# Development of a Multi-Sensor GNSS-IoT System for Precise Water Surface Elevation Measurement
## 1 Introduction
Their 63 lake surveys and additional river profiles demonstrated that the WaSP system provides sufficient accuracy for validating the decimetre-level precision of both SWOT and AirSWOT. [PERSON] and [PERSON] [19] explored the use of low-cost multi-GNSS single-frequency RTK averaging for marine applications, focusing on its ability to achieve accurate stationary positioning and vertical tide measurements. The vertical component of their Otago Harbour trials achieved a \(\leq\)0.016 m standard deviation (STD) over a 7.3 km baseline and a \(\leq\)0.022 m STD over a 27.4 km baseline. Their research also demonstrated the benefits of leveraging observations from multiple GNSS constellations instead of just GPS. [PERSON] et al. [20] integrated Ginan, an open-source GNSS toolkit developed by Geoscience Australia, with the Dark-water Inland Observatory Network at Googong reservoir, demonstrating real-time and post-processed PPP workflows. While real-time PPP was impacted by process noise and BeiDou SSR reliability, post-processed PPP achieved accuracy of 4.8 cm overall and 2.2 cm for daily average solutions. [PERSON] et al. [21] analysed long-term (2007-2020) GNSS and tide gauge data over French Polynesia to monitor absolute vertical land motions and absolute sea level (ASL) changes. Their study provided critical insights into the regional variations in sea level rise and land subsidence, contributing to a better understanding of the dynamics affecting coastal areas with long-term data. The GNSS data processing method was PPP and conducted by using PANDA software [22]. Despite the previously demonstrated effectiveness of GNSS buoys for WSE estimation, there are several challenges with their practical operation and maintenance and in terms of building this into a low-cost package that is commercially viable for large-scale deployment. These problems include accuracy and precision [16; 19], signal interference [18], power and maintenance [16], data fusion and complexity [15; 23]. These challenges underscore the need for ongoing research and development to enhance the reliability, accuracy and usability of low-cost GNSS technologies for monitoring WSE. Recent advances in low-cost GNSS and Internet of Things (IoT) technologies have enabled the development of compact automatic long-term surface displacement monitoring [24; 25; 26]. Building on these innovations, we designed a multi-sensor floating platform that integrates GNSS, ultrasonic ranging and accelerometer data to achieve high-precision WSE measurements. In this study, we develop a multi-sensor GNSS-IoT system (hereafter, the \"system\") to provide low-latency WSE measurements with sufficient accuracy and temporal resolution that will be useful for SWOT product validation and other hydrological applications. We evaluate the accuracy by comparing its measurements with reference data while analysing key error sources, such as ultrasonic sensor performance, and assessing its robustness under different weather conditions. In this contribution, we first introduce the details of the integrated multi-sensor GNSS-IoT water level measuring platform, including its components, mathematical model and error budget. Then, we describe an experiment at the Googong reservoir, Australia, that we designed to test the accuracy of proposed water level measurement platform. Following discussion of the system's overall performance, we conclude the main findings and propose future work.