The area of passive millimeter wave imaging is still in development, but both commercial and military applications abound for the technology. A conference on passive millimeter wave imaging technologies will be held at Aerosense in March. "Most of the world leaders and renown people [in this area] will be there," says Chair Roger Smith of the Air Force Wright Laboratory (Eglin AFB, Florida).
Desire and past
Active radar at longer wavelengths and infrared (and optical) systems at shorter wavelengths are more mature technologies, but passive millimeter wave sensing could add some imaging capability, either in use alone or as one part of an imaging system that allows sensor fusion (The technology has been shown to be compatible with sensor fusion systems.). Millimeter waves can penetrate many sorts of inclement weather, as well as opaque solids, and offers a lot of contrast. The emissivity of objects in this region is over a range about 10 times that in the infrared. Millimeter wave sensing can detect metal targets well because they reflect the sky, which is very cold. "On a nice clear day," says Smith, "there's nearly 300 degrees K temperature difference to work with," between the metal object and its background.
Although there are applications in which active millimeter-wave sensing is more appropriate, passive sensing can avoid some problems of active sensing, including glint. Another advantage for military applications is that passive sensing is covert.
Work on passive millimeter-wave sensing was strong during the 1960s and 1970s, says Smith, but attention was diverted by the advent of FLIR (forward-looking infrared) systems. At the time, equipment for this region of the spectrum was bulky, but now MMIC (microwave and millimeter wave integrated circuit) technology allows sensing in this region from small integrated chips. Single-element scanners and imagers exist now for sensing at 35, 94, 140, and 220 GHz, says Smith.
Current work is pushing the technology in several areas. TRW (Redondo Beach, CA) has been actively involved in developing a passive millimeter wave camera. A report on the camera will be given at the conference by Larry Yujiri and others.
The development of focal plane arrays are needed to advance the field, and several papers deal with this issue during the sessions that focus on components. In another TRW paper, G. S. Dow and others report on a focal plane arrays for millimeter wave sensing. A notable development also reported by A. Rahman and others at MIT in Cambridge, MA is a room-temperature microbolometer array for this part of the spectrum. Other developments, says Smith, include superheterodyne techniques, direct detection, and MMIC component technology advances.
The science of modeling and understanding the phenomena of millimeter wave images also requires development. Researchers from the Air Force, Nichols Research Corporation, the University of Saint Andrews, TRW, and the Institute of Radio Engineering and Electronics in Russia all report work in this area. In addition, a program on millimeter wave analysis of passive signatures (MAPS), by Millitech Corp. in South Deerfield, MA and the Wright Lab provides a mobile testbed system consisting of three radiometers operating at frequencies of 35, 60, and 95 GHz. The conference's opening paper is by Doc Ewen of Millitech about MAPS.
The final session of the conference is concerned with increasing the resolution of images. Higher resolution is needed because the pixel size is larger than microwave (and shorter wavelength) sensors. David Gleed at the Defence Research Agency Malvern, in England, "is doing some tremendous work in resolution enhancement techniques," says Smith. At the conference, A. H. Lettington of the University of Reading in England, and Gleed report on a "new high-speed method for super-resolving passive millimeter wave images."
Millimeter imaging has, "tons of applications" says Smith. The ability to penetrate fog, dust, smoke, and light rain is at the root of several potential applications, including military target acquisition and aircraft navigation. The military would like a weatherproof imaging system to avoid situations such as sorties during Operation Desert Storm that had to turn back because the laser targeting systems would not work in inclement weather. Such a system could also be valuable for a covert unmanned autonomous vehicle.
For military airborne applications, says Smith, systems must build an image more quickly than current systems can-his group is working toward building a system that provides an image in 1 s as a demonstration of the technology.
For passive millimeter wave imaging to work for military, Smith cautions, it needs to be exploited in the commercial sector first, to drive costs down. There are numerous civilian applications. Civilian air transportation would benefit from systems, such as the autonomous landing guidance systems under development, that could aid pilots in landing during Category III conditions. In such conditions now, landings are not permitted. TRW is actively working on in this field. Because of the penetration at millimeter wave frequencies, imaging systems could be used to fight fires, by seeing through smoke. It might also be used for inland waterway navigation in fog.
Applications that make use of penetration of solids include concealed weapon detection for airport security-Smith says that tests in this area have detected plastic as well as metal weapons through clothing and even through 0.5 in. of sheetrock. This same ability might make the system useful for remote sensing of earth resources or ice.
Passive millimeter wave sensing has already been used by TRW to detect oil spills. The contrast between oil and water in the millimeter wave regime is sufficient to differentiate the two.
As the technology advances-and the activity of research in this area assures that it will -- other applications are likely to become apparent.
Yvonne Carts-Powell, based in Boston, writes about optoelectronics and the Internet