Passive millimeter wave imaging growing fast

The International Society for Optical Engineering ^ | 2000 | Yvonne Carts-Powell

[theFIRMbss]

Passive millimeter wave imaging growing fast

by Yvonne Carts-Powell

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.

Developments

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."

 

Applications

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.

[theFIRMbss]

From another site,
notice this technology
can "see" ceramics!

[Tallguy]

I wonder if this type of sensing could negate stealth technology? Needless to say, we're heavily invested in stealth.

[fire_eye]

The question is whether it can be jammed...

[datura]

It would have no bearing on stealth aircaft - at least not in an operational sense. Stealth is designed to fool radar frequencies.

Visual stealth, OTOH, hasn't been made public yet. But it's definitely here.

[theFIRMbss]

>I wonder if this type of sensing could negate stealth technology?

Passive millimeter wave imaging considerations for tactical aircraft

Abstract:
Air Force, Army, Navy, and NASA Research Laboratories, in addition to major aerospace companies, are considering millimeter wave (MMW) imaging technology as an enhancement to sensor suits on both occupied and unoccupied vehicles. This is a review of the basic technology involved in MMW imaging and some of the programs and products that might benefit from passive day or night imaging through mist, haze, fog, clouds, smoke and/or dust. Potential applications include UAV surveillance of ground vehicles, airborne approach to airfields, tankers, flight leaders, and detection of airborne targets, including stealth aircraft.

[TC Rider]

wow, a Glock 7!

[datura]

Any electromagnetic signal can be jammed. From ELF to EHF - all you need is to transmit the corresponding signal to cancel the other one.

It's when you have multiple signals and their harmonics riding a carrier wave that jamming becomes more difficult.

[Blueflag]

Any receiver in a given frequency band(s) can be spoofed or jammed.

[The_Media_never_lie]

Interesting post!

[dartuser]

I didn't think there was a natural source for millimeter wave radiation. The article kept mentioning passive millimeter wave processing, but never mentions the source of the radiation.

Im curious about the image of the cars. Is there an active source and what we are seeing is the millimeter reflection?

Peaking my interest here ...

Anyone with a physics background that can answer some questions here ?

[datura]

But if it's in visual or passive sensing range, it's too late.

[bcoffey]

"Im curious about the image of the cars. Is there an active source and what we are seeing is the millimeter reflection? "

This is not a radar configuration. You can find electromagnetic energy throughout the spectrum, though we usually call it "noise." This method uses the "noise" as a source and performs incoherent imaging on the return.

So in a sense energy is "reflected," but it's the ambient signal that's reflected not an active radar signal.

Also, 94 GHz is selected as the frequency in order not to be absorbed by water vapor in the atmosphere.

[ArrogantBustard]

Any object or substance emits radiation according to Planck's blackbody equation. Note that emitted radiation intensity is a function of temperature and wavelength. The peak wavelength for any curve is a function of the temperature ... The peak wavelength for "Earth" at 289K is around 11 microns (or 0.011 mm) but that the curve drops fairly slowly ... there's still some energy out there at 1000 microns (1mm) to work with. Passive microwave radiometry has been done since the 1970s in the cm wavelength realm. Look up the USAF SSM/I sensor for an example of an orbital microwave sensor.

[theFIRMbss]

>But if it's in visual or passive sensing range, it's too late

I've read of "passive"
systems that monitor wide
areas. Perhaps

multiple units
can be forward-placed, and then
integrated well . . .

[dartuser]

I follow everything you said, and that makes sense. But the source question still plagues me. What in nature would emit 94 GHz radiation that could be reflected and detected? ... Are you saying that the radiation, being just background, is atomic? ... or cosmic, or solar?

See what Im saying ???

[ArrogantBustard]

It's not all reflected, some of it's emitted. Shiny objects tend to reflect the sky, which (if clear) is very cold. "Dull" surface objects are glowing at that wavelength, due to their temperature. Heat them up enough, and you'll start to see significant glowing at visible wavelengths.

[the invisib1e hand]

when gets to the passive-meter stage, the world will end.

[ArrogantBustard]

What in nature would emit 94 GHz radiation

Everything, including you! See Planck's equation.

[dartuser]

You really have me interested now ... I worked with IR at Wright Pat when I was in grad school. The EO group had active millimeter wave systems they were conducting experiments on ... I thought I remember one of the folks telling me there were no natural sources of millimeter wave radiation ... and all work was active ...

Of course, that was in the early 1980s ...

I will surely waste a few hours tonight on the internet reading some stuff.

Thanks for the info ...

[bcoffey]

See post #13. The Planck Law radiation curve gives a brightness of about 10^-13 watts per meter squared per Hz per steradian for a 300K blackbody at 94 GHz.

See Krauss, "Radio Astronomy" for more info.