We present an analysis of atmospheric propagation effects on current and proposed imaging systems. Effects such as extinction, resolution, beam spreading, anisotropy, and heterodyne efficiency are discussed. We also discuss a new DARPA program that will use atmospheric turbulence to an advantage to achieve super-resolution imaging. This super-resolution imaging phenomenon exploits a micro-lensing effect caused by atmospheric turbulence through use of a fast camera and signal processing to achieve better-than-diffraction limited resolution.
Several existing and potential future programs at the Defense Research Projects Agency (DARPA) are developing novel and highly sophisticated sensors. In order to make the most of these capabilities, the Department of Defense is developing means of distributing the information efficiently and sharing it with users who must make timely decisions. The goal is for the sensors to operate within network-centric architectures, which require high data bandwidths, flexibility and robustness. This paper does not deal with the communications between sensors, but rather the sensor technologies that fit within that framework. In particular, it describes imaging systems that exploit significantly different technologies such as speckle imaging, synthetic aperture ladar, and super-resolution. Techniques such as synthetic apertures are migrating between disciplines and it is important for developers in one field to be able to communicate effectively with those in other fields. The paper attempts to address issues in both the RF and optical disciplines, describing both the commonalties and differences.
Active systems, because they provide their own illumination, are capable of operating 24 hours a day and are not dependent upon the angle of the sun. Unlike passive systems, they can provide three-dimensional imaging. DARPA is currently developing systems, technologies, and signal processing to pioneer new or improve existing capabilities that employ active imaging capabilities. These involve both radar and ladar, ranging from a few MHz for foliage penetration to near-visible IR to achieve ultra-high resolution at long range. These capabilities would improve Battlefield Awareness (BA) and provide persistent Intelligence, Surveillance, and Reconnaissance (ISR) to perform target detection, recognition, and identification. This paper discusses two different approaches to active optical imaging. One is a coherent approach that uses synthetic aperture techniques with infrared laser radar, and another approach uses only the intensity of the speckle pattern in the aperture plane. Both are capable of producing ultra-high resolution at long range.