# Analysis of the Influence of Flood on the L4 Combination Observation of GPS and GLONASS Satellites
###### Abstract  
With global warming, extreme weather such as floods and waterlogging occurs more frequently and seriously in recent years. During the flood, the surrounding environment of the GNSS (Global Navigation Satellite System) station will change as the volume of water increases. Considering the multipath error is directly relevant to the observation environment, thus, the influence of flood on the L4 combination observation (a geometry-free ionosphere-free linear combination of carrier phase) which is related to the multipath error of GPS (Global Positioning System) and GLONASS satellites is investigated in depth. In addition, the ground track repetition periods of GPS and GLONASS satellites are analyzed in the sky plot to illustrate the rationality of chosen reference day. Based on the results of the satellite sky plot, one and eight days are adopted to demonstrate the influence of flood on L4 combination observation for GPS and GLONASS satellites, respectively. Real data sets collected at the ZHNZ GNSS observation station during the flood from DOY (Day of Year) 193 to DOY 204, 2021 are used. Experimental results show that the flood has a significant impact on the L4 combination observation of GPS and GLONASS satellites, and the fluctuation of L4 under flood performs much larger than that of without flood. For GPS satellites, the maximum RMS (root mean square) increase rate of L4 under flood is approximately 186.6% on the G31 satellite. Even for the minimum RMS increase rate, it can reach approximately 23.52%, which is the G02 satellite. Moreover, the average RMS increase rate of GPS and GLONASS satellites can reach approximately 109.53% and 43.65%, respectively. In addition, the influence of rainfall and hardware device are also investigated, which can further demonstrate that the fluctuation of L4 is mainly caused by the flood but not by the rainfall and hardware device elements. Thus, based on the above results, the influence of flood on L4 observation should be taken into account during the applications of L4 used, such as the retrieval of soil moisture and vegetation water content based on GNSS L4 combination observations  
G 2023 2  
multipath error is caused by different transmission paths of signal, which is mainly related to the reflection and refraction coefficient of objects around the station [6; 7; 8]. The influence of water on GPS (Global Positioning System) pseudorange multipath has been analyzed by [PERSON] et al. and the results show that the multipath error in the water environment performs more seriously compared with without water [9]. After investigating the relationship between multipath error and common surfaces at normal incidences on GPS L1 frequency, [PERSON] indicated that the reflection coefficient of water is approximately three times greater than that of dry soil, and the attenuation factor of water is approximately 9.45 dB larger than that of dry soil [10]. [PERSON] et al. demonstrated the relationship between flood and multipath error in theory, both theoretical and experimental results show that the multipath error performs more seriously during the flood compared with dry soil [11]. Based on the above analysis, the characteristics of multipath error are closely related to the environment of observation and will be influenced during the flood. Thus, the GNSS observation related to multipath error should be re-evaluated and analyzed during the flood. In the field of GNSS, although original pseudorange and carrier phase observations are largely affected by the multipath error, it is difficult to extract multipath errors from these two observations [12]. Thus, considering the characteristics of multipath error, two types of observation named CNR (Carrier-to-noise Ratio) and L4 are usually used to reflect the multipath error in the field of GNSS. The CNR can be obtained directly from the observation file, which is denoted as SNR (Signal-to-noise Ratio) in the RINEX (Receiver Independent Exchange Format) file [13].