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A 3-D pattern-space representation is presented for volumetric arrays. In this representation, the radiation pattern of an array is formed by the evaluation of the 3-D pattern-space on a spherical surface. The scan angle of the array determines the position of this surface within the pattern-space. This pattern-space representation is used in conjunction with a genetic algorithm to minimize the sidelobe levels exhibited by a thinned volumetric array during scanning.

We present performance bounds obtained from the optimization of the sidelobe levels of aperiodic linear arrays. The antennas comprising these arrays are large compared to the distance between neighboring antennas, a case not addressed in previously published work. This optimization is performed in pattern-space and is applicable over a wide range of scan angles. We show that grating lobes can be suppressed even when the elemental antennas are several wavelengths in size, provided that the ratio of the antenna size to the average spacing between the antenna center-points does not exceed 80%.

An increasing number of commercial applications are being promoted for future Ka-band satellite communication systems. Many of these systems will involve low-margin very small aperture terminals (VSATs). These systems are subject to important atmospheric propagation degradations that affect the quality of transmission and the link availability. The objective of this paper is to characterize the Ka-band channel and evaluate the performance degradation in VSATs resulting from atmospheric propagation, impairments. In particular microwave propagation through a turbulent atmosphere is discussed, and the statistical characterization and modeling of tropospheric scintillation is reviewed. Moreover, the paper extends the method proposed by Filip and Vilar (1990) for the long-term characterization and modeling of the combined effect of rain impairments and scintillation. Specifically, the increase in noise temperature during rain events is added to the Filip-Vilar model. This leads to a five-parameter global fading distribution that is used to predict typical Ka-band satellite link outage time, the mathematical formalism is illustrated by applying the method to the selected case of the Advanced Communications Technology Satellite (ACTS)-Georgia Tech experimental downlink. Numerical results confirm that both rain impairments and scintillation are important factors in the design of Ka-band VSAT systems.

The authors observed Jupiter at two wavelengths near 1.4 mm in an attempt to detect gaseous hydrogen sulfide (H/sub 2/S) or place new upper limits on its abundance in Jupiter's atmosphere. Although they were not successful in detecting H/sub 2/S, they report the first brightness temperature observations of Jupiter at 1.4 mm with a spectral resolution of approximately 1 GHz using Mars as the calibration standard. The methodology and results of a laboratory experiment in which the authors measured H/sub 2/S absorption at 1.4 mm in a simulated Jovian atmosphere (predominantly H/sub 2/ and He) are discussed. The results of the laboratory measurements are applied to a radiative transfer model, which is used to interpret the observations of Jupiter. >

For the original article see ibid., vol.1, no.12, p.374-5 (1991). The commenters point out several errors in a paper by G.C. Giakos and T.K. Ishii in which they claim to have developed conclusive laboratory evidence that demonstrates microwave pulse propagation in waveguides, and in air, at velocities exceeding that of light. The authors defend their observations. >

Early seminal contributions by Staelin helped initiate the field of microwave remote sensing as a key tool for the study of planetary atmospheres. Recent studies of the microwave emission from the neutral atmosphere of Venus have been used to identify the abundance and spatial distribution of microwave absorbing constituents such as sulfuric acid vapor and sulfur dioxide. A new mission to Jupiter, called Juno, will employ a six-channel microwave radiometer to measure the abundance and distribution of ammonia and water vapor in the Jovian atmosphere.

A genetic wire antenna is developed for a low-N phased array for use as a satellite-communications ground terminal. The radiation pattern of this antenna is optimized to compensate for the angularly-dependent path loss exhibited by a communications link with a low-earth-orbiting (LEO) earth-environmental satellite (EES).

This paper presents numerical design models and predicted performance for a parasitic dipole antenna array tile that contains multiple feeds. This tile is designed for end-to-end concatenation with identical tiles to construct a phased array with a large number of feeds. The tile is optimized so that the radiation pattern of each feed is similar to the desired radiation pattern. The tile is simultaneously optimized so that the array exhibits low sidelobe levels for a range of scan angles.

The increasing usage of radio spectrum in both frequency and power, coupled with the vulnerability of passive remote sensing systems to interference, necessitates a better understanding of the existing and predicted usage of the spectral environment. A Radio Spectrum Evaluation System (RSES) has been developed which provides coverage from 400 MHz to 7.2 GHz and offers the capability to observe and analyze spectral usage in multiple dimensions: frequency, power, location space, azimuth, polarization and time. Since passive radiometry can suffer interference at extremely low power levels, this system's sensitivity has been enhanced by reducing intermodulation distortion and thermal noise. A study of the radio frequency usage in and adjacent to the remote sensing bands at 4.5 cm and 21 cm was undertaken to determine current usage levels and characteristics. These bands are of interest to instruments such as NPOESS/CMIS and ESA/SMOS. While this study was performed in an urban area, additional studies are measuring spectrum usage in suburban and rural environments to assist in predicting future spectral usage and characteristics

Tropospheric scintillation can seriously affect satellite links operating at the K/sub a/-band (20/30 GHz). The average bit error probability for a multiuser binary direct-sequence spread-spectrum satellite communication network operating under amplitude scintillation is evaluated. Numerical results are presented to illustrate performance comparisons of systems operating under typical scintillation statistics. As in the single user case, it is shown that the average system performance gradually degrades as the scintillation intensity increases and is worse for a high variability of these events. The results also indicate that the impact of amplitude tropospheric scintillations becomes less important as the number of asynchronous active users increases.