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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Multibeam single frequency synthetic aperture radar processor for imaging separate range swaths

A single-frequency multibeam synthetic aperture radar for large swath imaging is disclosed. Each beam illuminates a separate ""footprint'' (i.e., range and azimuth interval). The distinct azimuth intervals for the separate beams produce a distinct Doppler frequency spectrum for each beam. After range correlation of raw data, an optical processor develops image data for the different beams by spatially separating the beams to place each beam of different Doppler frequency spectrum in a different location in the frequency plane as well as the imaging plane of the optical processor. Selection of a beam for imaging may be made in the frequency plane by adjusting the position of an aperture, or in the image plane by adjusting the position of a slit. The raw data may also be processed in digital form in an analogous manner.

Jain, A.↗
NASA NTRS1981

Amplitude calibration of synthetic aperture radars - The effects of nonlinearities

Nonlinearities within a synthetic aperture radar (SAR) present significant obstacles to the calibration of the output imagery. The problem of calibrating a SAR is considerably more involved than that of calibrating a real aperture radar (RAR) or scatterometer, since the SAR relies heavily on the principle of superposition, which fails in nonlinear systems. It is shown in this paper that non-linearities can distort SAR imagery to such an extent that two areas of equal backscatter on the ground can be imaged to have significantly different intensities in the image.

Held, D. N.↗
NASA NTRS1985

Target Simulator for Synthetic-Aperture Radars

In proposed Target Simulator for Testing Synthetic-Aperture Radars, radar transmitter output received, mixed down to video frequencies, and passed through variable digital delay. Delayed signal mixed up to original carrier frequency and transmitted back to radar. Synthetic-aperture radar (SAR) generates return signal that contains range-curvature information in addition to Doppler-frequency information.

Zebker, H. A.↗
NASA NTRS1983

Synthetic aperture radar observation of ocean roughness from rolls in an unstable marine boundary layer

Simultaneous synthetic aperture radar (SAR) and cloud photographic observations of the Atlantic Ocean off the coast of Florida were made from a high-altitude aircraft when there was an unstable marine boundary layer. The synthetic aperture radar images show unusual kilometer-sized features on the ocean surface which are related to clouds. The ocean near shore was cloud-free and had no radar features, while from 30 to 330 km offshore there were clouds and prominent kilometer-sized features in the SAR image. These radar features are most prominent when the radar was looking upwind, are less prominent when the radar was looking downwind, and disappear entirely when the radar was looking crosswind. Since ocean radar echo strengths are believed to be controlled primarily by ocean waves satisfying the Bragg relation, these radar features most likely resulted from local enhancements of short gravity waves with 17- to 34-cm wavelengths, which in turn are surface expressions of roll convections in a kilometer-thick unstable marine boundary layer.

Thompson, T. W.↗
NASA NTRS1985

Hexagonal uniformly redundant arrays for coded-aperture imaging

Uniformly redundant arrays are used in coded-aperture imaging, a technique for forming images without mirrors or lenses. This technique is especially important for the high energy X-ray and gamma-ray region above 20 kev. In this technique, a mask consisting of opaque (closed) and trasparent (open) areas is placed between the photon sources to be imaged and a position sensitive detector or a detector array. Each source casts a shadow pattern of the mask or aperture onto the detector. This shadow pattern may be viewed as an encoded signal for that source direction. If each possible source code is unique, the detected composite of overlapping shadow patterns may be decoded to produce an image of the source distribution.

Finger, M. H.↗
NASA NTRS1985

Secondary gamma-ray production in a coded aperture mask

The application of the coded aperture mask to high energy gamma-ray astronomy will provide the capability of locating a cosmic gamma-ray point source with a precision of a few arc-minutes above 20 MeV. Recent tests using a mask in conjunction with drift chamber detectors have shown that the expected point spread function is achieved over an acceptance cone of 25 deg. A telescope employing this technique differs from a conventional telescope only in that the presence of the mask modifies the radiation field in the vicinity of the detection plane. In addition to reducing the primary photon flux incident on the detector by absorption in the mask elements, the mask will also be a secondary radiator of gamma-rays. The various background components in a CAMTRAC (Coded Aperture Mask Track Chamber) telescope are considered. Monte-Carlo calculations are compared with recent measurements obtained using a prototype instrument in a tagged photon beam line.

Owens, A.↗
NASA NTRS1983

Coded aperture imaging of X-ray and gamma-ray sources

Coded aperture telescopes employing arrays of a small number of discrete detector elements for hard X-rays or gamma-rays are discussed. Aperture patterns are described that permit a unique reconstruction of the image with high contrast, and that exhibit a specific rotational antisymmetry to suppress systematic distortions. It is shown that high flux sensitivity can be achieved in this fashion as well as good angular resolution.

Kroeger, R.↗
NASA NTRS1984

An E-plane analysis of aperture-matched horn antennas using the moment method and the uniform geometrical theory of diffraction

The moment method and the uniform geometrical theory of diffraction are utilized to obtain two separate solutions for the E-plane field pattern of an aperture-matched horn antenna. This particular horn antenna consists of a standard pyramidal horn with the following modifications: a rolled edge section attached to the aperture edges and a curved throat section. The resulting geometry provides significantly better performance in terms of the pattern, impedance, and frequency characteristics than normally obtainable. The moment method is used to calculate the E-plane pattern and BSWR of the antenna. However, at higher frequencies, large amounts of computation time are required. The uniform geometrical theory of diffraction provides a quick and efficient high frequency solution for the E-plane field pattern. In fact, the uniform geometrical theory of diffraction may be used to initially design the antenna; then, the moment method may be applied to fine tune the design. This procedure has been successfully applied to a compact range feed design.

Heedy, D. J.↗
NASA NTRS1985

An aperture-matched compact range feed horn design

The moment method and the uniform geometrical theory of diffraction are used to obtain two separate solutions for the E-plane far field pattern of an aperture-matched horn antenna. This particular horn antenna consists of a standard pyramidal horn with the following modifications: a rolled edge section attached to the aperture edges and a curved throat section. The resulting geometry provides significantly better performance in terms of the pattern, impedance, and frequency characteristics than normally obtainable. The moment method is used to calculate the E-plane pattern and voltage standing-wave ratio (VSWR) of the antenna. However, at higher frequencies, the moment method requires large amounts of computation time. On the other hand, the uniform geometrical theory of diffraction provides a quick and efficient high frequency solution for the E-plane field pattern. This procedure has been successfully applied to design a compact range feed horn.

Heedy, D. J.↗
NASA NTRS1985

Spillage and flux density on a receiver aperture lip

In a dish-type point-focusing solar thermal collector, the spillage and the flux density on the receiver aperture lip are related in a very simple way, if the aperture is circular and centered on the optical axis. Specifically, the flux density on the lip is equal to the spillage times the peak flux density in the plane of the lip.

Jaffe, L. D.↗
NASA NTRS1987

Gamma-ray observations of the Crab Region using a coded-aperture telescope

The region of the Galactic anticenter, including the Crab Nebula, was observed during a balloon flight of the University of New Hampshire Directional Gamma-Ray Telescope employing the coded-aperture imaging technique to image celestial gamma-radiation between 160 keV and 9.3 MeV. The background systematics are treated with a simple and relatively straightforward correction procedure. The results demonstrate that the coded-aperture procedure is a viable approach for imaging not only point sources of radiation, but also extended sources of emission. The results for the Crab's photon spectrum are consistent with a power-law spectrum. Upper limits on the flux levels of line emission at 405 keV and 1050 keV and on the flux from the X-ray binary source A0535 + 26 and diffuse Galactic emission from the anticenter region are derived.

Mcconnell, M. L.↗
NASA NTRS1987

Performance characteristics of phased array and thinned aperture optical telescopes

While phased telescope arrays for general-purpose broadband imaging applications suffer severe sensitivity losses and field-of-view limitations, thinned-aperture telescopes consisting of a dilute, segmented primary mirror with a common secondary mirror are viable second-generation space telescope configurational possibilities yielding resolution and sensitivity an order of magnitude greater than those of the Hubble Space Telescope. Attention is given to thinned-aperture optical systems' image quality characterization problems; the 'practical resolution limit' image quality criterion proposed is defined as the reciprocal of the spatial frequency within which no zeros occur in the modulation transfer function.

Harvey, James E.↗
NASA NTRS1988

Small-aperture infrared photometry of Seyfert 1 galactic nuclei

Multiaperture infrared photometry of the nuclei of five Seyfert 1 galaxies is presented. When apertures approaching the seeing limit were used, it was found that all of the nuclei have observed power law energy distributions with F(nu) proportional to 1/nu sq from 1.25 to 3.45 micron. After correcting for reddening, the spectral indices had a range of alpha = 1.4-1.9. These results are supported through infrared imaging of one of the program galaxies, and they demonstrate the importance of properly isolating the nuclear component from the stellar disk through the use of extremely small apertures. Although the nature of the infrared emissions cannot be determined, results indicate that a common energy source exists among the observed galaxies.

Simons, D. A.↗
NASA NTRS1988

Interferometric synthetic aperture microwave radiometry for the remote sensing of the earth

Interferometric aperture synthesis is presented as an alternative to real aperture measurements of the earth's brightness temperature from low earth orbit. The signal-to-noise performance of a single interferometric measurement is considered, and the noise characteristics of the brightness temperature image produced from the interferometer measurements are discussed. The sampling requirements of the measurements and the resulting effects of the noise in the measurements on the image are described. The specific case of the electronically steered thinned array radiometer (ESTAR) currently under construction is examined. The ESTAR prototype is described in detail sufficient to permit a performance evaluation of its spatial and temperature resolution. Critical aspects of an extension of the ESTAR sensor to a larger spaceborne system are considered. Of particular importance are the number and placement of antenna elements in the imaging array. A comparison of the implementation methodologies of radio astronomy and earth remote sensing is presented along with the effects of the source brightness distribution, the antenna array configuration and the method used for array scanning.

Ruf, Christopher S.↗
NASA NTRS1988

Large aperture scanning airborne lidar

A large aperture scanning airborne lidar facility is being developed to provide important new capabilities for airborne lidar sensor systems. The proposed scanning mechanism allows for a large aperture telescope (25 in. diameter) in front of an elliptical flat (25 x 36 in.) turning mirror positioned at a 45 degree angle with respect to the telescope optical axis. The lidar scanning capability will provide opportunities for acquiring new data sets for atmospheric, earth resources, and oceans communities. This completed facility will also make available the opportunity to acquire simulated EOS lidar data on a near global basis. The design and construction of this unique scanning mechanism presents exciting technological challenges of maintaining the turning mirror optical flatness during scanning while exposed to extreme temperatures, ambient pressures, aircraft vibrations, etc.

Smith, J.↗
NASA NTRS1988

Design of a large-aperture lens antenna usable over a +/- 15 deg scanning sector

A lens antenna design is described which requires the formation of both on-axis and far off-axis beams. The aperture is about 100 wavelengths in diameter, and the off-axis beams are displaced +/- 20 beamwidths from the axis. The results are indicative of the much greater scan capabilities of lensatic antennas compared to paraboloidal designs when both have single-aperture nonarray feeds.

Parsons, Chester L.↗
NASA NTRS1988

Compact wide-aperture hyperbolic analyzers

Compact electrostatic analyzers that employ a new geometry are described. They accept collimated beams of ions through relatively wide (non-slit-like) inlet apertures and focus them at various locations, depending on the ion energy per charge, along rectangular position-sensing detectors. Each analyzer has a dynamic range of 10 or more for a fixed deflection voltage. The focal line width at a given energy is a small fraction of the inlet aperture width, and the focal lines are approximately coplanar.

Curtis, C. C.↗
NASA NTRS1990

A finite element-boundary integral formulation for scattering by three-dimensional cavity-backed apertures

A numerical technique is proposed for the electromagnetic characterization of the scattering by a three-dimensional cavity-backed aperture in an infinite ground plane. The technique combines the finite element and boundary integral methods to formulate a system of equations for the solution of the aperture fields and those inside the cavity. Specifically, the finite element method is employed to formulate the fields in the cavity region and the boundary integral approach is used in conjunction with the equivalence principle to represent the fields above the ground plane. Unlike traditional approaches, the proposed technique does not require knowledge of the cavity's Green's function and is, therefore, applicable to arbitrary shape depressions and material fillings. Furthermore, the proposed formulation leads to a system having a partly full and partly sparse as well as symmetric and banded matrix which can be solved efficiently using special algorithms.

Jin, Jian-Ming↗