Combined infrared and visible imaging speeds airborne tracking

By Lawrence J. Curran, Contributing Editor

Massachusetts State Police helicopters carry dual-sensor system on search and rescue missions.

Using infrared (IR) imaging to visualize thermal profiles is important for airborne law-enforcement missions. When augmented with CCD color cameras on-board helicopters, such systems prove indispensable in carrying out search-and-rescue missions and finding hot spots in fires.

During a recent deadly arson fire in a Worcester, MA, warehouse, for example, helicopters from the Air Wing of the Massachusetts State Police (MSP; Norwood and Chicopee, MA) flew over the fire scene, relaying images via downlinks to ground-based fire and police posts (see Fig. 1). "Firefighters wanted IR images to see the heat signature of the building," reports Joe Gura, chief tactical flight officer for the Air Wing. "Those images indicated temperature differences in sections of the structure that helped firefighters decide where to direct water more effectively." Images from the color camera also provided the police with a visual update on the situation on nearby streets so that they could better direct traffic.

System design

To accomplish both thermal and visual imaging, the MSP uses a dual-sensor Mark III from FLIR Systems (Billerica, MA) mounted in an external gimbal on an AS355N TwinStar helicopter built by American Eurocopter (Grand Prairie, TX). At the heart of the dual imaging system is an autotracking system developed by Octec (Bracknell, Berks, England). This digital system operates with either an IR sensor or color camera without losing lock on the target.

FIGURE 1. In-flight cockpit view over the shoulder of chief tactical flight officer Joe Gura of the Massachusetts State Police Air Wing operating the Mark III IR/visible imaging system shows fire scene on the ground. Click here to enlarge image

There are two processor boards in the system—the system control computer and the autotracker processor. The system control computer is a PowerPC PC-104 board from Octec. The autotracker processor is an Intel 386-based 6U VME board purchased from Octec. The latter performs most of the video processing using Octec-developed algorithms, according to Nick Lagadinos, FLIR senior design engineer.

In the MSP system, the two sensors send signals directly to the autotracker processor. The FLIR sensor sends an RS-170 signal, and the CCD color camera generates an NTSC signal. The system control computer communicates with the Octec board. Autotracker-processed video is then sent to the display. Lagadinos says the display autodetects which sensor signal is coming in.

The PC-104-based system also accepts signals from the helicopter's global-positioning-system receiver and transfers the data over a serial link so that it can be incorporated into video data generated by the Octec board. The system controller also allows the system to interface to the helicopter's searchlight system and to use multiple video recorders so that one sensor's output can be recorded while the scene detected by the other sensor is being displayed.

Sensor design

The dual-sensor system enclosure is 13.5 in. high and weighs 26 pounds. It can provide 24-hour-a-day search and surveillance for rotary and fixed-wing aircraft. Its water-tight 9-in.-diameter externally mounted gimbal carries a payload that provides the system's vision for crime-spotting, search, and rescue (see Fig. 2). An indium antimonide (InSb) IR focal-plane-array detector from Santa Barbara Focalplane (Goleta, CA) operating in the 3- to 5-µm range senses thermal signatures in atmospheric conditions.

The system features a 10X continuous-zoom IR lens, built by FLIR Systems, which has a focal range of 25 to 250 mm. The long focal range allows crews to fly higher and see targets with better detail without losing lock on a target when switching between magnification levels. Housed beside the IR imager is a 1/3-in. color CCD camera from Watec America (Las Vegas, NV) that features a 10X continuous-zoom lens from Fujinon (Wayne, NJ). The CCD camera complements the system's IR capability, providing visible color pictures for daytime surveillance.

It was important to have a 250-mm IR lens for high-altitude imaging. "Once we settled on the focal length, we designed a full-zoom IR capability." Without the zoom feature, the view of a target could be lost as the operator switched from one magnification to another. With the zoom feature, the target can be continuously tracked while zooming.

FIGURE 2. The system's two key processor boards are the system control computer and the autotracker processor. The system control computer is an Octec board based on the PC-104 industry-standard using PowerPC microprocessors. The autotracker processor is a VME-based board from Octec that performs video processing and displays video with an overlay on the system monitor. Click here to enlarge image

But the greater focal length means the system is more sensitive to the aircraft's bounce and jitter movements, requiring better stabilization than in earlier systems. That led the company to develop fiberoptic gyros for the Ultra 7000, which provide greater bandwidth than conventional quartz rate sensors for stabilization and a steadier ride for the cameras. In addition, the IR system requires cryogenic cooling, so the basic design of the cooler was modified to have it fit inside the gimbal, along with the IR sensor and CCD color camera.

A video display shows the flight officer a view of the scene that includes autotracker status, position, and gimbal tilt angle in the corners. A tilting arrow on the left of the display points to the depression angle of the gimbal without having to count lines on a vertical scale, as in previous displays. The zoom position of both the IR and color cameras is shown as a sliding arrow.

The police use a 10.4-in.-diagonal, liquid-crystal, flat-panel cockpit display for the Mark III. The 800 x 600-pixel resolution SVGA LCD 98-100 monitor is from Amcomm International (Birchington, Kent, England) and was installed by McAlpine Helicopters (Oxfordshire, England)—a company that customizes helicopters for police use. This monitor was selected because it is compatible with the Air Wing's mapping system.

Meeting the challenges

Trooper Gura says the MSP Air Wing selected the Mark III for two reasons: "It's much smaller and weighs a lot less than the others, and that's a major advantage in helicopters, where you trade weight for fuel." Also, the location of the Billerica facility in northeastern Massachusetts makes it a quick and easy flight from either Norwood or Chicopee to the plant for maintenance and servicing the system.

The Air Wing's computer-based global-positioning-system mapping system, furnished by Sky Force Avionics (West Sussex, England), can display aviation charts, topographical maps, or street maps, as desired. "The screen has pushbuttons that allow us to toggle through any of these to select or identify the search area," Gura points out. A selector switch on the screen also enables the flight officer to toggle back and forth among the CCD color camera, IR sensor, and/or the mapping system to present the desired display on the monitor.

Gura says the dual-sensor system is used for police missions that include aerial surveillance, pursuit, and search and rescue. When the Air Wing is alerted to a need for its service, once airborne, the aircraft is guided to the area of interest by the mapping system. In the case of a search for a missing person, the flight officer usually makes radio contact with a ground unit, where another agency is coordinating the search operation.

"Depending on the terrain, we perform either a grid box or a pattern search," Gura says. "If we identify a target's thermal signature with the IR sensor, we may go in for visual identification or use the color CCD camera, then direct the ground searchers to the precise location of the person" by radio communication.

The Air Wing unit has been flying two Mark III-equipped TwinStars since August of last year, "and the advanced electronics of the color camera and FLIR system have enhanced our ability to see into areas we can't see with the naked eye," Gura points out. The dual-sensor system "allows us to more effectively provide services to other agencies, including police and fire departments."

Combined infrared and visible imaging speeds airborne tracking

System design

Sensor design

Meeting the challenges

Company Information

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