This paper describes the identification of regions of ice, cloud droplets, rain, mixed-phase hydrometers, and insects in stratiform clouds using 33 and 95 GHz radar measurements of reflectivity, linear-depolarization ratio (LDR), dual-wavelength ratio, and velocity from a single-antenna radar system. First, the radar system, experiment, and data products are described. Then, regions are classified using a rule-based classifier derived primarily from LDR, velocity, and altitude. Next, a region-dependent attenuation-correction algorithm is developed to remove attenuation biases in the reflectivity estimate, and histograms of the corrected data are presented for each data product and class. The labeled regions and attenuation-corrected data are then used to train a neural net and maximum likelihood classifier. These agree with the rule-based classifier 96% and 94% of the time, respectively. Finally, the paper evaluates the importance of measuring dual-frequency parameters, velocity, and depolarization ratio.

A unique focused array imaging Doppler radar was used to measure directional spectra of Ocean surface waves in a nearshore experiment performed on the North Carolina Outer Banks. Radar images of the ocean surface’s Doppler velocity were used to generate two dimensional spectra of the radial component of the ocean surface velocity field. These are compared to simultaneous in-situ measurements made by a nearby array of submerged pressure sensors. Analysis of the resulting two-dimensional spectra include comparisons of dominant wave lengths, wave directions, and wave energy accounting for relative differences in water depth at the measurement locations. Limited estimates of the two-dimensional surface displacement spectrum are derived from the radar data. The radar measurements are analagous to those of interferometric synthetic aperture radars (INSAR), and the equivalent INSAR parameters are shown. The agreement between the remote and in-situ measurements suggests that an imaging Doppler radar is effective for these wave measurements at near grazing incidence angles.

FOPAIR, a FOcused Phased Array Imaging Radar, provides high-resolution X-band images of the ocean surface. The system is designed to provide high-speed imagery (up to 180 frames/s) for short range applications (50-400 m) from a fixed platform such as a pier or tower. FOPAIR employs a fast, sequentially sampled antenna array and uses a software-based beamforming technique to generate high resolution imagery without the need for multiple radar receivers or beamforming hardware typical of active phased arrays. A summary of the principles of operation and the design of the instrument is given, followed by examples of FOPAIR's imaging capability. To the authors' knowledge, these examples include the highest-resolution, highest-speed microwave images of the ocean surface produced to date. A brief comparison between FOPAIR and synthetic aperture radar techniques is also included.>

Ocean radar backscatter in the presence of large waves is investigated using data acquired with the Jet Propulsion Laboratory NUSCAT radar at Ku band for horizontal and vertical polarizations and the University of Massachusetts C-SCAT radar at C band for vertical polarization during the Surface Wave Dynamics Experiment. Off-nadir backscatter data of ocean surfaces were obtained in the presence of large waves with significant wave height up to 5.6 m. In moderate-wind cases, effects of large waves are not detectable within the measurement uncertainty and no noticeable correlation between backscatter coefficients and wave height is found. Under high-wave light-wind conditions, backscatter is enhanced significantly at large incidence angles,with a weaker effect at small incidence angles. Backscatter coefficients in the wind speed range under consideration are compared with SASS-II (Ku band), CMOD3-H1 (C band), and Plant's model results which confirm the experimental observations. Variations of the friction velocity, which can give rise to the observed backscatter behaviors in the presence of large waves, are presented.>

Attempting to understand and predict weather on a local and global basis has challenged both the scientific and engineering communities. One key parameter in understanding the weather is the ocean surface wind vector because of its role in the energy exchange at the air-sea surface. scatterometers, radars that measure the reflectivity of a target offer a tool with which to remotely monitor these winds from tower-, aircraft-, and satellite-based platforms. This paper introduces three current airborne scatterometer systems, and presents data collected by these instruments under low-, moderate-, and high-wind conditions. The paper focuses on airborne scatterometers because of their ability to resolve submesoscale variations in wind fields. Discrepancies between existing theory and the observations are noted and the concerns in measuring low-wind speeds discussed. Finally, the application of using this technology for estimating the surface-wind vector during a hurricane is demonstrated.>

Millimeter-wave radars have been used since the early 1950s to study clouds and precipitation, but until recently these early instruments were limited to simple backscatter power measurements and were plagued by hardware problems. However, development of solid-state millimeter-wave componentry and high-power klystron amplifiers has spurred the evolution of reliable, coherent radars operating up to 95 GHz. In addition, advances in digital signal processing technology have resulted in single-card processors that can simultaneously execute algorithms to compute reflectivity, Doppler, and polarimetric quantities in real time. A review of the current state of the art in millimeter-wave cloud radars is presented, including a discussion of transmitters, antennas, and receiver components. Two radar systems built by the University of Massachusetts are described, including a mobile, dual-frequency (33- and 95-GHz) polarimetric radar, and an airborne 95-GHz polarimetric radar that was recently flown in a cooperative experiment with the University of Wyoming. Spaceborne applications are also discussed, especially the use of satellite-based 95-GHz radars for measuring the vertical distribution of clouds.>

A 95 GHz dual-polarization radar system was developed and flown on the University of Wyoming King Air research aircraft, from which it measured reflectivity, depolarization, and Doppler-derived velocity mean and standard deviation of a variety of clouds. This paper describes the radar and a data acquisition system that uses commercially available digitizers, signal processors, and signal generators. The authors also describe the tradeoffs between spatial resolution and ability to estimate reflectivity and velocity. This paper presents the first known airborne measurements of clouds made at 95 GHz; these are thought to be the most highly resolved millimeter-wave cloud images made to date. Depolarization, measured in terms of the linear depolarization ratio (LDR), was especially high in the melting band and in regions containing pristine ice crystals. These measurements demonstrate the advantages that high-spatial-resolution airborne millimeter-wave radars offer for the study of cloud microphysical properties.>

Range measurements made by satellite radar altimeters experience an electromagnetic (EM) bias toward the troughs of ocean waves. Measurements taken with the NASA altimeter on the TOPEX/Poseidon satellite in a series of aircraft flights during the Surface Wave Dynamics Experiment (SWADE) indicate that EM bias is slightly higher at 5.3 GHz than at 13.6 GHz, and that the magnitudes of both biases increase with increasing wind speed, as does their difference. Tower, airborne, and satellite measurements show a consistency in the characteristics of the wind speed dependence but suggest that bias decreases with increasing altitude. The airborne measurements appear to be the most reasonable basis for correcting the NASA altimeter range data from the TOPEX/POSEIDON satellite. A preliminary analysis of data acquired at 20.3 m/s in the Southern Ocean Waves Experiment (SOWEX) has given confidence that the quadratic models for the prelaunch EM bias corrections are more appropriate for wind speed dependence than linear models.>

A stepped-frequency delta-K (SFDK) radar has been developed to remotely sense ocean surface characteristics. This C-band radar uses frequency diversity and real-time signal processing for improved Delta K measurements. It is shown how frequency diversity can be implemented to enhance the energy of the resonant peak of the cross-product spectrum relative to the background energy. This feature is essential for making real-time measurements of resonant peak frequency over extended time periods. SFDK was used in a month-long field experiment at North Truro, MA. The results presented show that the phase velocity of ocean surface waves could be precisely measured 87% of the time.>

An airborne C-band scatterometer system (C-Scat) has been developed to remotely sense ocean surface winds and improve upon the present understanding of the relationship between normalized radar cross section (NRCS) and ocean surface roughness influences such as wind speed and direction, wave height and slope, and the air-sea temperature difference. The scatterometer utilizes a unique frequency-steered microstrip array antenna that is installed beneath the fuselage of an airplane. The antenna is electronically scanned in elevation, from 20 degrees to 50 degrees off-nadir, and mechanically spins in azimuth. The system is capable of measuring ocean surface NRCS from altitudes as high as 25000 ft. The system is divided into four subsystems: the transmitter and receiver, the spinning antenna, the computer control and data acquisition subsystem, and the digital and analog interface electronics.>

Polarimetric radar measurements of foliage made in the 220-GHz transmission window are described. A noncoherent, 60-W-peak-power polarimetric radar was used to measure the target Mueller matrix directly by measuring the scattered polarization state for either four or six incident polarizations. Measurements of deciduous trees indicate that waves scattered from erectophil trees (vertically oriented leaves) are more highly polarized than waves scattered from planophil trees (horizontally oriented leaves) for near-grazing incidence angles. White pine trees, which have thin needles, where the least polarized of all trees studied. A comparison of the Mueller matrices of 11 independent footprints taken from seven white pine trees showed a high degree of consistency between measurements.>

Communication systems for high-speed ground vehicles which employ coupled surface waveguides are considered in this paper. We determine communication errors resulting from AM and FM transmission for the coupling configuration of two randomly separated Goubau lines as a function of the various system parameters. Results are given which indicate that filtering of the transmitted and received signals leads to enhanced communications.