Developed originally by Ericsson Microwave Systems (now part of Saab Electronic Defence Systems’ business area), the Carabas foliage penetration (FOPEN) radar has been around for some time, but it has been improved significantly so it can see under the ground, and has been miniaturized so it can be carried by unmanned air vehicles (UAVs).

Carabas (coherent all radio band sensing) began trials in the early 1990s and was initially developed as a low-band (20 to 90 MHz) VHF synthetic-aperture radar that could see manmade objects, such as vehicles, through foliage and manmade camouflage. In its original guise the radar’s antennas were mounted in two inflatable fairings, each around 18 feet long, that projected aft from a Rockwell Sabreliner test bed aircraft. Trials with the system, and subsequent developments, showed significant promise and it has been proposed as a strategic system for use with large UAVs or business jet platforms.

Two Major Changes
Now the system has undergone two significant developments. First, the radar has been scaled down, so a useful tactical Carabas capability could be provided by a small helicopter or lightplane, or a small UAV such as Saab’s vertical takeoff Skeldar. The system has been decreased to around 33 to 44 pounds in weight, and has a power consumption of around 100 Watts.

Second, Saab has developed a ground penetration mode using high-band VHF and a changed polarization. The depth at which it can spot solid objects under the surface depends on radar depression angle and the nature of the ground. In wet soil areas a depression angle of 10 degrees gives a penetration depth of just over 11 feet, while an angle of 17 degrees into dry soil gives penetration of almost 20 feet. At 10-degree angle, Carabas can see to a depth of 85 feet into sand. The high-band mode also gives improved foliage penetration capability against targets such as humans.

Both modes can be accommodated in the same radar system, allowing areas to be searched for hidden targets either beneath trees or beneath the surface. The low-band VHF mode can be used at a standoff range of up to 3.7 miles and an altitude of almost 6,600 feet, in turn allowing an area of almost 105 sq mi to be surveyed every hour. The high-band mode can be used from 490- to 9,840-foot standoff and from 100- to 2,300-foot altitude. The hourly area coverage ranges from one to 17 sq mi.

Scaleable Carabas can provide a single-pass look at an area, but its capabilities are enhanced when used in conjunction with change detection algorithms. Newly hidden vehicles, or recently buried IEDs, can be detected rapidly by comparing the latest radar imagery with previously gathered data of the same area.

Saab has so far tested the new radar only from a mast, but is planning a series of flight tests in the fall, using a Schweizer 300 helicopter as platform. The company suggests that it could be in production with the system by the end of 2011, for service-entry in early 2012.

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