water Surface Elevation Measurement
## 1 Introduction  
In December 2022, the American and French space agencies, NASA and CNES, launched the Surface Water and Ocean Topography (SWOT) satellite mission. This innovative new imaging radar mission hosts a bi-static Ka-band Synthetic Aperture Radar interferometer that is being used for the first time to generate maps of ocean and in-land water topography with unprecedented spatial resolution [1, 2, 3]. Early analysis of SWOT data products for Australia by [PERSON] et al. found that water surface elevation (WSE) for on-farm water storages as small as 0.17 km\({}^{2}\) can be resolved [4]. This opens up new possibilities in remote monitoring of in-land water usage and storage by water managers. However, if these observations are to be used in compliance, trust in the remotely derived SWOT measurements needs to be established by undertaking robust ongoing validation with in situ measurements for a range of water body types and environments. Mission requirements for the WSE measurements from SWOT are 10 cm accuracy or better (1\(\sigma\)) for open-water areas larger than 1 km\({}^{2}\), and water surface slope (WSS) accuracy of 1.7 cm/km or better (1\(\sigma\)) over a maximum flow distance of 10 km [5, 6]. To assess theperformance of SWOT against these requirements, in situ validation measurements with centimetric accuracy are required. A range of different in situ instruments can be used to measure WSE and therefore validate satellite-derived measurements. These include manual readings, pressure gauges and acoustic gauges that have been widely used but often come with limitations in precision, response time, spatial coverage and cost [7]. The advent of Global Navigation Satellite System (GNSS) technology has opened new possibilities for near-real-time, high-precision WSE and WSS monitoring. There are two main methods to measure WSE using GNSS technology: one utilizes GNSS radio signals reflected from the water surface, called the GNSS Reflectometry technique (GNSS-R), and the other deploys GNSS instruments on floating buoys and platforms to directly measure water surface vertical positioning. GNSS-R can calculate WSE based on either signal-to-noise ratio (SNR) or phase delay observations [8; 9; 10]. GNSS-R has been successfully demonstrated to retrieve WSE data on various types of water bodies, including those of seas, lakes, rivers and reservoirs [11; 12; 13]. While these studies have demonstrated that GNSS-R technology can achieve a mm to cm level precision under calm water conditions, it has limitations including the undefined datum of measurements and the requirement of installation on a fixed location. On the other hand, this need for precision and flexibility has driven the development of various GNSS buoy systems. [PERSON] et al. [14] explored the use of GPS offshore buoys and tide gauge benchmark control by using GPS. [PERSON] et al. [15] present the development and testing of a GNSS buoy specifically designed to monitor WSE in estuaries and coastal areas, providing an efficient and accurate method for capturing WSE changes. The authors caution against using Precise Point Positioning (PPP) for real-time monitoring of tides and ocean waves due to its long convergence times, which render it unsuitable for such applications. [PERSON] et al. [16] developed a low-cost (\(\sim\)GBP 300) GNSS buoy for measuring coastal sea levels, demonstrating the potential for precise WSE measurements to achieve a mean difference of RMSE 1.4 cm between the GNSS buoy and reference tide gauge. The data processing procedure for [PERSON] et al. 's experiment is based on the post-processing kinematic (PPK) method using RTKLIB software [17]. Prior to the satellite SWOT mission, [PERSON] et al. [18] developed and deployed the GNSS-mounted floating Water Surface Profiler (WaSP) system, which efficiently and accurately measures WSE and WSS in various surface water environments using Precise Point Positioning (PPP). This system was instrumental in validating the performance of the experimental airborne prototype AirSWOT.