Passive mm-wave imaging from UAVs using aperture synthesis

Abstract

Passive imaging using mm-waves offers very significant advantages in scientific and military surveillance. However, the relatively long wavelengths mean that for the resolutions that are typically sought, the input aperture of the imager needs to be quite large; typically in excess of one metre. Deployment of conventional dish antennae of these dimensions on small aircraft is highly problematic. The use of snapshot synthetic aperture interferometric radiometry (SAIR) offers an attractive route to integrating a two-dimensional antenna array into the structure of an aircraft so that the transverse effective dimensions of the antenna can be almost as large as the aircraft. We report here a study into the feasibility of deployment of a SAIR on unmanned airborne vehicles and the achievable performance parameters. Critical considerations are the achievement of acceptable sensitivity and angular resolution from a SAIR that does not require excessive complexity. It is shown that traditional approaches based on fully sampling the spatial frequencies in the scene are unable to simultaneously meet all of these criteria, but that a SAIR based on thinned sampling of the spatial frequencies shows promise.

Doping Effect on Crystal Structure and Magnetic Properties of Highly Al-Substituted Strontium Hexaferrite Nanoparticles

“…In this paper we report the structural changes (unit cell volume, X-ray density, crystallite size and cell constants) may occur due to substitution of aluminum as much as possible into the ferrite lattice and also it's effect on the magnetic properties (saturation magnetization, coercivity and remanence) of the nanoparticles synthesized by the sol-gel auto-combustion method using citric acid. High levels of substitution render the materials unsuitable for use as magnets, but aluminum ferrites show promise for use in mm-wave imaging applications, potentially a very useful application as mm-wave radiation penetrates aerosols, dust and fabric [7,8].…”