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.
================================
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.
