Engineering topics

Rahmat-Samii, Yahya

Publications and source records attributed to Rahmat-Samii, Yahya.

At least 37 records · Page 2

NASA NTRS2006

Compact, Lightweight Dual-Frequency Microstrip Antenna Feed for Future Soil Moisture and Sea Surface Salinity Missions

The development of a compact, lightweight, dual-frequency antenna feed for future soil moisture and sea surface salinity (SSS) missions is described. The design is based on the microstrip stacked-patch array (MSPA) to be used to feed a large lightweight deployable rotating mesh antenna for spaceborne L-band (approx.1 GHz) passive and active sensing systems. The design features will also enable applications to airborne soil moisture and salinity remote sensing sensors operating on small aircrafts. This paper describes the design of stacked patch elements and 16-element array configuration. The results from the return loss, antenna pattern measurements and sky tests are also described.

microstrip antennas↗

NASA NTRS2005

Very Long Microstrip Array Feeds of a Membrane Reflector for the Advanced Precipitation Radar

Very long microstrip arrays have been developed at the Ku- and Ka-band frequencies. Each array having an electrical length of about 110 free-space wavelengths is used to feed a deployable thin-membrane cylindrical reflector for a spaceborne precipitation radar application. These arrays, designed for 0(deg) and 30(deg) beam directions, achieved peak sidelobes of -20 dB and average sidelobes below -30 dB with peak cross-pol levels below -20 dB. Several unique challenges were encountered during the development of these very long arrays, such as the strong coupling between very long power divider lines, the strong leakage radiation from the lengthy transmission lines, and the lack of computer analysis capability of these electrically large arrays.

array antennas↗

NASA NTRS2005

A Range Compensating Feed Motion Concept for Spaceborne Radar

Tropical cyclones can cause major loss of both life and property, so that improvements in forecasting motion, intensity, and rainfall are needed. Such forecasting requires accurate measurements of the current state of the cyclone. Ground-based Doppler radars have long been used as an effective means of monitoring severe precipitating storms. Because of the oceanic nature of tropical cyclones, remote monitoring from space is desirable. Recently, the Precipitation Radar (PR) [1] aboard the Tropical Rainfall Measuring Mission (TRMM) [2] satellite has demonstrated an unprecedented capacity for 3-D imaging of precipitating storms. Nonetheless, due to the relatively long TRMM return cycle (less than once per day) the value of PR data has primarily been limited to the understanding of climatological properties of tropical cyclones [3]. The return cycle can be substantially reduced by sensing from a geostationary platform.

reflector antennas↗

NASA NTRS2004

Compact, Lightweight Dual- Frequency Microstrip Antenna Feed for Future Soil Moisture and Sea Surface Salinity Missions

The development of a compact, lightweight, dual frequency antenna feed for future soil moisture and sea surface salinity (SSS) missions is described. The design is based on the microstrip stacked-patch array (MSPA) to be used to feed a large lightweight deployable rotating mesh antenna for spaceborne L-band (approx. 1 GHz) passive and active sensing systems. The design features will also enable applications to airborne sensors operating on small aircrafts. This paper describes the design of stacked patch elements, 16-element array configuration and power-divider beam forming network The test results from the fabrication of stacked patches and power divider were also described.

sea surface salinity↗

NASA NTRS2004

Radar for Monitoring Hurricanes from Geostationary Orbit

A document describes a scanning Doppler radar system to be placed in a geostationary orbit for monitoring the three-dimensional structures of hurricanes, cyclones, and severe storms in general. The system would operate at a frequency of 35 GHz. It would include a large deployable spherical antenna reflector, instead of conventional paraboloidal reflectors, that would allow the reflector to remain stationary while moving the antenna feed(s), and thus, create a set of scanning antenna beams without degradation of performance. The radar would have separate transmitting and receiving antenna feeds moving in spiral scans over an angular excursion of 4 from the boresight axis to providing one radar image per hour of a circular surface area of 5,300-km diameter. The system would utilize a real-time pulse-compression technique to obtain 300-m vertical resolution without sacrificing detection sensitivity and without need for a high-peakpower transmitter. An onboard data-processing subsystem would generate three-dimensional rainfall reflectivity and Doppler observations with 13-km horizontal resolution and line-of-sight Doppler velocity at a precision of 0.3 m/s.

Im, Eastwood↗

NASA NTRS2004

Advanced geostationary radar for hurricane monitoring and studies

The current Geostationary Operational Environmental Satellites (GOES) are equipped to make cloud top measurements only. In contrast, a millimeter-wave radar allows 3-D measurements of precipitation associated with hurricanes and other convective systems.

precipitation↗

NASA NTRS2004

Radar for Measuring Soil Moisture Under Vegetation

A two-frequency, polarimetric, spaceborne synthetic-aperture radar (SAR) system has been proposed for measuring the moisture content of soil as a function of depth, even in the presence of overlying vegetation. These measurements are needed because data on soil moisture under vegetation canopies are not available now and are necessary for completing mathematical models of global energy and water balance with major implications for global variations in weather and climate.

Moghaddam, Mahta↗

NASA NTRS2003

Conceptual design of a geostationary radar for hurricane studies

A novel 35-GHz Doppler radar instrument concept and the associated critical technologies are being developed for detailed monitoring of hurricanes and severe storms from a geostationary orbit. This instrument is designed to make quantitative rainfall rate profiling measurements at 13-km horizontal resolution and 300-m vertical resolution, and the radial Doppler velocity at 0.3 m/s precision, of the 3-D hurricane structure once per hour throughout its life cycle.

precipitation↗

NASA NTRS1999

ARISE: A Challenging 25-m Space Antenna Design

ARISE (Advanced Radio Interferometry between Space and Earth). Future scientific and communications missions are conceptualizing using very large reflector antennas in space. The antenna dimensions are targeted from lOm to 25m beyond. Frequencies coveting L-band through W-band are being considered. these challenging missions, one may refer to ARISE (Advanced Radio Interferometry between Space and Earth), which is projected to utilize a 25-m dual Gregorian optics operating from 8 GHz to 86 Gl-lz . An artist's renditions Gregorian antenna concept is shown . The objective of this mission is to create a space-based VLBI system (integrated with ground based antennas) for high resolution mapping of black holes. Successful realization of ARISE necessitates development of several technologies . The most crucial technology is that of the deployable 25-m reflector that must work at frequencies as high as 43 and 86 GHz. The current baseline selection for ARISE is an inflatable antenna, under development for several other applications in communications and remote sensing.

Rahmat-Samii, Yahya↗

NASA NTRS1998

Anomalous Results from PO Applied to Reflector Antennas: The importance of Near Field Computations

There is no doubt among the designers of reflector antennas that the physical optics (PO) analysis technique is the most popular numerical technique. Powerful computer codes are available for the analysis of single or multi reflector antenna systems. Additionally, ever increasingly demand on the antenna performance necessitates the computation of antenna far field patterns under various situations. For example, in using multi reflector antennas such as, Gregorian or Cassegrain, it may become necessary to determine the total fields including the feed radiation pattern, subreflector scattered pattern and the main reflector scattered pattern. In these situations, the common practice is to sum up various scattered fields and the incident field contributions to obtain the desired total field, it is the purpose of this paper to demonstrate that the typical approach based on the far field pattern of the feed would result into erroneous result and special care must be exercised to obtain the correct result. This will be demonstrated through a detailed investigation of a representative test case.

Rahmat-Samii, Yahya↗

NASA NTRS1997

Application of Cylindrical Near-Field Measurement Technique to The Calibration of Spaceborne Radar Antennas: NASA Scatterometer and SeaWinds

Modern spaceborne radar scatterometers such as the NASA Scatterometer (NSCAT) and SeaWinds radar instruments require precise determination of the normalized backscattered radar cross section within a few tenth of a decibel in order to achieve the desired wind velocity and direction measurement accuracy of 2 m/s and 20 degrees respectively.

Spaceborne Radar Antennas SeaWinds Scatterometer↗

NASA NTRS1993

Probe Compensation Characterization in Cylindrical Near-Field Scanning

This paper presents a useful computer simulation methodology to properly characterize the role of probe compensation in cylindrical near-field scanning. The methodology is applied to a linear test array antenna and has been applied to other antenna configurations.

probe compensation near-field scanning waveguide r↗

NASA NTRS1993

Diffraction analysis of frequency selective reflector antennas

In this paper, a unified computational technique is presented which allows the incorporation of the curved frequency-selective surface (FSS) geometry in the computation of the reflector antenna radiation pattern. The scattered fields from an illuminated FSS reflector are formalized using Huygens' principle in such a way that the 'reflecting' and the 'transparent' FSS subreflector cases are treated identically and the thickness of the FSS subreflector remains arbitrary. The analysis utilizes local surface coordinates to describe the reflection/transmission matrices of the FSS subreflector where it is assumed that these matrices are available. In most cases one may use the local tangent plane for approximating the plane of the FSS in the local coordinate surface of the reflector. The paper demonstrates how the local curved coordinate system can be introduced in the diffraction modeling of FSS reflectors and its importance in accurately predicting the side-lobe and crosspolarization levels. Results of numerical simulations are presented for several FSS subreflector configurations.

Rahmat-Samii, Yahya↗

NASA NTRS1993

Probe compensation in cylindrical near-field scanning: A novel simulation methodology

Probe pattern compensation is essential in near-field scanning geometry, where there is a great need to accurately know far-field patterns at wide angular range. This paper focuses on a novel formulation and computer simulation to determine the precise need for and effect of probe compensation in cylindrical near-field scanning. The methodology is applied to a linear test array antenna and the NASA scatterometer radar antenna. The formulation is based on representing the probe by its equivalent tangential magnetic currents. The interaction between the probe equivalent aperture currents and the test antenna fields is obtained with the application of a reciprocity theorem. This allows us to obtain the probe vector output pickup integral which is proportional to the amplitude and phase of the electric field induced in the probe aperture with respect to its position to the test antenna. The integral is evaluated for each probe position on the required sampling point on a cylindrical near-field surface enclosing the antenna. The use of a hypothetical circular-aperture probe with a different radius permits us to derive closed-form expressions for its far-field radiation patterns. These results, together with the probe vector output pickup, allow us to perform computer simulated synthetic measurements. The far-field patterns of the test antenna are formulated based on cylindrical wave expansions of both the probe and test antenna fields. In the limit as the probe radius becomes very small, the probe vector output is the direct response of the near-field at a point, and no probe compensation is needed. Useful results are generated to compare the far-field pattern of the test antenna constructed from the knowledge of the simulated near-field with and without probe pattern compensation and the exact results. These results are important since they clearly illustrate the angular range over which probe compensation is needed. It has been found that a probe with an aperture radius of 0.25(lambda), 0.5(lambda), and 1(lambda) needs a little probe compensation, if any, near the test antenna main beam. In addition, a probe with low directivity may provide a better signal-to-noise ratio than a highly directive one. This is evident in test antenna patterns without probe compensation at wide angles.

Hussein, Ziad A.↗

NASA NTRS1993

Isolated and coupled superquadric loop antennas for mobile communications applications

This work provides an investigation of the performance of loop antennas for use in mobile communications applications. The analysis tools developed allow for high flexibility by representing the loop antenna as a superquadric curve, which includes the case of circular, elliptical, and rectangular loops. The antenna may be in an isolated environment, located above an infinite ground plane, or placed near a finite conducting plate or box. In cases where coupled loops are used, the two loops may have arbitrary relative positions and orientations. Several design examples are included to illustrate the versatility of the analysis capabilities. The performance of coupled loops arranged in a diversity scheme is also evaluated, and it is found that high diversity gain can be achieved even when the antennas are closely spaced.

Jensen, Michael A.↗

NASA NTRS1991

On the reflectivity of complex mesh surfaces

Poorer than expected surface reflectivity was observed in an early Tracking and Data Relay Satellite System antenna utilizing a tricot mesh weave. This poor reflectivity was determined to be caused by inadequate electrical contact at wire crossover points. A proper mathematical and numerical approach to assess the impact of wire junctions on reflectivity performance is developed. A mathematical method is presented for computing the surface reflectivity of complex mesh configurations like those on unfurlable-type spacecraft antennas. The method is based on the Floquet mode expansion to establish an integral equation for mesh wire currents. The equation is solved using the method of moments with triangular basis functions. It is observed that it is necessary to give special attention to the junction treatment among different branches of the mesh configurations. A vector junction current approach that resulted in satisfactory solutions for the current is described. The results of numerical simulations are compared against measured data and excellent agreement is observed.

Imbriale, William A.↗

NASA NTRS1991

Effects of measurement errors on microwave antenna holography

The effects of measurement errors appearing during the implementation of the microwave holographic technique are investigated in detail, and many representative results are presented based on computer simulations. The numerical results are tailored for cases applicable to the utilization of the holographic technique for the NASA's Deep Space Network antennas, although the methodology of analysis is applicable to any antenna. Many system measurement topics are presented and summarized.

Rochblatt, David J.↗

NASA NTRS1991

Novel array-feed distortion compensation techniques for reflector antennas

Degradation of antenna performance by reflector surface distortion, which lowers gain and increases sidelobe levels, is addressed. Distortion compensation concepts based on the applications of properly matched array feeds are presented. Results of conceptual developments, numerical simulations, and measurement verifications are presented in support of this approach, with particular attention to the measurement technique. It is shown that the concept is most useful for overcoming the deterioration effects of slowly varying surface distortions, which would make the method very useful for future large space and ground antennas. It is further shown that for a typical, slowly varying thermal or gravitational surface distortion, a 19-element array feed can improve the reflector performance considerably.

Rahmat-Samii, Yahya↗