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This paper describes the development and application of a highly integrated Global Positioning System (GPS) receiver that employs reflected GPS signals to measure the floodwater levels, sea levels, and soil moisture of riverbeds. Both right- and left-hand circular polarization antennas are employed to simultaneously obtain direct and reflected signals. The objective of this paper is to use the carrier-phase Doppler shifts reflectivity of L1 and L2 band reflected signals and direct signals to determine the floodwater and sea levels in riverbeds, as well as to distinguish wet soil from bare soil. In monitoring coastal tidal currents, water levels, and floodwater levels, the reflection heights of this system are accurate to within 29 ~ 31 cm. Furthermore, the currents at reflection points are estimated using differential carrier Doppler shifts and a coordinate rotation correction model, which provide velocity vectors for flows speed of approximately 5 ~ 265 cm/s.