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Recent advances in millimeter-wave radar component technology have led to the development of a new generation of radars above 30 GHz. These radars are more reliable and more versatile than previous millimeter-wave radars and consequently are gaining popularity among the remote sensing community, particularly among atmospheric scientists. High frequency operation allows the use of smaller antennas for the same spatial resolution, decreases the far field distance for the same beamwidth, and, during atmospheric measurements, significantly increases the backscattering efficiency of small particles. This report summarizes recent advances and the need for future improvements in millimeter-wave radar technology.
II. Millimeter-Wave Technology
The military was the first to take advantage of the small, high gain antennas at high operating frequency by developing millimeter-wave radars for aircraft in the 1950s and 60s. This new technology was quickly adopted by atmospheric scientists to study clouds and precipitation, but those early radars were limited to simple backscatter measurements and plagued by hardware problems. Research and development of millimeter-wave radar components continued, and by the early 1980’s technology was available to build reliable radars with Doppler and polarimetric capability. The technology that made these next generation millimeter-wave radars possible included: 1) compact Extended Interaction Klystron Amplifiers (EIKAs) with kW-level pulses suitable for operation in the 35 and 95-GHz atmospheric transmission window; 2) low and medium-power low-noise solid-state oscillators and amplifiers; 3) low-loss, high-power switches, which allow fast T/R and polarization switching.
Since 1990, at least 24 millimeter-wave cloud radars have been built worldwide for atmospheric science applications. Many of these radars operate near 35 GHz, where atmospheric attenuation due to water vapor and liquid water is considerably reduced as compared to attenuation near 95 GHz. Cloud radars operating at 35 GHz generally employ comparatively energetic transmitters (10-100W average power) and large antennas (1-3 meters diameter). 95 GHz cloud radars, which benefit from the increase in small droplet scattering efficiency, typically operate with less average power (0.1 to 15 W) and smaller antennas (0.3 to 2 m diameter). The most sensitive systems can detect clouds with reflectivities of -50 dBZ at several km range.
In addition to designs based on EIKA powered transmitters, solid-state 95 GHz FMCW radars are now being built for applications requiring somewhat less demanding sensitivity. FMCW systems usually require separate transmit and receive antennas to avoid saturating the receiver. The modest transmit power of these systems, typically 0.1-1 W, is offset by the two-antenna design, which eliminates 5-6 dB of T/R network losses. Using traditional FMCW processing techniques, these radars can achieve very fine range resolution (less than 1 meter) without the need for high bandwidth digitization.
At the time of this writing (2004), millimeter-wave cloud radars are being used exclusively in research applications. Radars such as the Department of Energy’s 35 GHz MMCR, http://www.arm.gov/docs/instruments/static/mmcr.html, are operated continuously at multiple sites, gathering statistics on the spatial and seasonal distribution of clouds in support of DOE’s Atmospheric Radiation Measurement program. Others, such as the University of Wyoming airborne W-band cloud radar, http://www-das.uwyo.edu/wcr/, are deployed in field campaigns that require high spatial resolution sampling of clouds with a focus on cloud structure, dynamics and composition.
At present, no observational systems exist to measure the vertical distribution of clouds on a global basis. The first system planned for such measurements is CloudSat, http://cloudsat.atmos.colostate.edu/, an experimental 95 GHz spaceborne cloud radar with a launch planned for no earlier than January 2005. CloudSat’s primary mission is to improve the parameterization of clouds in global circulation models.
The relatively high attenuation rate experienced by millimeter-waves propagating through liquid water clouds can be exploited by multi-frequency radars intended to measure cloud liquid water content. NASA Glenn has recently acquired a three frequency (10, 35 and 95 GHz) wing-pod mounted cloud radar in support of aircraft icing avoidance research. Also, the National Research Council of Canada is developing an airborne side-pod mounted 10 and 94 GHz polarimetric Doppler radar system. Differences in the backscattered power at multiple frequencies will be used to map the distribution of liquid water.