Eric Loew

Polarization lidar at 1.54 m and observations of plumes from aerosol generators

The ability to detect relative changes in backscatter polariza- tion from a scanning high-pulse-energy lidar system at 1.54-m wave- length is demonstrated. The new capability was tested during the dis- semination of various biological aerosol simulants and other particulate emissions at the U.S. Armys Dugway Proving Ground. Results demon- strate that the lidar is sensitive to different types of aerosols, and depar- tures from the atmospheric background depolarization ratio are consis- tent with the limited amount of information available on the degree of particle sphericity. We conclude that the polarization-sensitive coatings of the beam-steering unit mirrors are presently the largest source of error and that this error is minimized when scanning with a near-zero elevation angle. This is an encouraging result for aerosol source surveillance ap- plications, where the depolarization information may be useful in deter- mining the aerosol generation mechanism or provide an additional scalar variable for use in delineating the plume from the background.

Design of a bistatic dual-Doppler radar for retrieving vector winds using one transmitter and a remote low-gain passive receiver

A bistatic dual-Doppler network consisting of an existing single transmitting pencil-beam weather radar and a remotely located, nontransmitting, passive bistatic receiver with a low-gain antenna was constructed and tested during 1993. High-quality dual Doppler vector winds were retrieved from this system. The windfields were compared with those collected with a traditional two-transmitter dual-Doppler system. Several interesting engineering challenges relating to frequency and timing synchronization were resolved in order to retrieve successfully Doppler velocities at a remote bistatic site. Frequency synchronization was achieved by using extremely stable local oscillators, linked by both Global Positioning Satellite (GPS) signals and transmitter sidelobe coupling. Both methods provided the necessary one part in 10/sup 9/ coherence necessary for calculating accurate Doppler velocities. Timing/gating synchronization with submicrosecond accuracy was achieved by using local oscillators at each site linked with GPS and sidelobe coupling. The successful testing of this system demonstrates that inexpensive and practical bistatic multiple-Doppler networks can be deployed. These systems can provide three-dimensional vector winds for a number of purposes in research, aviation, media, weather prediction, education, meteorological modeling and severe weather detection. >

Design trade-offs for airborne phased array radar for atmospheric research

This paper discusses the design options and tradeoffs of the key performance parameters, technology, and costs of dual-polarized and two-dimensional active phased array antenna for an atmospheric airborne radar system. The design proposed provides high-resolution measurements of the air motion and rainfall characteristics of very large storms that are difficult to observe with a ground-based radar system. Parameters such as antenna size, wavelength, beamwidth, transmit power, spatial resolution, along-track resolution, and polarization have been evaluated. The paper presents a performance evaluation of the radar system. Preliminary results from the antenna front-end section that corresponds to a Line Replacement Unit (LRU) are presented.

T/R modules for active phased array radars

This paper presents a summary of the three T/R modules developed for weather active phased array radars. Critical factors that influence the system performance are discussed in terms of the technology and tradeoffs that includes cost. Performance is considered for deployable dual-polarized PAR radar systems. The three T/R modules and antenna aperture Line Replacement Units (LRU) have been designed, integrated and tested. Performance and cost are derived from affordability constraints imposed by the latest technology is presented.

S-Pol’s Polarimetric Data Reveals Detailed Storm Features (and Insect Behavior)

Capsule Summary:Polarimetric radar data from NCAR’s S-Pol are given for a severe, hail-producing convective storm, and for wide-spread migrating insects.

Airborne polarimetric Doppler weather radar: Trade-offs between various engineering specifications

NCAR/EOL is investigating potential configurations for the next generation airborne phased array radar (APAR) that is capable of retrieving dynamic and microphysical characteristics of clouds and precipitation. A number of antenna aperture configurations, and signal processing algorithms will be used for realizing overall performance of the APAR. In the case of polarimetric measurements, alternate transmit with alternate receive (single channel receiver) and simultaneous reception (dual channel receiver) is considered. The above-mentioned engineering options will be evaluated for realizing an optimal APAR system suitable for measuring the high temporal and spatial resolution of Doppler and polarimetric measurements of precipitation and clouds when compared to a mechanically scanning radar.

Transmit/receive (T/R) modules architectures for dual-polarized weather phased array radars

A discussion of dual-polarized T/R module architectures and technologies for active phased array radar (PAR) for remote sensing application is presented in this paper. Three types of T/R modules developed for weather phased array radar show the evolution and trend of T/R module technology for civil applications. Factors influencing system performance are discussed in terms of the allowable technology and tradeoffs that includes cost. Performance is considered for deployable dual-polarized PAR radar systems. T/R module and antenna aperture Line Replacement Units (LRU) have been evaluated. Performance and cost are derived from affordability constraints imposed by the radar application.

On the development of a C-band active array front-end for an airborne polarimetric radar

MIT Lincoln Laboratory (MIT LL), in an effort sponsored by the National Center of Atmospheric Research (NCAR), has engaged in the development of C-Band active element phased array front-end hardware for a polarimetric radar application. This polarimetric active element scanning array (AESA) radar, when fully developed, would be deployed on a C-130 for use in Hurricane Hunter-like missions. The combination of mission and platform imposes requirements on the RF hardware that are often in conflict. These specifications include minimum range and resolution, maximum prime power, and a challenging cross-polarization isolation requirement. The RF hardware developed included a dual polarization (pol) radiator test panel, an alternating transmit simultaneous receive (ATSR) transmit/receive (T/R) switch designed and manufactured by M/A-COM Technical Solutions (MTS), a three channel T/R multichip module (MCM), and a four-pack T/R module assembly. This paper describes the mission and platform requirements, presents a notional design for the phased array aperture, discusses the design trades, and provides an overview of the design and performance of the radiator test panel, the MCM and the four-pack T/R module. This work borrowed heavily in terms of materials, technologies, and lessons learned acquired over the past five years from the MPAR Panel [1] development efforts of MIT LL and MTS co-sponsored by the Federal Aviation Administration (FAA), the Air Force, and the National Severe Storms Laboratory.

A new high-altitude airborne millimeter-wave radar for atmospheric research

A high-altitude airborne millimeter-wave radar for atmospheric research is being developed by the National Center for Atmospheric Research. The radar will be mounted on the High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER). We present simulations of the minimum detectable signal, the reflectivity accuracy, the polarimetric purity, and the expected accuracy of cloud liquid water retrievals to describe the expected performance of the radar. We also describe the phased approach to implementing the system which starts with a W-band Doppler radar, and through phases, adds pulse compression, polarimetric capability, and a second wavelength (Ka-band) radar.