The United States has spent hundreds of billions of dollars to study fifth generation of stealth fighters, such as the F-22 "Raptor" fighter of Lockheed Martin and the F-35 "joint attack aircraft". However, relatively simple signal processing enhancements, as well as equipping large warheads and terminal guided missiles, can potentially lock and fire the most advanced fighters of the US Army.
01. Low frequency radar can find stealth aircraft
It is well known that low frequency radar at very high frequency (VHF) and ultra high frequency (UHF) frequency bands can be detected and tracked to low visibility aircraft, but such radar can not help the missile attack targets, that is, to produce "weapon level" tracking. However, this statement is not entirely correct. Some experts say that there are ways to solve this problem.
Generally speaking, the low frequency radar guided weapons are limited by two factors: one is the width of the radar beam, the other is the width of the radar pulse. These two can be solved by means of signal processing.
The width of the radar beam is directly related to the design of the radar antenna. In view of the low frequency factor, the design of the antenna must be large. The early low frequency radar, such as the Soviet Union's P-14 very high frequency (VHF) radar, has a huge parabolic type designed to limit its beam width. Later radar, such as P-18, uses the Yagi Uda antenna array (Yagi-Uda). The antenna is relatively light and smaller. However, in terms of range and direction accuracy, these early low frequency radars are seriously deficient. In addition, these radar can not determine the height, because the radar beams produced by these systems generate a few degrees of migration at the azimuth, and a large amount of offset will occur at altitude.
Another traditional disadvantage of VHF and UHF radar is that their pulse width is too long, which leads to low pulse repetition rate, which means that such radar systems can't give accurate distance. The US professional test pilot said that the pulse width of 20 microseconds would generate a pulse of about 19600 feet. The radar resolution is only half the length of the pulse. This means that there is no more accurate measurement within 10000 feet. In addition, two very close targets can not be effectively separated.
Engineers have solved this problem, and their practice is to use phased array antennas, which reject parabolic matrices. Unlike the old mechanical scanning matrix, phased array radar can control the radar beam electrically. Such a radar can produce multiple beams while shaping the beam width, scanning frequency and other characteristics. In fact, in 1970s, there was enough powerful computing power to accomplish the task, which was later used in the "Aegis" combat system equipped on the US Navy's "Te Kant Roca" cruiser and the "Burke" destroyer. The active electric scanning array performs better and more accurately.
02. The big warhead can also strike a stealth fighter
If the missile's warhead is large enough, the resolution of the radar does not need to be precise enough. For example, the outdated S-75 "de la Na" (NATO code SA-2 "Gade Lai") has a 200 kilogram warhead with a killing radius of more than 30 meters. Therefore, a nominal 20 microsecond compression pulse with a resolution of 45 meters should be enough for the missile to approach the target. For a target with a distance of 30 nautical miles, the directional and elevation resolution must be similar to the angular resolution of about 0.3 degrees, since the launching radar is the only system that can guide the SA-2 missile. That is to say, an infrared sensor equipped with a scanning volume of cubic kilometers will pose a greater threat to F-22 and F-35 fighters. (authorship: Defense Science and technology important news / Li Xiang)
S-75 "de Van"
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