The Advanced Multi-Frequency Radar (AMFR) for remote sensing of clouds and precipitation

The University of Massachusetts has developed an advanced Multi- Frequency Radar (AMFR) system for studying clouds and precipitation. This mobile radar consists of three polarimetric Doppler subsystems operating at Ku-band (13.4 GHz), Ka-band (35.6 GHz) and W-band (94.92 GHz). This combination of frequency bands allows measurement of a wide range of atmospheric targets ranging from weakly reflecting clouds to strong precipitation. Beamwidths are matched at each frequency ensuring consistent sampling volume, and all systems are mounted on a programmable scanning pedestal.

AMFR employs high-power klystron amplifiers at each frquency as well as pulse compression (linear FM) in order to improve sensitivity. Without pulse compression, calculated sensitivities of the AMFR system at 1 km distance are -54 dBZ, -57 dBZ, and -52 dBZ for Ku-, Ka-, and W-bands respectively. Using pulse compression the respective sensitivities at 10 km distance are -47 dBZ, -52 dBZ, and -47 dBZ. Range sidelobe suppression level of 45 dB is achieved using a windowing function in the receiver, while sidelobe suppression up to 60 dB is possible using amplitude tapering in both transmitter and receiver. Range resolution is 30 m (5 MHz BW). The hybrid polarization scheme is employed. Circuits for tuning the transmitted polarization and a fully complex (amplitude and phase) calibration loop have been employed together for the first time.

Although multi-frequency radar remote sensing techniques are reported in the literature, they are not widely used because few multi-frequency radars are available to the science community. One exception is the 33 GHz/95 GHz UMass Cloud Profiling Radar System (CPRS), which AMFR replaces. AMFR's multi-parameter capabilities are well suited for characterizing the complex microphysics of layer clouds and precipitation processes in winter storms. AMFR will also play an important role in developing algorithms and validating measurements for an upcoming generation of space-borne radars. The frequency bands selected for AMFR match those of several sensors that have been deployed or are under development. These include the Tropical Rainfall Measuring Mission (TRMM) satellite Ku-band (13 GHz) radar, the CloudSat W-band (95 GHz) radar, and the Global Precipitation Mission (GPM) satellite radars at Ku-band and Ka-band (35 GHz).