A New Method to Monitor Flood Dynamics Using GPS Dual-Frequency CNR and a Strength-Based Threshold Constraint Strategy

The strength of a GPS carrier-to-noise ratio (CNR) signal is closely influenced by the multipath effect. This effect becomes more pronounced during flood events, as the reflection coefficient of water is significantly higher than that of dry soil. Consequently, the CNR measurements of GPS signals are impacted by floods. Based on this theory, the fluctuation of the CNR during a flood can be used to accurately monitor the process of a flood from occurrence to recession. Considering that the strength of the CNR largely depends on the satellite and frequency, and the characteristics of the influence of a flood on the CNR are different for each frequency, a new method based on the GPS dual-frequency direct signal CNR and the strength constraint threshold strategy was developed to increase the accuracy of the flood dynamics monitoring process. By using 64 MGEX (Multi-GNSS Experiment) stations distributed globally, an accurate direct-signal CNR threshold model of GPS dual-frequency was established. The threshold model demonstrated that the average difference of the direct-signal CNR, which is larger than 45 dB-Hz, between adjacent days at GPS L1 and L2 frequencies is 0.0659 dB-Hz and 0.0661 dB-Hz, respectively. Moreover, GPS real datasets in Zhengzhou city, China, from DOY (day of year) 199 to DOY 203, 2021, were collected to assess the proposed method. Based on the fluctuation of the direct-signal CNR threshold, the experimental results show that the flood appeared at about 16:04 PM on DOY 200, 2021, reached a peak at approximately 5:05 AM on DOY 202, and totally subsided at about 8:54 AM on DOY 202. Thus, the experiment results reveal that the proposed method accurately monitors the entire process of a flood from occurrence to recession, which provides valuable insights into operational flood dynamics, warning, and monitoring based on the GPS technique.