# Real-Time Water Level Monitoring Based on GNSS Dual-Antenna Attitude Measurement
## 1 Introduction
However, when the sea surface is rough, the continuity of the reflected signal phase is disrupted, making it difficult to track [26]. In contrast to complex sea conditions, the water surface of inland water bodies is relatively calm. This makes the characteristics of the reflected signals more stable and their phase information easier to measure accurately. As a result, this paper focuses on the carrier phase altimetry technique. The key to phase altimetry is resolving the integer ambiguity. However, this is a complex problem. Scholars have proposed methods to avoid calculating the integer ambiguity [27; 28; 29], and these methods can achieve a temporal resolution of up to 1 min. Nonetheless, during emergency situations such as flash floods, water authorities require even higher temporal resolution of water level data to effectively develop emergency response measures. [PERSON] and colleagues achieved 1 Hz temporal resolution and 1 cm accuracy in water level monitoring by introducing an atomic clock [30]. However, the high cost of atomic clocks limits their practical application. To address these issues, this paper investigates real-time water level monitoring using GNSS dual-antenna attitude measurement. In recent years, GNSS dual-antenna attitude measurement has played an important role in real-time carrier attitude measurement with the advantages of low cost, high accuracy, short initialization time and high reliability. GNSS dual-antenna attitude measurement determines the relative direction and distance between two antennas by measuring the phase difference between different GNSS signals [31]. So GNSS dual antenna attitude measurement algorithm can be used to achieve real-time monitoring of water level. When the water surface is rough, the reflected signal may lose its lock and exhibit cycle slips phenomenon. In GNSS-R carrier phase altimetry, cycle slips can significantly impact the accuracy of water level monitoring [32]. However, detecting and correcting cycle slips is time-consuming and not well-suited for real-time applications. GNSS single-epoch calculation is a positioning technique that processes satellite measurement data at a single time point. GNSS single-epoch calculation is not affected by cycle slips and allows for immediate reinitialization after satellite signal loss. In harsh observation environments, single-epoch calculation offers higher data availability compared to multiple-epoch data, enabling the efficient utilization of observation data. To avoid the impact of cycle slips on real-time applications, this study introduces a single-epoch calculation method. When the reflected signal enters the right-hand circularly polarized (RHCP) antenna, signal crosstalk will occur [33]. [PERSON] installed signal clapboards between the RHCP and left-hand circular polarized (LHCP) antennas to mitigate the impact of signal crosstalk on the carrier phase [34]. Inspired by his work, we installed a signal clapboard at the midpoint of the connection axis between the RHCP and LHCP antennas. The primary aim of this paper is to enhance the temporal resolution of GNSS-R phase altimetry, thereby facilitating its widespread implementation in practical scenarios. We present a novel approach for real-time water level monitoring using GNSS dual-antenna attitude measurement and develop a model to retrieve water level based on baseline vector. For the first time, this approach determines the position of the virtual antenna using the attitude angle information of the baseline vector. This approach offers a new idea for monitoring water levels in remote areas, including key lakes and rivers.