Inexpensive FM-CW Radar for Boundary-Layer Precipitation Studies

A vertically pointing C-band (5.8-GHz) frequency-modulated (FM) continuous-wave Doppler radar was developed for less than U.S. $6000, which can estimate the Doppler velocity power spectra from 20 to 300 m above the ground with a 5-m resolution. Three key design elements kept the costs low. First, the data acquisition system (DAS) commanded a direct digital synthesizer to start the FM sweep and then collected all voltage samples. This architecture insured phase coherence between the transmit FM sweep and the sampled voltages, which is critical for the Doppler processing that detects target phase changes over several successive FM sweeps. Second, a 5-m range resolution was achieved by increasing the FM bandwidth to over 36 MHz and decreasing the DAS sample rate to 500 kHz, enabling the use of an inexpensive DAS. Third, the Doppler processing at each range was achieved using two fast Fourier transforms (FFTs), i.e., a range FFT converted FM sweep voltages into in-phase and quadrature voltages I and Q at each range, and a Doppler FFT converted I and Q voltages from consecutive sweeps into the Doppler velocity power spectra. Observations show precipitation variability at 1-s and 5-m resolutions in the lowest 300 m of the atmosphere.