The U.S. Air Force is spending over $15 billion to apply Block 4 upgrades to its current 900 and subsequent F-35s. This collection of upgrades, most of them electronic and many of them classified, include a major upgrade to the AESA radar system that is the key element to the unprecedented situational awareness (knowledge of what is going on around you) F-35 pilots have. F-35 pilots report that this situational awareness is what makes the F-35 such an effective aircraft and very popular with its pilots. Block 4 upgrades include several improvements in the range and capabilities of the AESA radar along with upgrades to fire control and electronic warfare systems. Upgrades to the cockpit controls will ensure that the pilot workload does not increase with all the new capabilities. Pilots consider the efficient and intuitive cockpit controls a key aspect of pilot acceptance of all these capabilities. The air force has long paid attention to the pilot interface with aircraft capabilities, especially for single seat aircraft that can spend many hours in the air before landing. Block 4 increases the passive sensor capabilities. This system monitors heat and electronic sources some distance from the F-35 and enhances the F-35 ability to spot the enemy first. This capability is also used to simultaneously monitor ground activity when the F-35 is seeking or tracking ground targets. This enables an F-35 to maintain its stealth by sending brief, encrypted messages to non-stealth warplanes with target information. The non-stealth aircraft can launch weapons at ground targets they are not equipped to detect and track as effectively as an F-35. This capability keeps older, non-stealth aircraft relevant. These aircraft are also a lot cheaper to build and operate than F-35s.
The Block 4 upgrades also include a new engine, one that is more reliable and generates more electrical power to support all the electronic upgrades. Because so many of the upgrades are classified and only described in very general terms, there was some confusion over the use of different names for the F-35’s APG-81 AESA radar. So far a thousand APG-81s have been built and 2,000 more are planned for future F-35 production. APG-81 is standard equipment for all current and not yet produced F-35s. The AGP-85 appears to be a version only for American F-35s.
It’s common to have U.S.-only versions of some high-tech equipment. This is a security measure that is sometimes waived on a case-by-case basis. For example, the F-22 was never exported while the F-35 was very much built with export customers in mind. Some export customers, like Israel, get their way because they are the only F-35 user that regularly uses their F-35s in combat. Israel will share their experiences with their F-35s and any special modifications they made. Israel is a unique case and may get the APG-35 or be allowed to create their own variant (the APG-81I or 85I). For the moment those details, like so many others regarding Block 4 upgrades, are classified. The radar is only one of 75 upgrade items. It's quite possible that some of these items were developed by Israel or inspired by Israeli combat experience. The Americans and Israelis often trade new aircraft developments which is one reason why Israel is usually the first export customer to receive new products. This collaboration has led to many new items for combat aircraft. Some are still classified but most are not, if only because their use is quite visible and obvious. The inner workings of the F-35 electronics are less obvious.
Early on (2019) many American, Israeli and British pilots had flown the F-35 in combat. Most of these missions were flown over Syria and Iraq although marine F-35Bs have served in Afghanistan and Israel had Iranians thinking these stealth aircraft have ventured into Iran. By then dozens of pilots had spent hundreds of hours with their F-35s in combat zones. All these F-35 combat pilots found that the main advantage of the F-35 was its ease of operation and much enhanced situational awareness. Stealth is useful but not as much as those first two items. When flying in beast mode (non-stealthy because there are lots of bombs and fuel tanks carried externally), the ease of use and situational awareness enable pilots to operate much more effectively than any other aircraft they had flown. Because of these reviews by American and foreign pilots, export customers ordered more F-35s and new customers were interested. Other export customers who have just started pilot training are receiving the same reactions from their pilots and many of those nations are increasing their F-35 orders because of it.
The advantages of the F-35 create additional capabilities for pilots. For example, the ease of flying enables F-35 pilots to concentrate on something that still requires a lot of decision making by the pilot; stealth management and threat management. The stealth characteristics of the F-35 make it more difficult, but not impossible, for radar to detect it. How the pilots fly in a combat zone can improve the effectiveness of stealth. That is done by learning to manage the flood of “threat management” data that F-35 pilots have access to. By being able to concentrate on stealth and threat management, F-35 pilots achieve what has been the key element in air combat since 1914; getting in the first shot. From 1914 into the 1940s the key to success in air-to-air combat was knowing how to fly into a position where you would see the enemy first and carry out a surprise attack. The earliest of these tricks was the World War I tactic of trying to have the sun behind you to make it more difficult for the enemy to see you coming. Another tactic was trying to get higher and out of sight (for as long as possible) until you could dive on the enemy aircraft in a high speed, and unexpected, attack. In effect, “stealth” and the resulting surprise was always the key to victory. The F-35 was designed with that in mind. The radar stealth and maneuverability is not as good as the F-22, but the F-35 “situational awareness” is much better. Pilots who have flown the F-22 and F-35 always note this and point out that, in the hands of an experienced pilot, it makes the F-35 a more effective aircraft than the older and more expensive F-22.