- Tactical Airborne Reconnaissance Goes Dual-Band and Beyond
Multispectral imaging technologies are satisfying the need for a "persistent" look at the battlefield.
André G. Lareau
The military action following
the events of Sept. 11 has put intelligence, surveillance and reconnaissance in
the spotlight. It has highlighted the need to persistently monitor a battlefield
to determine exactly who and what are there. Terrorists have become more successful
in developing not only unique methods of destruction, but also more sophisticated
means of eluding detection.
Tactical reconnaissance and surveillance cameras
must be able to find enemy targets by day or by night, whether they are moving,
fixed or camouflaged. Intelligence officers have discovered that infrared imagery
offers additional useful information when it is correlated with visible imagery.
Figure 1. An F/A-18 E/F equipped with a dual-band tactical reconnaissance camera flies
over the Pentagon in an October 2001 demonstration. Courtesy of the US Navy. Image
by Randy Hepp.
For example, infrared imaging can be
used to expose the fuel status of an aircraft on the runway. A daytime, visible-spectrum
image of the same aircraft would offer information about external details, such
as the plane’s markings and paint scheme. A dual-band, common-aperture camera,
however, enables the precision registration of the two images through a process
called fusion, which frequently yields more information than is possible by evaluating
the images separately (Figures 1 and 2).
Figure 2. Dual-band, common-aperture
imagery detects targets by day or night. The images were taken simultaneously with
Recon/Optical Inc.’s CA-270, mounted in a P-3 aircraft, from an altitude of
10,000 feet during a flight test with the Naval Research Laboratory in December
2001. The camera’s visible channel (left) uses a 25-megapixel CCD detector,
and the IR channel (right) uses a 3- to 5-μm band, four-megapixel focal plane
Image analysts are demanding this added
dimension of content to confirm a suspected target. The technology in these dual-band
cameras pushes the limits of focal plane array development, semiconductor fabrication,
airborne image processing, stabilization accuracy and optical system performance.
Visible-spectrum silicon CCD arrays
of 25 megapixels and larger have been in service for several years. The arrays collect large areas of reconnaissance imagery in a single frame. After they were deployed in Bosnia during the mid-1990s, battlefield commanders sought the rapid imaging of even larger areas. In 1996, this
led to the digital step-frame camera, in which a pechan prism and scan mirror in
front of the camera body allow imagery to be collected from various depression angles
as a plane flies over the battlefield.
To be comparable with synthetic aperture
radar, the arrays must offer coverage areas of 10,000 square nautical miles per
hour. The addition of a precision stepping mechanism in front of the traditional
framing camera enables the imaging of much larger areas, depending on the aircraft’s
altitude and speed.
The digital step-frame camera captures
a mosaic pattern of images, which are then electronically stitched together. In
this manner, high-speed reconnaissance aircraft can take multiple across-line-of-flight
images, resulting in collection capabilities of up to 10,000 square nautical miles
per hour from altitudes of about 25,000 feet and at typical velocities of 480 knots.
Massive IR arrays
To create a world in which there is no place for
an enemy to hide, one must be able to find targets at night. Thus, massive IR arrays
that can capture nighttime images are needed (Figure 3).
Figure 3. A nighttime IR image exposes the interior of an aircraft’s
wings, enabling an intelligence officer to determine whether it is fueled for launch.
Eastman Kodak Co. fabricated the first
of this type of array for Recon/ Optical Inc. from 1996 to 1998. The 1968 x 1968-element
PtSi array was wafer-scale — that is, 60 mm on a side — so that each
4-in. silicon wafer yielded one focal plane array. These arrays have been integrated
into a CA-265 IR framing camera and a CA-270 dual-band camera for flight tests sponsored
by the US Naval Research Laboratory in Washington and featured quantum efficiencies
of approximately 7 percent, with NEDTs of 0.1 °C.
It was realized, however, that a higher
quantum efficiency was required to meet the demanding specifications for airborne
tactical reconnaissance. Using an indium bump-bonding process on a silicon CMOS
readout integrated circuit, Cincinnati Electronics (now CMC Electronics Cincinnati
Inc.) fabricated a wafer-scale, high-quantum-efficiency array that is being integrated
into the latest generation of dual-band cameras (Figure 4).
Figure 4. Using indium bump bonds on a silicon CMOS readout integrated
circuit (left) boosts the quantum efficiencies of InSb IR focal plane arrays. Courtesy
of CMC Electronics Cincinnati Inc.
The final element necessary for dual-band
operation is a lens system permitting coincident collection of both visible and
IR images. This is illustrated in a catadioptric lens system in which both optical
channels are active, yielding true simultaneous visible and IR imaging (Figure 5).
Figure 5. A lens system that enables the simultaneous collection of visible and IR images is necessary for high-performance dual-band operation. In this camera, the reimaging
optics in the visible channel can be varied from 1:1 to 2:1, yielding a total effective
focal length of 50 to 100 in. The resolution of the IR channel depends only on the
50-in.-focal-length catadioptric primary lens.
The layout includes a 50-in.-focal-length
catadioptric primary lens that is reimaged into the visible and IR channels through
a CaFl beam divider/prism. The catadioptric primary lens has an elliptic first surface
and a parabolic secondary mirror. The 1:1 reimaging optics in the IR channel produce
an image resolution that is dependent solely on the primary lens. The reimaging
optics in the visible channel can be varied from 1:1 to 2:1 to yield a total effective
focal length of 50 to 100 in. Because the pupil diameter of the camera is fixed,
the aperture varies proportionally with the change in focal length (Table 1).
For pointing, the camera moves the
primary mirror by up to ±8° in azimuth. The camera body rolls to provide
depression-angle coverage from horizon-to-horizon through nadir. The camera can
be cued and pointed either by preplanned/preprogrammed mission data or by operator
intervention. Once cued, the time required to reach any point in the field from
any other point is less than 2 seconds. The camera pointing accuracy is within ±0.2°
in depression and ±0.2° in azimuth, and a solid-state stabilization system
provides stabilization in the roll and azimuth axes so aircraft motion does not
Further technology is under development to overcome
camouflage, concealment and deception. Instead of looking for targets in two spectra,
airborne imaging spectrometers will image in dozens, perhaps hundreds (Figure 6).
Figure 6. Spectrometer-based imaging systems
represent the next step in tactical surveillance. Such systems will offer spectral
as well as spatial information about a potential target.
These “hyperspectral” cameras
have been around for years, but their lack of robustness and their limited capability
made them impractical for tactical reconnaissance. This, however, is changing as
higher-speed electronics, precision stabilization and pointing, sophisticated diffraction
optics and larger focal plane arrays are becoming more available and affordable.
In these tactical imaging systems,
a slit is placed at the focal plane of the primary lens. As a mirror scans an image
across the slit, a diffraction grating within the spectrometer spreads the spectrum
of each line of the image and projects it onto a focal plane array, which can be
sensitive to a broad range of spectral energy from the visible to the far-IR. For
each “line” coming through the slit, a frame of data is recorded that
represents the spatial and spectral dimensions of the image. As the data is recorded,
an “image cube” is created that represents the scene with a complete
spectral signature for each spatial component.
Still newer technologies are on the
horizon. Advanced CMOS image-processing chips are being manufactured that will
enable teraFLOP-speed processing of dual-band imagery in real time, aboard the aircraft.
This will provide onboard fusion of dual-band imagery, as well as enhanced features
such as moving-target indications or real-time cueing against reference target data.
Precision geolocation information of cued targets determined from each fused image
will embed global positioning system tags with each target, enabling true “sensor-to-shooter” operations.
It will no longer be necessary for
data to be passed to a ground station to be turned into targeting information. The
process will occur at the camera, enabling the immediate detection and targeting
of an enemy. Networks of cameras and weapons platforms will fan out across wide
Through a combination of onboard and
offboard systems, such networks will offer a “persistent” look at the
battlefield, satisfying the need for information by day or night.
Meet the author
André G. Lareau is senior vice president
of advanced technology and programs at Recon/Optical Inc. in Barrington, Ill. He
earned a BS in electrical engineering at the University of Illinois in Urbana and
a master’s degree in engineering management from Northwestern University.
He has been with Recon/Optical for more than 20 years and holds seven US patents
relating to the application of electro-optical imaging to tactical reconnaissance.
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