According to an article on the Japanese Ministry of Defence website:
The JASDF detects and identifies aircraft approaching the Japanese airspace with assets such as warning and control radars, and airborne early warning and control aircraft.
By such means, the JASDF ensures the security of Japan’s airspace. When there is the possibility that aircraft violates the territorial airspace of Japan, the JASDF scrambles its fighter aircraft to gather detailed information of the aircraft, and if necessary, track and monitor it.
The JASDF scrambled 947 times in the FY2019 (April 1st, 2019 – March 31st, 2020). Although this was a decrease by 52 times compared to the FY2018, the record was the third most since Japan first implemented the measures against airspace violations in 1958.
Including estimates, the flag countries of the aircraft, forcing the JASDF to take actions, were China with 675 scrambles (approx. 71%), Russia with 268 scrambles (approx. 28%), and other countries with less than 1%. By regional air defense forces, the Northern Air Defense Force scrambled 198 times, the Central Air Defense Force 35 times, the Western Air Defense Force 133 times, and the Southwestern Air Defense Force 581 times.
The Chinese military flights marked as unusual include the flight of H-6 bomber that flew between the main island of Okinawa and Miyako Island and the new Y-9 surveillance aircraft, which was visually confirmed for the first time.
Unusual flights by Russia include the violations of Japan’s airspace by Tu-95 bomber and A-50 airborne early warning and control; and the first visual confirmation of Su-34 fighter bomber. In the FY2019, all three publicly announced cases of airspace violation were the acts by Russian aircraft.
This was published on the Japanese Ministry of Defence website on June 2020.
Recently, the Japanese government decided to ramp up their airpower capabilities by buying 105 additional F-35s which will make it the second largest operator of the F-35s.
The additional aircraft will replace 100+ pre-MSIP F-15Js
According to the Australian Defence Business Review:
The US Defense Security Cooperation Agency (DSCA) has announced that the State Department has approved the acquisition of 105 Lockheed Martin F-35 Lightning IIs by Japan.
The July 9 announcement says Japan plans to acquire 63 more F-35A conventional take off and landing (CTOL) versions and 42 of the vertical take-off and landing (VTOL) variant. These numbers line up with a December 2018 announcement by Japan of its intention to acquire additional F-35s, including the F-35B for operations from its Izumo class helicopter destroyers.
The DSCA notice says the 105 F-35s will cost an estimated US$23.11bn ($A33.11bn), and will include five spare P&W F135 engines, and all of the usual US Government and contractor training, logistics, spares, ground support equipment, and delivery support.
Japan already has about 20 F-35As in service of a total of 42 aircraft on order, 40 of which will be built by Mitsubishi Heavy Industries at its final assembly and checkout (FACO) facility before Japan’s production switches to US-built aircraft.
The 27,000 tonne Izumo was recently reported as being part-way through its modification cycle to accommodate the F-35Bs, work which will include the installation of heavier deck plating, additional fuel bunkerage, communications, and landing and air traffic control systems.
There is little doubt that the significant buys of F-35s by Japan and Australia will provide then with significant opportunities to enhance their bilateral cooperation and evolution of common concepts of operations,