Department of Electrical & Computer Engineering
University of Massachusetts - Amherst
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. The 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, however, 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 high-power Extended Interaction Klystron Amplifiers (EIA-s) 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) solid-state, low-loss, high-power switches, which allow fast T/R and polarization switching.
The radars using these new components are compact, sensitive and can measure the full polarimetric scattering characteristics and Doppler velocity spectrum of the scatterers. Although, their primary civilian use is with the atmospheric research community, there are ongoing studies which aim to remotely characterize snow cover, foliage, the ocean surface and to monitor ship traffic in coastal regions. One of the recent successful applications of 95 GHz millimeter-wave radars was to cloud microphysics research as part of a joint Univ. of Massachusetts and Univ. of Wyoming airborne experiment. The compact, portable, light weight U. Mass. 95 GHz cloud radar is ideal for airborne installation, and with a small, high gain antenna and short far-field distance is able to resolve spatial variations in clouds and precipitation on the 15 m scale within a range of 1 km. University of Wyoming recently acquired their own 95 GHz cloud radar which soon will be available for the atmospheric research community as part of the pool of NSF facility instruments.
III. Recommendations for Future Work
In spite of the decade long development and numerous successful airborne and ground-based experiments with millimeter-wave radars with EIA transmitters, their full capability is still unexplored. Their resolution, range and sensitivity is often limited by the speed of their data acquisition systems; furthermore, techniques such as pulse compression and frequency hopping have not been widely implemented. Pulse compression would allow ultra high resolution (<10m) measurements while maintaining good sensitivity, and frequency hopping would reduce the dwell time that is necessary to estimate the radar parameters. This would improve along track resolution during airborne measurements and allow faster scanning.
Multiple frequency millimeter-wave observation of the atmosphere has recently become possible with the University of Massachusetts Cloud Profiler Radar System (CPRS). CPRS can image clouds and precipitation from the ground using a dual-frequency antenna that allows co-located polarimetric measurements at 33-GHz and 95-GHz. Millimeter-wave radars could complement low frequency radars by resolving spatial variations on a finer scale, while measuring the difference between radar parameters at multiple frequencies.
The maximum range of millimeter wave radars is often severely degraded by absorption and scattering by water drops and vapor in the atmosphere. From space, however, millimeter-wave radars could penetrate the upper, dry region of the atmosphere and could detect clouds that might be obscured by attenuation when observed from the ground. The distribution of ice and liquid water in clouds is a critical parameter in the Earth's radiation budget, yet no existing spaceborne sensors can measure the vertical structure of clouds. Millimeter-wave radars would be well suited for this task due to their low weight, low power consumption and high sensitivity. Potential opportunities for a spaceborne millimeter-wave cloud radar include a Japanese J-1 rocket mission as early as 1997, the Japanese Experiment Module on the Space Station around the year 2000, or a future NASA Delta-class mission.
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