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Silent Knight Shines In the Dark

November 2007
By Henry S. Kenyon

The Silent Knight terrain-following/terrain-avoidance (TF/TA) radar will allow special operations aircraft to fly low-altitude missions with a low probability of detection. The radar also features new capabilities such as color weather displays and tactical data to enhance pilot’s situational awareness.
New technologies enhance low-level flying for special operations.

Flying at very low altitudes at night or in bad weather entails a range of challenges not encountered in other types of military missions. Whether an operation involves a strike aircraft penetrating heavily defended national airspace or special operations forces covertly inserting personnel, these flights require highly capable radar equipment designed to guide pilots over and around terrain they cannot see. A new tactical radar system will help warfighters navigate safely through hostile terrain under a variety of atmospheric conditions.

The essence of low-level flight is staying below terrain to avoid detection. Nap-of-the-Earth missions involve flying between hills and mountains through valleys and other low areas. Although such operations are difficult to carry out in full daylight and good weather, they become more challenging at night or in poor visibility. To carry out these flights successfully, pilots need terrain following/terrain-avoidance (TF/TA) radar systems.

Under development for use by the U.S. Special Operations Command (SOCOM), the Silent Knight radar is a multi-mode TF/TA system for fixed- wing and rotary wing aircraft. It is intended to serve on a variety of platforms, including MH-47G Chinook and MH-60M Blackhawk helicopters, MC-130 transports and CV-22 Osprey tilt-rotor aircraft.

The K-band radar includes a variety of new applications, such as navigation support systems, ground mapping and weather data. A color display highlights weather intensity, and a ground map mode picks out prominent terrain features. Silent Knight will have airborne and maritime functions to detect and locate aircraft and ships.

The radar’s mapping and weather capabilities are new to terrain- following systems, and they permit special operations forces to perform low-level insertion missions in all types of atmospheric environments. Silent Knight is 30 percent lighter and requires less power than previous airborne systems.

According to U.S. Defense Department budget documents, Silent Knight also could include or interact with several situational awareness technologies that SOCOM is developing for its aviation assets. These capabilities encompass digital terrain elevation data and electronic order of battle information, digital maps and enhanced situational awareness. Other network-centric capabilities may include data fusion, threat detection and avoidance applications, and electronic support systems to locate threats geographically and to identify specific radar emitters such as those used by antiaircraft systems.

One budget document noted that SOCOM is requiring new TF/TA radars to have reduced power needs. Cutting power also decreases the system’s radar emissions and reduces the potential for signals to be intercepted or detected. The document indicated that low-emission radars are also more capable of avoiding detection from passive sensor systems.

Because of the unique requirements for low-level flight, SOCOM is developing an on-board enhanced situational awareness (OBESA) system for its aircraft. OBESA will collect and consolidate threat data from on- and off-board sensors to create a single mission picture for the aircrew. It contains the Below-Line-Of-Sight Electronic Support Measures (BLOSEsM) processing software. BLOSEsM is an advanced receiver system designed to provide location data on threats that are below the line of sight of current special operations forces’ threat warning systems.

Another important aspect of Silent Knight is that it is intended for use on many different platforms. Traditionally, special operations aircraft used radars specifically designed for them. Mark S. Alderson, a Raytheon Company program manager, McKinney, Texas, explains that while the system may have to be slightly modified every time it is installed in a new platform, it remains the same basic equipment. “Instead of taking a system and adapting it for other platforms, we’re building this radar to be available to all platforms at one time,” he says.

SOCOM awarded the contract, valued at $164 million, to Raytheon in December 2006. Although SOCOM ordered it, other commands also will use Silent Knight. Raytheon is developing the radar with an industry team consisting of AIC, Crestview, Florida; DRS Technologies, St. Louis, Missouri; and Rockwell Collins, Cedar Rapids, Iowa. Alderson adds that Rockwell Collins is providing additional expertise in systems engineering, aircraft integration and flight test expertise. The first system is scheduled for delivery in 2012. The award also has an option for six low-rate initial production units, but Raytheon is not contractually obligated to produce them. The Silent Knight radar has not yet been assigned a model number.

The Silent Knight TF/TA radar is designed to operate in a variety of aircraft, such as the CV-22 Osprey, with little or no modification.
Silent Knight follows in a long history of TF/TA radar development dating back to the late 1950s. During the Cold War, terrain avoidance systems were designed to allow bombers and other strike aircraft to penetrate heavily defended Soviet airspace. But current U.S. military operations emphasize special operations missions such as inserting and removing reconnaissance teams from hostile territory. 

For flying at night and in low visibility, radar has a major advantage over electro-optical systems because it can penetrate clouds, rain and dust. However, a fundamental design challenge continues to be low-altitude ground clutter, says William L. Harland, Raytheon’s senior manager for new business. Most aircraft radars try to eliminate ground clutter, but TF/TA systems must be able to pick out key details, such as electrical towers, while providing pilots with enough warning to avoid them. “Our customers don’t get to choose when to fly their missions. Actually there are many times when they want to use the weather to cover their operations, so they want to be able to see through it,” he says.

Two key elements of TF/TA systems are the ability to read the ground the aircraft is flying into accurately and the capability to generate pilot commands to maneuver around or over objects. But safety remains the major design challenge, Harland points out. Low-altitude radars are built for high reliability because they cannot fail during a mission. He notes that the major distinction between terrain avoidance and other types of radars is that terrain mapping systems constantly test themselves. These systems run continuous diagnostic functions to ensure that they are emitting and receiving signals. When in the air, terrain avoidance radars spend 30 to 40 percent of their time conducting self-tests. “Every element of the system is tested not less than once every two seconds,” he shares.

If a problem is detected, the system will alert pilots to climb to a higher altitude. Ensuring crew confidence is an important part of the design philosophy for this type of radar. “For young people to sit in the pilot’s seat and do what this thing [the radar] tells them when they can’t see out the window, don’t know where they’re going and are in strange territory—this is faith,” Harland maintains.

TF/TA systems use guidance cues to tell pilots to climb or dive. “When you’re at low level and you can’t see outside the cockpit, and you’re flying at 100 to 500 knots, depending on the type of aircraft you’re in, there’s a lot of competence that has to be brought into the system. There are a lot of details. Even though we try to make it simple, it’s not an easy task for the aircrew. It’s stressful just being in that environment, knowing that they can’t make a mistake, and they can’t have the system make a mistake,” Alderson says.

Crew training and confidence are important parts of this mission. Well-trained pilots will quickly respond to the system’s commands. Alderson notes that crews are less worried about the terrain-following system providing them with the right cues than they are about reacting in time to the data. Reaction time is a critical issue. The dominant hazard in terrain-following flight is detecting smaller manmade objects. Fast combat jets do not typically fly as low as slower aircraft such as propeller driven platforms and helicopters. Because slower airplanes are more vulnerable to anti-aircraft fire, their crews desire to fly as low as possible to avoid detection. But at very low altitudes, a variety of manmade objects, especially in urban areas, become the primary danger.

Recent technological developments have made TF/TA systems even more reliable. Improvements in digital signal processing hardware and software allow systems to carry applications running more complex algorithms. These new programs enhance overall safety because they permit additional performance measurement applications to be installed to perform faster and more thorough system tests.

New technologies that soon will enter service include stored map, electronic warfare and weather databases. System testing is another important tool for ensuring reliability. Alderson notes that modern recording technologies allow large amounts of flight data to be captured for post-mission analysis.

Information sharing among aircraft conducting low-level flight is another area where development is taking place. Because of the difficult environment of terrain-following flight, maintaining formations is particularly challenging and dangerous. Aircraft currently rely on visual methods to remain safely in formation. However, Alderson notes that networked datalinks could provide new opportunities for low-altitude formation flying.

The requirements of low-altitude flight make simulators essential tools for crew training. Alderson explains that 15 to 20 years ago, terrain-following missions were simulated with cameras moving over model landscapes. Modern simulators allow pilots to practice on and operate equipment such as forward-looking infrared systems and radar. Improved training technologies provide such fidelity that aircrews feel they are in a real mission, Alderson says. He notes that simulators also can reproduce a variety of radar images and ground clutter.

Training applications save time and money while developing crew skills. Other new technologies include digital map databases that allow pilots to simulate flying over any part of the Earth. This capability allows the military to rehearse missions safely prior to deployment. “We used to have to go out and train new pilots to fly terrain-following [missions]. You might spend five or six weeks teaching, and the first couple of weeks were just getting them used to the environment. That takes a lot of aircraft hours, and there’s a lot of cost there. Now that simulators have gotten better, you can go and do all this practice at low cost to the units and teach better crew coordination in the cockpit,” Alderson says.

Web Resources
U.S. Special Operations Command:
Raytheon Company: