An even simpler system that is even harder to counter than a long wavelength search radar is a passive detection system (PDS) that detects and tracks the radio frequency (RF) signals emitted by an aircraft—radar signals, UHF and VHF radio signals, identification-friend-or-foe (IFF) signals, data link signals like Link-16, and navigation transponder signals like TACAN.
A good example of a modern PDS is the VERA-NG, a Czech system being sold internationally that uses three or more receiving antennas spaced well apart to detect and track and identify the RF signals emitted by fighters and bombers. The system’s central analysis module calculates the time difference of the signals reaching the receivers to identify, locate, and track up to 200 aircraft transmitting radar signals. The VERA-NG is only one of many types of PDS used throughout the world: the Russians, Chinese, and others produce PDSs, as well, and these have been widely fielded for several years.
The beauty of a PDS, from the perspective of an adversary employing one, is that radar stealth is irrelevant to it ability to detect and track aircraft. If the aircraft has to use its radar, radios, data links, or navigation systems to accomplish its mission, the PDS stands a good chance of being able to detect, track, and identify it by these emissions. Every aircraft in the world is susceptible to PDS, stealth and non-stealth alike, and the F-35 is no exception.
The F-35’s main air-to-air weapon, the AIM-120, is a beyond visual range radar missile; as a result, the F-35 has to use a large radar transmitting high-power signals in order to detect airborne targets and then guide the missile to them . Likewise, the aircraft has to employ high-powered ground mapping radar signals to find ground targets at long range. Moreover, if the plane’s systems have to communicate with other aircraft in the formation or with off-board supporting aircraft like AWACS, it has to use its radios and data links. The F-35 is thus likely susceptible to detection by passive tracking systems. Several of these passive detection systems are significantly less expensive than search radars—and they are virtually undetectable electronically.
The DOT&E report also lists several major reasons for the limited interdiction usefulness.
One such reason is that the F-35’s Block 2B (USMC) and Block 3i (USAF) software prevents it from detecting many threats and targets while severely limiting the kinds of weapons it can carry. For example, the F-35 can currently only carry a few models of large guided direct attack bombs . None of these can be launched from a distance like a power guided missile. Rather they fall on a ballistic trajectory from the aircraft to the target, which means they can only be released at relatively short ranges in view of the target. For now F-35 pilots “will be forced to fly much closer to engage ground targets and, depending on the threat level of enemy air defenses and acceptable mission risk, it may be limited to engaging ground targets that are defended by only short-range air defenses, or by none at all.”
The small number of weapon types the F-35 can carry also limits its flexibility in combat . The current software can only support one kind of bomb at a time, which DOT&E says is only useful when attacking one or two similar targets. So, for example, when a flight of F-35s departs loaded with bombs designed to destroy surface targets, they wouldn’t be able to also destroy any hardened or bunker targets because they wouldn’t have the heavier bombs required.
The F-35 is projected to carry a larger variety of weapons as more software, bomb racks, and testing to validate these are developed—but we will not know until 2021 which of those weapons are actually combat suitable. Moreover, in order to carry something other than two large guided bombs it will have to use external weapons and racks, significantly reducing the plane’s already disappointing range and maneuverability—and, of course, more or less eliminating stealth.
The ability to penetrate heavily defended airspace to destroy fixed targets deep in enemy territory is an often-cited justification for the F-35. Of course, the F-35’s limited range —less than legacy F-16s—means that it is unlikely to be able to perform what the Air Force likes to call “deep strikes” well inside the homeland of large nations such as Russia and China.
The 2016 DOT&E report describes some official foot-dragging that has delayed putting the F-35’s penetrating ability to the test. For instance, the program is only now starting to receive the critical ground radar simulator equipment, which mimic enemy radar systems, that are needed to conduct robust testing of the F-35’s effectiveness in highly contested, near-peer, scenarios. It’s only receiving that equipment because it was sought and procured by DOT&E when it became clear that the Services and the JSF Program Office were not going to pursue a test infrastructure adequate for replicating the near-peer threats the F-35 is expected to be able to counter. Deliveries of this equipment have begun but will not complete until early 2018. The JPO has not planned or budgeted for developmental flight-testing against it.
The military does developmental and operational testing of stealth aircraft at the Western Test Range at Nellis Air Force Base in Nevada. The tests are conducted against the ground radar simulator equipment and surface-to-air missile (SAM) launchers. Aircraft being tested fly over these arrays to see if the aircraft’s onboard sensors—in particular its electronic warfare systems and ground mapping radar—combined with offboard intelligence provided via data links can detect the threats and respond appropriately, such as by warning the pilots, jamming the signals, or firing defense suppression missiles.
The problem is a complicated one because the radar signals that reveal the presence of a SAM, for instance, thereby allowing the aircraft to either target the SAM or avoid it, are not necessarily distinctive and often closely resemble the signals of radars that pose no immediate threat to the aircraft. The F-35 can't carry enough weapons to bomb everything. Its sensor and sensor fusion system must be able to tell the difference between enemy SAM radars that pose a genuine threat and the many innocuous radars that may be within range of detection—general purpose air surveillance radars, short-range, low-altitude air defense radars targeting weapons and not aircraft, and even nearby civilian air traffic control and weather radar systems.
Equally crippling, until the ground radar simulator equipment is in place, the F-35 program will be unable to properly develop, validate, and update the F-35’s mission-critical onboard software files, called Mission Data Loads (MDLs). MDLs are huge files specifying all target and threat locations together with their individual electronic and/or infrared signatures and all relevant mapping data. Without accurate, up-to-date MDLs, the F-35 cannot find targets or evade and counter threats—nor can it carry out the networking and sensor fusion functions that are said to be its primary strengths. The F-35 cannot go to war without its MDLs. The MDLs also need to be updated continuously with information concerning such things as threats, targets, and signals that is gathered on every F-35 mission. F-35 pilots can only be sure the MDLs they need to survive work properly after they have been tested over ranges equipped with the necessary ground radar simulator equipment.