Topic
Angular aperture
About: Angular aperture is a research topic. Over the lifetime, 1771 publications have been published within this topic receiving 27257 citations.
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13 May 1997
TL;DR: In this article, the effects of a deterministic ionosphere on SAR imaging for a radar operated with a 100 MHz bandwidth centered at 250 MHz and over an angular aperture of 23/spl deg/.
Abstract: Synthetic aperture radars (SAR) operated from airplanes have been used at very high frequency (VHF) because of their enhanced foliage and ground penetration compared to radars operated at higher frequencies. A satellite-borne VHF SAR would have considerable utility but in order to operate with high resolution it would have to use both a large relative bandwidth and a large aperture. The presence of the ionosphere in the propagation path of the radar will cause a deterioration of the imaging because of dispersion over the bandwidth. In this paper we present calculations of the effects of a deterministic ionosphere on SAR imaging for a radar operated with a 100 MHz bandwidth centered at 250 MHz and over an angular aperture of 23/spl deg/. A daytime ionosphere induces a point spread function with an approximate width of 100 m in slant-range compared to the nominal resolution of 1.5 m.
13 citations
Patent•
12 Nov 1992
TL;DR: In this article, a zoom lens system comprising, in order from the object side, a first lens unit having a negative refractive power as a whole, a second lens unit being a negative this article as a overall, and a third lens unit with a positive this article was presented.
Abstract: A zoom lens system comprising, in order from the object side, a first lens unit having a negative refractive power as a whole, a second lens unit having a negative refractive power as a whole and a third lens unit having a positive refractive power as a whole. This zoom lens system has a very wide field angle of approximately 102° to 93°, a high aperture ratio of F/2.8 and a relatively simple composition.
13 citations
11 Jul 2010
TL;DR: In this paper, an alternative procedure based on power measurements is proposed and evaluated for a double-shell dielectric lens, compared to those obtained for the same lens with the usual method based on measured phase pattern.
Abstract: One critical performance parameter associated with feed systems for reflectors or other focused antennas is the feed phase center position. Maximum aperture efficiency is obtained when the feed phase center is positioned at the reflector focal point. Although horn antennas are the most common feeds for reflectors, lens antennas with bolometer detectors integrated at its base are the most common configuration [1] at millimeter and, especially, at sub-millimeter waves imaging and radiometric applications. Unlike horns, where the phase center position is normally located close to the aperture plane, the phase center of integrated lenses is very dependent upon the lens shape and may even be far out from the lens body. However, when bolometers or other detectors are used at the lens under test, only amplitude measurements can be obtained, preventing the usual phase center calculation from radiation pattern phase information. In this communication an alternative procedure based on power measurements is proposed and evaluated. Corresponding results obtained for a double-shell dielectric lens are compared to those obtained for the same lens with the usual method based on measured phase pattern. The alternative method uses an auxiliary collimated lens and can be used in small anechoic chambers.
12 citations
TL;DR: In this article, the authors computed the near-field radiation of several electrically large apertures based on the aperture field convolution method, including a square aperture, a circular aperture, and a serrated edge aperture.
Abstract: The near-field radiation of several electrically large apertures has been computed based on the aperture field convolution method, including a square aperture, a circular aperture, and a serrated edge aperture. The frequency range is 8.0-12.0 GHz. The aperture diameters are limited to 5.00 m and the corresponding electrical size is 133-200 wavelengths. The time domain spectrum has been obtained by chirp z-transform with a Hamming window and the mechanism for the generation of the aperture near-field radiation in time domain is analyzed. The calculation results demonstrate that the near-field radiation of the aperture can be approximately seen as the synthesis of a plane wave from the aperture, cylindrical waves from the edges, and spherical waves from the corners. By this method, the direction, position, and magnitude of incoming waves in the near-field region can be estimated, and the aperture design can be modified to meet the requirements of the near-field radiation. The near field of the three apertures mentioned above has been compared, and it is shown that the serrated edge aperture has more uniform direct wave and lower diffraction waves, making it a candidate for compact range (CR) aperture design.
12 citations
TL;DR: In this article, the propagation of higher-order Gaussian (HOG) beams through ABCD paraxial optical systems and their diffraction by a finite aperture are studied by means of the generalized Huygens-Fresnel integral.
Abstract: The propagation of higher-order Gaussian (HOG) beams through ABCD paraxial optical systems and their diffraction by a finite aperture are studied by means of the generalized Huygens-Fresnel integral. Numerical calculations have demonstrated that the field distributions of apertured HOG beams vary with the beam and aperture parameters, and with the propagation distance, the diffraction effects can be ignored only when the relative aperture a/w 1 (i.e. the ratio of the aperture radius a to the spot radius w 1 of the fundamental mode at the aperture position) is greater than 3 for our calculation HOG modes.
12 citations