Sunday, March 06, 2011

An Answer for the AN-2

Boeing's latest airborne warning and control system (AWACS) aircraft, mounted on an 737 airframe. Later this year, the South Korean Air Force (ROKAF) will become the third customer for the platform, taking delivery of the first of four aircraft. Acquisition of an AWACS capability will improve the detection of low-altitude threats, particularly North Korean AN-2 SOF insertion aircraft (Boeing photo via the Chosun Ilbo)

For years, one of the most vexing problems posed by North Korea has been an airplane that dates from the 1940s, and has a top speed of less than 120 knots. We refer to the AN-2 Colt, a Soviet design that dates back more than seven decades, but it remains a key insertion platform for Pyongyang's special forces.

Why is the AN-2 such a challenge. For starters, the bi-plane is built mostly from wood and fabric; the engine represents the only significant portion of the aircraft made from metal. So, the Colt has a very small radar signature, and detection is further complicated by its slow speed and low altitude flight profile.

Even against modern, 3-D air defense radars (like those in South Korea), the AN-2 can often avoid detection by dropping to low level and blending in with ground terrain) while keeping their airspeed below 120 knots. That puts them below the "velocity gates" of many ground-based and airborne radar systems, meaning they won't show up on the scope. Adjusting the velocity gate improves detection prospects, but it also introduces more clutter into the system, which also works to the AN-2's advantage.

Realizing that, North Korea has more than 200 AN-2's in its inventory. By some estimates, Pyongyang's Colt fleet could insert more than 2,000 commandos into South Korea for a preliminary attack, dropping them over allied airfields, C2 nodes, air defense sites and other priority targets.

But South Korea's defenses against the AN-2 (and other airborne threats) will soon receive a major upgrade. In June, the ROKAF will take delivery of South Korea's first AWACS aircraft, the Boeing E-737. Eventually, the South Koreans will receive a total of four 737 AWACS, enough to provide round-the-clock coverage--assuming high mission-capability rates and operations that don't last more than a few weeks.

South Korea will be the third nation (after Turkey and Australia) with an AWACS system mounted on a 737 airframe, instead of the 707s used by the United States and NATO, and the 767 jets used for Japan's AWACS. The migration towards smaller airframes is the result of several factors; first, with the primary threat in close proximity to its borders, South Korea doesn't need a long-haul jet that must cruise for several hours to reach its orbit area. Operating from airfields anywhere on the peninsula, the Korean AWACS will be on station in a matter of minutes after takeoff, so a larger airframe (with more fuel capacity) made little sense.

Additionally, the ROKAF is very aware that the 707 production line shut down years ago and the 767 will close after the U.S. Air Force acquires its next-generation tanker. After an aircraft goes out of production, parts become more scarce and maintenance becomes more difficult. Meanwhile, the Boeing 737 has been rolling off the assembly line for more than 40 years, and the airframe will remain in production for decades to come, ensuring the ready availability of spare parts, at affordable prices.

Use of the 737 also reflects the evolution of AWACS technology. The Northrop-Grumman Multi-mode Electronically Scanned Array (MESA) radar is smaller and doesn't require the larger, rotating rotodome found on earlier AWACS. Instead, the radar signal is transmitted (and received) through a fixed, vertical antenna mounted on the top of the 737. Additionally, the computer processors and related hardware are smaller (but more powerful) than on previous models, allowing them to fit easily inside the 737 airframe.

With advanced signal processing and a detection range of up to 500km. the E-737 will give the ROKAF a powerful tool for detecting the AN-2 and other low altitude threats. Of course, the new jet is also useful in a variety of other roles, ranging from maritime surveillance to the direction of air defense strike aircraft. Equipped with secure datalinks, the E-737 can share information with both ROKAF and allied aircraft.

In response to the E-737, North Korea may dust off its intercept tactics for high-value airborne assets (HVAAs) and ramp-up activity at its near-dormant SA-5 SAM sites. Optimized for engaging stand-off platforms (including tankers and battle management platforms), the DPRK's two SA-5 sites could force the E-737--and other aircraft--to orbit well south of the DMZ, decreasing their coverage of the battlespace.

Still, it would take the north a minimum of several months to beef up their SA-5 coverage; in recent years, analysts have rarely observed more than a single launcher-mounted missile at both sites, and radar tracking activity in those locations has been sporadic at best. As for the airborne intercept of the E-737, Pyongyang has that capability (they managed to surprise a USAF RC-135 over the Sea of Japan a few years ago), but the NKAF needs more practice--and there's little guarantee they would get past the Patriot belt and HVAA cap to actually complete their mission.

Meanwhile, we're guessing that North Korea will closely study the training patterns and orbit areas of the E-737 once it joins the ROKAF inventory. Such information can be used to alter the ingress routes and operational tactics of the AN-2, trying to take advantage of perceived coverage gaps and technical limitations. The Colt remains a serious threat on the Korean peninsula, but with introduction of the ROK AWACS, the bi-plane's will lose some of its vaunted "stealthiness."

1 comment:

BK Price said...

I'm really surprised you wrote this. The An-2 Colt is neither made of wood nor invisible to radar.

You can walk up to and touch an An-2 Colt at the War Museum in Seoul, ROK. In fact, you can rap on its metalit fuselage and hear the inmistakable sound of metal. The wings are indeed covered with painted fabric but the wing struts are also made of metal.

The fact that it has a propellor also makes it nearly impossible to be "invisible to radar." A propellor is one of the most radar reflective features of all on this aircraft (as well as on many others). The propellor also makes it noticeably more detectable at low altitudes when flying through the moutainous terrain of the border area in northern South Korea. Which is why the area is littered with ROK listening posts designed specifically to support the air defense network against that threat.

If the An-2 Colt is flying slow enough, you are correct it can get lost in the doppler notch. Assuming that its prop is not turning. If it is turning, then no matter how slow the aircraft is flying, it is still detectable by doppler radars. But, you could turn off the prop and just glide. Unfortunately, you can't do that at low altitude (or you crash into a mountain pretty quickly) so ground clutter becomes almost useless for radar masking.

If anything, the new "answer" the ROK is developing would be far more useful for detecting maritime insertion vessels that the NODAKs are also likely to use in order to insert far more SOF troops than the An-2 can.

Unfortunately, I have spent a good part of my career trying to dispel this myth of the An-2 invisibility which appears to be initiated at basic intel training at Goodfellow.