# Real-Time Water Level Monitoring Based on GNSS Dual-Antenna Attitude Measurement
## 1 Introduction  
The Global Navigation Satellite System (GNSS) not only provides navigation, positioning, and timing services to users worldwide but also continuously transmits massive high temporal-spatial resolution and high-precision microwave signals (especially the 1-2 GHz L-band signals) to the Earth's surface [1]. Through GNSS studies, researchers have discovered that GNSS-reflected signals carry characteristic information about the reflective surface. Using GNSS-reflected signals, it is possible to estimate and retrieve the physical characteristics of reflective surfaces without a special radar transmitter [2]. This has led to the emergence of GNSS-R, an interdisciplinary field of GNSS and remote sensing. GNSS-R offers benefits such as low cost, low power consumption, and high temporal resolution compared to traditional satellite microwave remote sensing. In recent decades, with the continuous improvement of the theoretical architecture system, signal processing technology, and parameter retrieval model of GNSS-R, it has made some progress in fields such as ocean surface wind [3], ocean altimetry [4], sea ice [5], soil moisture [6], etc. Water level monitoring has always been crucial in hydrology and water resources, and water conservancy engineering management. In recent years, traditional manual water gauges have been replaced with float-type, pressure-type, ultrasonic, and other types of water level gauges, resulting in significant improvements in automation and measurement accuracy to meet general water level monitoring requirements. However, these methods have low coverage, particularly in complex terrains, such as high slopes in reservoir areas and harsh weather conditions, which can hinder their accuracy, reliability, and availability [7]. To address this issue, GNSS-R provides a new approach to achieve high coverage water level monitoring with its unique advantages. In 1993, [PERSON] first proposed the concept of Passive Reflectometry and Interferometry System (PARIS) and demonstrated the feasibility of using GPS reflection signals for altimetry [8]. Subsequently, various scholars conducted experimental studies to ascertain the viability of using GNSS-R technology for measuring water levels [9; 10; 11]. After over two decades of development, GNSS-R altimetry has been proven to be effective for water level retrieval in oceans [12], lakes [13], rivers [14], and reservoirs [15]. Currently, there are three commonly used methods in GNSS-R altimetry. The first is code-delay altimetry, which determines a time delay between the direct and reflected signals' peaks and then retrieves the water level based on this delay. The C/A code of GPS and the BII code of BDS can be used to invert sea surface height with high spatial and temporal resolution [16]. However, due to the relatively narrow bandwidth of GNSS signals, accuracy using this method is limited to the meter level [17]. The second altimetry method, proposed by [PERSON] et al. in 2013, utilizes signal-to-noise (SNR) ratio data. This method retrieves water level by processing signal-to-noise ratio data containing interference information from GNSS direct and reflected signals [18]. Numerous studies have demonstrated that the oscillation frequency of multipath in SNR data correlates with the height of the antenna above the reflecting surface [19; 20; 21]. They achieved millimeter-level accuracy in water level inversion for the first time by combining data from multiple stations and signals [22]. However, this method requires long continuous recording time and can only use GNSS signals from satellites with low elevation angles. As a result, its temporal resolution is low [23]. The third method is carrier phase delay altimetry, which confirms the path delay between direct and reflected signals using carrier phase observations [24]. In comparison to SNR analysis, phase altimetry not only enables centimeter-level sea-level altitude inversion but also provides high temporal resolution [25].