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Double-diaphragm radome drives raindrops away from antenna feed horn. Includes solid diaphragm at mouth of feed horn surmounted by perforated diaphragm. Pressurized air in plenum between diaphragms flows out through perforations. Eliminates increase in noise temperature occuring when water covers radome. Useful for X-band and higher frequencies, susceptible to effects of water.
During this period, the computations of the impedance elements were completed. These include interactions between the two electric current modes, the elecric current mode and the magnetic current mode, and the two magnetic current modes. An accurate and efficient formulation of computing interactions between electric current mode and magnetic current mode was accomplished. This, together with other subroutines allows for the fill-in of all the elements in the matrix. After the fill-in of the impedance elements in the matrix, the forward problem is accomplished. That is, given the specification of the horn and the excitating waveguide mode, the radiation pattern of the antenna based on the integral equation can be obtained. An example case was run for a standard X-band gain-horn (DBG-520). The H- and E-plane patterns of this horn antenna with perfectly conducting walls are compared with the gain pattern available from the manufacturer for up to the first side lobe. Good agreements are obtained although the cross polarization has not yet been accounted for. The effect of the lossy coating on the radiation pattern was also investigated. The resulting E-plane pattern shows about 3-dB improvement in the first sidelobe and 4-dB improvement in the second sidelobe.
The accuracy of solar system tests of gravitational theory could be very much improved by range and Doppler measurements to a Small Mercury Relativity Orbiter. A nearly circular orbit at roughly 2400 km altitude is assumed in order to minimize problems with orbit determination and thermal radiation from the surface. The spacecraft is spin-stabilized and has a 30 cm diameter de-spun antenna. With K-band and X-band ranging systems using a 50 MHz offset sidetone at K-band, a range accuracy of 3 cm appears to be realistically achievable. The estimated spacecraft mass is 50 kg. A consider-covariance analysis was performed to determine how well the Earth-Mercury distance as a function of time could be determined with such a Relativity Orbiter. The minimum data set is assumed to be 40 independent 8-hour arcs of tracking data at selected times during a two year period. The gravity field of Mercury up through degree and order 10 is solved for, along with the initial conditions for each arc and the Earth-Mercury distance at the center of each arc. The considered parameters include the gravity field parameters of degree 11 and 12 plus the tracking station coordinates, the tropospheric delay, and two parameters in a crude radiation pressure model. The conclusion is that the Earth-Mercury distance can be determined to 6 cm accuracy or better. From a modified worst-case analysis, this would lead to roughly 2 orders of magnitude improvement in the knowledge of the precession of perihelion, the relativistic time delay, and the possible change in the gravitational constant with time.
It was shown earlier that 1 x 10 to the -14th Doppler data and 3 cm accuracy range measurements to a small Mercury Relativity Satellite in a polar orbit with four-hour period can give high-accuracy tests of gravitational theory. A particular conceptual design has been developed for such a satellite, which would take less than 10 percent of the approach mass for a possible future Mercury Orbiter Mission. The spacecraft is similar to the Pioneer Venus Orbiter, but scaled down by about a factor four in linear dimensions. A despun antenna 30 cm in diameter is used for tracking. The transmitted power is roughly 0.2 watts at K-band and 0.5 watts at X-band. The orbit parameters for individual eight-hour arcs and the gravity field of Mercury through degree and order 10 are determined mainly from the Doppler data. A 50 MHz K-band sidetone system provides the basic ranging accuracy. The spacecraft mass is 50 kg or less.
A practical technique is proposed for calibrating single-antenna polarimetric radar systems using a metal sphere plus any second target with a strong cross-polarized radar cross section. This technique assumes perfect isolation between antenna ports. It is shown that all magnitudes and phases (relative to one of the like-polarized linear polarization configurations) of the radar transfer function can be calibrated without knowledge of the scattering matrix of the second target. Comparison of the values measured (using this calibration technique) for a tilted cylinder at X-band with theoretical values shows agreement within + or - 0.3 dB in magnitude and + or - 5 degrees in phase. The radar overall cross-polarization isolation was 25 dB. The technique is particularly useful for calibrating a radar under field conditions, because it does not require the careful alignment of calibration targets.
A radar development is presently funded by the NASA Mesoscale Atmospheric Research Program to build an X-band Doppler radar (EDOP) for the ER-2 aircraft for measuring the structure of updrafts and downdrafts in deep convective systems. In this paper, the radar's scientific objectives and characteristics are briefly reviewed. These objectives include documentation of deep isolated thunderstorms, examination of cloud top and anvil structure, structure of microscale convective systems, cloud microphysics, and testing of satellite precipitation estimation algorithms. System specifications for the radar are given.
Precipitation profiles retrieved from a dual frequency X- and Ka-band radar and from an X-band radiometer are analyzed. Profiles from combined radar and microwave radiometer measurements are compared to those from dual frequency radar measurements to study the performance of the combined radar-radiometer retrievals. A method for the retrieval of rainfall rate profiles from attenuated radar returns and passive radiometric data is presented. The results suggest that the operation of a single frequency radar with multifrequency passive radiometers may provide cost-effective spaceborne measurements of global tropical rainfall distributions. The possible use of this methodology for the Tropical Rainfall Measuring Mission is considered.
Propagation experiments as essential components of the general plan to develop an operational deep space telecommunications and navigation capability at Ka-band (32 to 35 GHz) by the end of the 20th century are discussed. Significant benefits of Ka-band over the current deep space standard X-band (8.4 GHz) are an improvement of 4 to 10 dB in telemetry capacity and a similar increase in radio navigation accuracy. Propagation experiments are planned on the Mars Observer Mission in 1992 in preparation for the Cassini Mission to Saturn in 1996, which will use Ka-band in the search for gravity waves as well as to enhance telemetry and navigation at Saturn in 2002. Subsequent uses of Ka-band are planned for the Solar Probe Mission and the Mars Program.
Very Long Baseline Interferometry (VLBI) measurements of the Magellan spacecraft's angular position and velocity were made during July through September, 1989, during the spacecraft's heliocentric flight to Venus. The purpose of this data acquisition and reduction was to verify this data type for operational use before Magellan is inserted into Venus orbit, in August, 1990. The accuracy of these measurements are shown to be within 20 nanoradians in angular position, and within 5 picoradians/sec in angular velocity. The media effects and their calibrations are quantified; the wet fluctuating troposphere is the dominant source of measurement error for angular velocity. The charged particle effect is completely calibrated with S- and X-Band dual-frequency calibrations. Increasing the accuracy of the Earth platform model parameters, by using VLBI-derived tracking station locations consistent with the planetary ephemeris frame, and by including high frequency Earth tidal terms in the Earth rotation model, add a few nanoradians improvement to the angular position measurements. Angular velocity measurements were insensitive to these Earth platform modelling improvements.
X-band (HH) synthetic aperture radar (SAR) data of sea ice collected during the Marginal Ice Zone Experiment in March and April of 1987 was statistically analyzed with respect to discriminating open water, first-year ice, multiyear ice, and Odden. Odden are large expanses of nilas ice that rapidly form in the Greenland Sea and transform into pancake ice. A first-order statistical analysis indicated that mean versus variance can segment out open water and first-year ice, and skewness versus modified skewness can segment the Odden and multilayer categories. In additions to first-order statistics, a model has been generated for the distribution function of the SAR ice data. Segmentation of ice types was also attempted using textural measurements. In this case, the general co-occurency matrix was evaluated. The textural method did not generate better results than the first-order statistical approach.
The Odden, a protuberance of sea ice in the Greenland Sea Basin, was studied using the NIMBUS-7 scanning multichannel microwave radiometer (SMMR) satellite and an X-band (3 cm) synthetic aperture radar (SAR) aircraft. The sea ice, meteorological, and oceanographic conditions within the northern portion of the Odden were also studied in March and April 1987. The SMMR data, which were first validated with in situ ship measurements and the SAR data, showed rapid 2-4 day oscillations of the Odden ice edge. The oscillations at 74-75 deg N were several hundred kilometers in extent. The rapid oscillation of the Odden does not appear to be a result of wind-induced ice drift, but rather results from the rapid formation of thin ice off the main ice edge.
A practical technique is proposed for calibrating single-antenna polarimetric radar systems using a metal sphere plus any second target with a strong cross-polarized radar cross section. It is shown that all magnitudes and phases (relative to one of the like-polarized linear polarization configurations) of the radar transfer function can be calibrated without knowledge of the scattering matrix of the second target. Comparison of values measured (using this calibration technique) for a tilted cylinder at X-band with theoretical values shows agreement with + or - 0.3 dB in magnitude and + or - 5 deg in phase. The technique is particularly useful for calibrating a radar under field conditions because it does not require careful alignment of calibration targets.
Precipitation-rate profiles derived from a single-frequency radar and a radiometer are compared with profiles derived from a dual-frequency radar, using data obtained during the 1985-1986 Communication Research Laboratory/NASA rain measuring experiment over the Atlantic Ocean from a dual-frequency X- and Ka-band radars and an X-band passive radiometer. Profiles were retrieved using a modification of the Hitschfeld-Bordan (1954) algorithm constrained by a measurement of the path integrated extinction derived from passive radiometry. It was found that the two techniques under comparison yielded average rainfall rates that agreed within about 25 percent. It is suggested that, while a single-frequency radar operating in conjunction with multifrequency passive radiometers might provide cost-effective spaceborne measurements of tropical rainfall distributions, a dual-frequency radar might be able to remove some ambiguities that were encountered in this study.
An analytical study to determine the feasibility of a rain-on-radome experiment at the Aircraft Landing Dynamics Facility (ALDF) at the Langley Research Center is described. The experiment would measure the effects of heavy rain on the transmission of X-band weather radar signals, looking in particular for sources of anomalous attenuation. Feasibility is determined with regard to multipath signals arising from the major structural components of the ALDF. A computer program simulates the transmit and receive antennas, direct-path and multipath signals, and expected attenuation by rain. In the simulation, antenna height, signal polarization, and rainfall rate are variable parameters. The study shows that the rain-on-radome experiment is feasible with regard to multipath signals. The total received signal, taking into account multipath effects, could be measured by commercially available equipment. The study also shows that horizontally polarized signals would produce better experimental results than vertically polarized signals.
Functional requirements for airborne windshear detection and warning systems are discussed in terms of the threat posed to civil aircraft operations. A preliminary set of performance criteria for predictive windshear detection and warning systems is defined. Candidate airborne remote sensor technologies based on microwave Doppler radar, Doppler laser radar (lidar), and infrared radiometric techniques are discussed in the context of overall system requirements, and the performance of each sensor is assessed for representative microburst environments and ground clutter conditions. Preliminary simulation results demonstrate that all three sensors show potential for detecting windshear, and provide adequate warning time to allow flight crews to avoid the affected area or escape from the encounter. Radar simulation and analysis show that by using bin-to-bin automatic gain control, clutter filtering, limited detection range, and suitable antenna tilt management, windshear from wet microbursts can be accurately detected. Although a performance improvement can be obtained at higher radar frequency, the baseline X-band system also detected the presence of windshear hazard for a dry microburst. Simulation results of end-to-end performance for competing coherent lidar systems are presented.
A proposed conceptual design for increasing the output power of an existing X-band planetary radar transmitter from 365 kW to 1 MW CW is discussed. The paper covers the basic transmitter system requirements as dictated by the specifications for the radar. The characteristics and expected performance of the high-power klystrons are considered and the transmitter power amplifier system is discussed. Also included in the discussion is the design of the exiter system. Two alternative feed systems for delivering the 1-MW CW signal to the antenna system are described. The expected performance of the beam supply, heat exchanger, and monitor and control devices is also presented. Finally, an assessment of the state-of-the-art technology to meet system requirements is given and possible areas of difficulty are summarized.
The Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) is the most advanced imaging radar system to fly in Earth orbit. Carried in the cargo bay of the Space Shuttle Endeavour in April and October of 1994, SIR-C/X-SAR simultaneously recorded SAR data at three wavelengths (L-, C-, and X-bands; 23.5, 5.8, and 3.1 cm, respectively). The SIR-C/X-SAR Science Team consists of 53 investigator teams from more than a dozen countries. Science investigations were undertaken in the fields of ecology, hydrology, ecology, and oceanography. This report contains 44 investigator team reports and several additional reports from coinvestigators and other researchers.
The Magellan spacecraft will be launched from Kennedy Space Center (KSC) within a 31-day overall launch period extending from April 28 to May 28, 1989. The launch will use the Shuttle Orbiter Atlantis to lift an Inertial Upper Stage (IUS) and the Magellan Spacecraft into low Earth orbit. After the Shuttle achieves its parking orbit, the IUS and attached Magellan spacecraft are deployed from the payload bay. After a short coast time, the two-stage IUS is fired to inject the Magellan spacecraft into an Earth-Venus transfer trajectory. The Magellan spacecraft is powered by single degree of freedom, sun-tracking, solar panels charging a set of nickel-cadmium batteries. The spacecraft is three-axis stabilized by reaction wheels using gyros and a star sensor for attitude reference. The spacecraft carries a solid rocket motor for Venus Orbit Insertion (VOI). A hydrazine propulsion system allows trajectory correction and prevents saturation of the reaction wheels. Communication with Earth through the Deep Space Network (DSN) is provided by S- and X-band telemetry channels, through alternatively a low, medium, or 3.7 m high-gain parabolic antenna rigidly attached to the spacecraft. The high-gain antenna also serves as the radar and radiometer antenna during orbit around Venus.