North Korea recently successfully tested its first cruise missile, one with a range of 1,500 kilometers and the ability to hit targets in Japan. Cruise missiles are not sanctioned by the UN, while North Korean ballistic missiles are. South Korea and Japan have been spending a lot of money on ABM (anti-ballistic missile systems) but little on cruise missile detection. South Korea has developed a growing number of detection systems for small UAVs that North Korea has been sending south for reconnaissance. As Iran has demonstrated, such UAVs equipped with explosives and sent on one-way pre-programmed missions can be very effective. North Korea mentioned Japan as a target because cruise missiles coming in low over water are the most difficult to detect. The U.S. developed a solution (JLENS) for that in 1998 but it was not produced and tested until 2014 and then cancelled in 2017 because it was too expensive ($175 million each) compared to smaller but similar systems that were much cheaper. The JLENS system uses two 75 meter (233 foot) long, helium filled, unmanned blimps equipped with radar and other sensors. A JLENS blimp is about 2.5 times the size as the more familiar advertising blimp. The JLENS blimp is an aerostat, a blimp-like vehicle designed to always turn into the wind and stay in the same place. The JLENS blimp is unpowered and secured by a cable (tether) that can keep the aerostat in position at its maximum altitude of 5,000 meters (15,000 feet). At that altitude the JLENS aerostat can carry a two-ton payload. The cable also supplies power, which means the blimp can stay up for about 30 days at a time before it is brought down for maintenance on its radars. Two radars are carried in each aerostat. One is a surveillance radar, the other is a PTIR (precision track and illumination radar). The surveillance radar provides long-range coverage that is more than 300 kilometers, although the exact range is secret. At 300 kilometers the cruise missile is still 20 minutes away. PTIR is a steerable system capable of tracking multiple targets, can focus on items of interest, and provide essential location data for air-defense missiles or manned interceptor aircraft. Each JLENS can cover a huge area and pass target data to airborne or ground based missile systems for interception. Japan would need something like JLENS because it has a huge coastline and cruise missiles can be programmed to take advantage of any gaps in radar surveillance.
The North Korean cruise missiles look remarkably like the American Tomahawk, the first modern cruise missile that entered service in 1983 and has been continually updated. About 8,000 Tomahawks have been produced since introduction. Although Tomahawk is a simple design using widely available components, many nations have paid for the wreckage of Tomahawks to obtain details of its design, especially the guidance system, so they can produce clones.
A new generation of cruise missiles are in the works that use a stealthier shape and electronic countermeasures to defeat detection. These double the $1.5 million cost of current designs. North Korea can build Tomahawk clones for less than $500,000 each. Meanwhile, neighboring South Korea is way ahead in cruise missile technology as well as defenses against them.
South Korea introduced its Hyunmoo-3 cruise missile in 2008 and it was similar to the Tomahawk. The initial version had a range of 500 kilometers which was subsequently increased to 1,000 and by 2012 was 1,500 kilometers. Hyunmoo 3s were deployed along the North Korean border, aimed at ballistic missiles, nuclear weapons, and other strategic targets to the north. The South Korean cruise missiles were a real threat to North Korea, which has elderly air defense radars that have a hard time tracking conventional aircraft.
North Korea did have three-hundred AN-2 single engine bi-plane transports. The AN-2 is a sturdy Russian aircraft which, although designed in the 1940s, is still very popular, so much so that about 18,000 were produced between 1947 and 2001. A well-maintained AN-2 can last for decades and many have. Few AN-2s saw military use but North Korea realized an AN-2 could carry ten passengers and were adapted to deliver parachute-equipped commandos into South Korea early on in a war. Several thousand of these troops could cause a lot of confusion as South Korea mobilized for war. South Korea eventually stationed mobile anti-aircraft weapons (stationary and self-propelled) in areas where the AN-2s were likely to be used in the event of a war, or simply individually to deliver spies or commandos into South Korea. In the last decade the North Korean AN-2s have been showing their age and suffering from lack of maintenance. Pilot training has also declined due to fuel and money shortages in the north. The AN-2s are still a threat, just a much less reliable one.
South Korea has been developing its own ballistic missiles in the last decade and more money goes into that than cruise missile production. This shift is a result of increased North Korea aggression that led the U.S. to agree to a modification of a treaty that guaranteed speedy and massive support for the defense of South Korea if North Korea attacked. In return South Korea agreed not to develop and build ballistic missiles and nuclear weapons. South Korea, like Japan, was much more capable of quickly developing such weapons and only refrained from doing so because, like many non-nuclear nations, the Americans offered protection. That worked against the Soviet Union and then Russia and China. North Korea has demonstrated tha it is not that easy to deter.