Japan has ordered four E-2D AWACS (Airborne Warning and Control System) aircraft for $425 million each (including spare parts, warranties and tech assistance). This is apparently the first step in an effort to replace its elderly (since 1987) E-2C aircraft. Given the age of the E-2Cs Japan considers it more efficient to order newly built E-2Ds than to refurbish the E-2Cs.
Japan is not the only E-2 user in the neighborhood. The other user went the upgrade route instead. In 2013 Taiwan received the last two of four E-2K AWACS aircraft from the U.S., where they have been sent for upgrading to the E-2C 2000 standard. The E-2C 2000 entered service in 2005, as an interim upgrade before the E-2D (with new engines, new phased array radar, new electronics, and many other improvements) arrived. The E-2K upgrade cost about $63 million per aircraft. Taiwan bought two E-2Ks new in 2006. The Taiwanese E-2K is very similar to the American E-2C. Taiwan received its first E-2s (four T models) in 1995. Meanwhile American aircraft carriers often operate in and around Japan and each of these ships carries E-2s.
The U.S. Navy received its first E-2D aircraft in 2010. This is the latest version of the E-2 Hawkeye radar aircraft that was originally introduced in 1964. The two engine, 24 ton E-2 was never produced in large quantities (fewer than a hundred are in use). In 2007 the E-2 fleet reached a milestone of a million flight hours.
The E-2C models began entering service in the 1970s, and are difficult to maintain because of their age. The E-2s always contained a large quantity of the most modern, and failure prone, electronics. Operating mostly off carriers, and thus constantly exposed to corrosive, salty ocean air, the aircraft takes a beating. The five man crew is mostly concerned with using the large radar carried atop the aircraft and keeping track of friendly, and hostile, aircraft and missiles up to 400 kilometers away.
The aircraft can stay in the air for 4-6 hours at a time and cruises at 450 kilometers an hour. Currently, the three "system operators" use large flat screen displays and many terabytes of disk storage each (for capturing and comparing data) to operate as a sea-going AWACS. It was the navy that developed the AWACS concept at the end of World War II, using Avenger light bombers equipped with radar to control large carrier strikes.
Each American aircraft carrier has four E-2s, and the U.S. Navy has a total fleet of about 70 E-2s. There are several dozen in service in other countries but only France operates them from carriers. Everyone else uses land based E-2s. About half the E-2s ever built are still flying, and the United States expects to keep using them (as the E-2D) into the 2020s. After that, an unmanned aircraft will probably replace the E-2.
Development and manufacturing of 75 E-2Ds for the U.S. Navy cost about $206 million per aircraft. The E-2D has longer range and more accurate radar, as well as much more efficient and reliable computer systems. Many of the current E-2Cs will get some of the electronics improvements, depending on how much money is available.