Japan already has bought Raytheon AIM-120 Amraams, so why is it spending ¥36 billion ($468 million) to upgrade about 60 F-2 fighters with the Mitsubishi Electric Corp. AAM-4B missile?

Although the benefits to Japanese industry are obvious, details of the upgrade and the missile itself suggest that the program is giving an enormous boost to the Mitsubishi Heavy Industries fighter's ability to counter enemy aircraft. The weapon has at least one advanced feature that other such missiles lack: a seeker with an active, electronically scanned array (AESA) radar.

The program will move into high gear in the financial year that begins April 1. The work is progressing in two parallel programs: integration of the AAM-4B missile, and upgrade of the J/APG-1 radar to a more powerful standard called J/APG-2. The improved radar, needed to exploit the new missile, will incidentally raise the capabilities of the aircraft by offering greater detection ranges.

Both systems have been developed by the Japanese defense ministry's Technical Research and Development Institute with considerable help from contractors, Mitsubishi Heavy Industries for the missile integration and Mitsubishi Electric for the radar. The same companies are contracted to do the installation work. Ministry officials tell Aviation Week that development went smoothly and is now complete.

Early in the development program, in 2001, the ministry gave rough indications of the AAM-4B's capabilities. It could be launched at a 20% greater range than could the then-current AAM-4 and at least as far as an “AIM-120B+,” a standard that was expected to appear around 2004. The crucial claim was that the AAM-4B could switch to autonomous guidance at a 40% greater range than either of the other two missiles and would similarly outperform what was expected to be the 2009 standard of the Russian R-77 (AA-12 Adder). In a 2010 paper, the ministry attributed the seeker's greater performance to the higher transmitting power available from the AESA.

The implication is that an F-2 firing AAM-4Bs can stop tracking the target for missile guidance much sooner than an unmodified F-2 can—and officials tell Aviation Week that the key aim of the project is indeed to increase the range at which an F-2 can turn away.

Referring to this detail, a former high-ranking U.S. Air Force officer says: “In the air-to-air realm, a 40% increase in range is very significant and would provide the [Japan Air Self-Defense Force] a very capable missile.” The same person, highly familiar with the electronic technology of air warfare, does not regard the advances claimed for the upgraded F-2 as improbable; they are to be expected, he says.

The ministry also says that the AESA seeker will have a better capability against a crossing target—one that reflects a radio signal with the same frequency as the ground. Fighter pilots can dive and turn at right angles to a threat to create that complication for enemy radars and missiles.

The number of F-2s that are already able to use the AAM-4B is undisclosed, but a budget allocation to do most of the radar upgrades in fiscal 2012 shows that the bulk of the installation program is only now getting underway. The missile integration, centered on an upgrade of the fire-control system, will be applied to 16 aircraft in that fiscal year, at a cost of ¥340 million per aircraft, while 40 will get the new radar, for ¥260 million each. Future budgets will determine when the planned 60 upgrades, enough to equip three squadrons, are completed. The work is being done in conjunction with heavy maintenance on each fighter.

Research and development of the F-2's fire-control upgrade began in 2004. The installation was mentioned in the fiscal 2010 budget, but little has been known about it. The ministry is always secretive about its air-to-air missiles.

The AAM-4, which has also been integrated in some of the Japan Air Self-Defense Force's (JASDF) Boeing F-15Js since 2007, has an active radar seeker and a data link. Like the Amraam, it can be launched at a range at which it could not detect the target. Instead, the fighter guides it toward the interception, making adjustments as necessary, with the missile seeker switching on partway through the flight—at which point the firing aircraft is free to evade, unlike one that must point at and illuminate the target for a semi-active missile.

Scant details released by the ministry show that the AAM-4 is larger than the Amraam, with the 200 mm (8 in.) diameter of the old, semi-active Raytheon AIM-7 Sparrow. It must have a correspondingly large antenna.

And in fitting an AESA antenna to the current production version, the AAM-4B, Japan appears to have made a notable advance. Made up of separate transmitter-receiver modules, such antennas are increasingly common in fighters but not, so far as is known, in air-to-air missiles. Among the advantages of the technology is greater detection range for a given antenna size.

There are no official hints as to how the AAM-4B would compare with a current-production AIM-120 version. An engineer from a Western missile company points out that its performance will also depend on guidance and control technology that he doubts would match the level of Raytheon's exportable AIM-120C-7.

Development of the AAM-4 was undertaken from the 1980s, with one aim being to advance Japan's indigenous missile technology. There seems to be no sign that the initial AAM-4 entered service, nor is it clear that the Amraam, which Tokyo bought as early as the late 1990s, has entered Japanese service in large numbers. The AAM-4B seems to be mainly replacing the vastly less-capable AIM-7M, which Mitsubishi Electric was still making as late as 2010.

Yet the ministry cites no specific urgency in the F-2 upgrade program. Rather, it is presented as just a natural enhancement of a type that will serve for 30-40 years. The F-2 became operational in 2000.

Japanese officials confirm that the upgraded F-2 will be able to handle several missiles in the air and several targets at once; in neither case is the exact number disclosed. The defense ministry has said that each upgraded F-2 can carry four AAM-4s.

The F-2's radar, before and after the upgrade, itself has an AESA. In other aircraft with AESA technology, messages are sent from the aircraft to the missile by the radar, but upgraded F-2s are fitted with a separate data link transmitter, the J/ARG-1. This is done because the guidance system was already developed for the F-15J, which has a mechanically scanned radar, and the ministry did not want to pay to adapt the system for the F-2.

The rest of the modification to the F-2 guidance system includes changes to the radar specifically for fire-control purposes, not as part of its separate upgrade to the J/APG-2 standard.

The ministry says the J/APG-2 is needed to exploit the capabilities of the AAM-4—specifically, to detect targets at ranges that match the kinetic performance of the missile, and to create sufficiently precise tracks at such ranges. For the missile to have any chance of hitting its target, the seeker must find the target in a limited volume of sky ahead of it.

The J/APG-2 is installed by upgrading, not swapping out, the J/APG-1. In its new form, the radar has greater radiating power and a new and faster signal processor that runs new algorithms. The original antenna is retained with undisclosed modifications. Officials declined to discuss specific performance and said they had no way of comparing the J/APG-2's capabilities with those of foreign radars. It was, however, the best radar that could be achieved with the technology at Japan's disposal, they said. A key limit on radar performance is the cooling system, but the ministry will not say whether it has been upgraded.

The J/APG-1 was the first AESA radar to go into service on a fighter, so program officials regard Japanese engineers as quite adept in the technology and say it was only natural that development of the J/APG-2 should present no great challenges. Be that as it may, the J/APG-1 has a reputation for poor performance, and it is not good enough to fully exploit the AAM-4B. There is a hint that the J/APG-2 is comparable to Raytheon's APG-79 on the F/A-18E/F Super Hornet: That U.S. radar is supposed to make full use of the Amraam, whose kinetic performance, according to the Japanese ministry, is comparable to the AAM-4B's.

The J/APG-1 is an X-band radar; the frequency band of the communications channel is not disclosed.

Beyond the missile-guidance and radar upgrade, the next step for the F-2 will probably be replacement of the mission computer, taking advantage of faster computer processors to support later upgrades, officials say. Research is underway and should be completed around 2018. No engine or cockpit enhancements are contemplated, nor has the ministry looked at fitting a stealthy weapons pod to the F-2.

The aircraft is not presenting any particular challenges as a base for upgrades, partly because it was designed with later modification in mind. Developers are finding it straightforward to rearrange and shrink components.

Maintenance, too, is presenting no big surprises, say officials, noting that its derivation from the F-16 has helped. Support arrangements are being gradually improved, in the Japanese spirit of kaizen, but no radical changes to them have been made and none is planned.

With Robert Wall in Singapore.

Japanese and U.S. Active-Homing AAMs
AAM-4B AIM-120 Amraam
Weight 220 kg (485 lb.) 151 kg (332 lb.)
Diameter 200 mm (8 in.) 178 mm (7 in.)
Length 3.7 meters (12 ft.) 3.7 meters (12 ft.)
Guidance Inertial with data link for midcourse, AESA active terminal radar Inertial with data link for midcourse, active terminal radar
Sources: Japanese defense ministry, U.S. Defense Department