Tra High Frequency River Surface Dynamics Parameters Radar Design And Experiment

To fully understand and utilize freshwater resources provides a strong support to our national security and economy. Conventional hydrological equipments used in freshwater monitoring adopt contact measurement, which is inconvenient to install and maintain. Microwave remote sensing will easily suffer the impact of bad weather, such as rain, cloud, fog and so on. Therefore, this article proposes an Ultra High Frequency Surface Dynamics parameters Radar (URSDR) to compensate the above disadvantages.UHF radars utilize the Doppler shift by Bragg scattering of radio waves to detect the river surface flow radial velocity. The URSDR system adopts Frequency Modulated Continuous Wave (FMCW) mechanism, utilizing the frequency and phase of echo signals to extract dynamics parameters. The center frequency of the URSDR system is340MHz, and the bandwidth is15.36MHz. Besides, the sweeping period is0.1032s, and a coherent integration period contains512frame periods. Based on the waveform parameters above, the range resolution of URSDR is9.77m and the velocity resolution is0.0084m/s.The URSDR system is composed of antennas, transmitter, receiver, computer host and linear power supply. The URSDR receiver is an all-digital radar receiver, echo signal is mixed using the local oscillator generated by FPGA on the digital board, and radio frequency carrier is directly moved to zero frequency. Compared with traditional super-heterodyne radar receiver, it eliminates mixer on the analog board, therefore simplifies system architecture and enhances system dynamic range. The transmitter amplifies the linear frequency modulated signal to the desired power. The antennas adopt "one transmitting, six receiving" method, transmitting and receiving antennas are replaceable yagis. The computer is responsible for initial parameters configuration, data storage and so on. Linear power supplies high quality power for the system.URSDR receiver is composed of analog board, frequency synthesis board and digital board. In the analog board, proper amplifiers, filters and switching circuits are designed to realize high sensitivity, high gain, wide dynamic range as well as good isolation. In the frequency synthesis board,80MHz high stable, low jitter clock is selected as clock source. Phase locked loop (PLL) device provides983.04MHz clock reference signal. Direct digital synthesis (DDS) device generates desired FMCW signal. FPGA stores configuration parameters in RAMs (Read random access memories) in advance, and writes PLL and DDS following specific timing sequence to accomplish configuration. In the digital board, clock management device distributes81.92MHz signal from PLL to FPGA (Field Programmable Gate Array) as system clock and six ADCs (Analog-Digital Converter) as sampling clock. ADC device is equipped with wide analog bandwidth, high samling rate, high resolution, and is appropriate for radio bandpass sampling of echo signal which is amplified and filtered by analog board. The FPGA device implements digital down conversion processing to the sampling signal:CORDIC (COordinate Rotate Digital Computer) algorithm realizes orthogonal demodulation, CIC (Cascaded Integrator Comb) decimation filter realizes high-rate decimation and filterting, DFT (Discrete Fourier Transform) module transforms the processed signal from time domain to frequency domain. The USB device transfers data to the computer host with a high speed. The computer host programs perform double FFT processing to extract target range and velocity information.In the URSDR close-loop test, the echo signal from target is simulated by delaying RF transmitting signal (-20dBm) for a certain period of time. Receiver performs demodulation, decimation and filtering. Range spectrum is obtained after the first FFT, different frequency offset corresponds to different range cell, and phase stability is within0.08degree, amplitude stability is within0.0025dB. Range-Doppler spectrum is obtained after the second FFT. As the simulation target has no movement, Doppler peak should appear in zero frequency.In March2013, URSDR system completes verification test on Wuhan East Lake, and successfully verified the basic law electromagnetic wave scattering effect from river surface. In July2013, URSDR system completes surface current velocity measurement experiment on Wuhan Yangtze River. We build a river echo spectrum model, analyze and compare the measured data and the simulation results, discuss the relationship between radar maximum dectection range and flow velocity, plot the radial current map, which verify the correctness of the system.