On Aug. 26, 2024, Professor Yi Jianxin's team from Wuhan University's School of Electronic Information in China published an article in the Signal Processing Journal stating that, provided it is applied correctly, Starlink can be used to help Beijing discover stealthiness of enemy aircraft.
To test this theory, the team also conducted experiments using the DJI Phantom 4 Pro drone, and the results, according to the scholars, were very positive.
Phantom 4 Pro V2.0 - Credit DJI official site
What happened during the experiment?
In the ground-based radar monitoring equipment brought by Professor Yi Jianxin's team, the flight path of the approaching “stealth fighter” was clearly shown without the radar emitting electromagnetic waves.
In terms of the current level of technology, it is known that aircraft stealth is still mainly based on reducing the signals of electromagnetic waves, infrared rays, acoustic waves and other signals emitted or reflected by aircraft to enemy detectors, and these measures are usually taken:
one is by altering the shape and structure of the aircraft, mainly to eliminate or reduce the parts of the aircraft that generate strong reflected signals, such as the engine inlet, cockpit, radar antennae, wing leading edges, and external hangers. These areas usually form corner reflectors or the Doppler effect, resulting in enhanced radar echoes. Stealth fighters usually use design techniques such as smoothing, tilting, and splitting to give the aircraft simple geometric shapes such as polygons or rhombuses, and try to make the surface of the aircraft perpendicular or parallel to the direction of incidence of the radar electromagnetic waves, so that the radar electromagnetic waves will glide along the surface of the aircraft or be reflected in other directions instead of returning to the radar receivers. In doing so, it drastically reduces the aircraft's reflectivity to radar electromagnetic waves, causing the aircraft to appear as a smaller or fuzzy echo on enemy radar screens, or simply to disappear altogether.
The second is to reduce the RCS by 1-2 orders of magnitude by coating special materials that have complex dielectric constants and magnetic permeability that enable radar electromagnetic waves to be reflected, refracted, and interfered with many times within them, like coatings that can convert electromagnetic waves into thermal energy absorption.
In addition to the control of radar electromagnetic waves, stealth technology also includes the control of other types of signals, such as infrared stealth technology, acoustic stealth technology, as well as optical stealth technology, which is not very relevant to our topic today, so I will not repeat.
Stealthy aircraft does not mean that it can remain invisible in all situations. Typically, a stealthy aircraft has the smallest frontal radar scattering cross-section, a relatively large side radar scattering cross-section, an even more significant radar scattering cross-section in the tail, and the largest radar scattering cross-section in the upper and lower portions of the belly, which is almost a flat plate.
And the aircraft's wave-absorbing coating does not absorb electromagnetic waves in all frequency bands, just mainly for X-band (8.5-10.68 gigahertz) so that the mainstream frequency of modern radar to have a strong absorption effect, if the use of the lower frequency of the meter-band as a radar frequency, then the stealthy aircraft's stealth effect will be significantly weakened, or even ineffective.
Although the stealth aircraft can also be detected by ground radar, but here is a bug, is that the stealth aircraft is a surprise defense with the mission of destroying the local air defense network, carrying anti-radiation missiles is not a vegetarian, along with the ground radar emitted by the electromagnetic waves can destroy the radar.
The detection of passive radar
Passive radar, also known as passive radar, compared with traditional active radar that emits electromagnetic waves, does not radiate electromagnetic signals of its own, but borrows external sources of radiation for detection and positioning.
In this way, enemy detection equipment and anti-radiation missiles cannot use electromagnetic signals to capture, track and attack passive radar, greatly improving wartime survivability.
Theoretically, passive radars can use third-party electromagnetic wave radiation from the environment, such as signals from cell phone towers, television and radio broadcasts, and use the reflections of these signals to detect targets moving through a given airspace.
Operating Principle of Passive Positioning Radar Systems
The concept of passive radar actually existed in World War II, and in 1935, Robert Watson Watt, in a single-base passive system, used the short-wave radio frequency transmitted by the British Broadcasting Corporation (BBC) to illuminate a Seaford bomber at a distance of 10 kilometers, but due to the low data-processing capability of the time, the precise coordinates of the target could not be calculated.
Later, in the early 1960s, F. Pech, a radar designer at the Tesla Arsenal in Czechoslovakia, was given the top-secret task of developing a radar system capable of detecting radar-guided cruise missiles, such as the U.S. Matador, in the shortest possible time.
He took a different approach to the use of passive radar systems, and finally developed the world's first passive detection radar in 1963, named “Kopachi”, and since then, F. Pech and then again between 1979-1998, the development of successful “Ramona” radar, “Tamara” radar and “Vela” radar, that is, “Vela” series of passive radar. Radar, “Tamara” radar and “Vila” radar, also known as the “Vila” series of passive radar.
Photo: “Ramona” radar
In March 1999, during the NATO air campaign against the Federal Republic of Yugoslavia, the Federal Republic of Yugoslavia used the Tamara passive radar to detect and lock onto a United States F-117A fighter-bomber, which was eventually shot down by two SAM-3 missiles.
Passive radar can usually be divided into two kinds, one is based on the target's own radiation to locate the passive radar, in the detected target itself is a radar, communication radio, transponders, such as radiation sources or carry radiation sources, can use the detection of the target's radiation of electromagnetic waves for detection and tracking, a typical representative of the “Vera” series passive radar.
The Vera series passive radar
Another is based on the external radiation source on the target irradiation passive radar, this radar detection target itself does not directly radiate electromagnetic energy, passive radar is mainly through the antenna to receive from the external third-party direct wave, as well as the external radiation source irradiation of the target after the formation of the reflected wave or scattering wave, and then processed by the extraction of the target information and the elimination of useless information and interference, so as to complete the detection, localization and tracking.
Third parties in this case include radio and television stations, communication stations, GPS, active radars on various platforms, etc., and the radar that was used to detect the stealth aircraft with the star chain described at the beginning of this article is this passive radar.
Disadvantages of passive radar
The disadvantages of traditional passive radar are still relatively obvious, first, due to the heavy reliance on third-party electromagnetic signals, including cell phone signals, television stations, radio and other civilian electromagnetic waves, in the most important need to search for stealth aircraft outside of the border line, the sea line, there is a lack of these signals, and these civilian signals are unstable, and if the wartime was to be attacked or shut down, the radar will be affected by detection.
Secondly, passive radar can only provide two-dimensional or even one-dimensional data in most cases, and only the dimension of orientation is more reliable, which is far inferior to the traditional active radar to provide accurate three-dimensional data of the target's orientation, altitude and distance. And passive radar scanning frequency is relatively low, the continuous tracking ability is relatively poor. Therefore, passive radar can not be used as a guidance radar for air defense weapons.
The third-party signal source utilized by Prof. Yi Jianxin's team
Since the star chain needs to communicate with the user end, it must constantly send electromagnetic waves to the ground, and when the vehicle traverses the signal area, the electromagnetic waves hit the surface of the vehicle and produce reflected electromagnetic waves, which is the third-party signal source utilized by Prof. Yi Jianxin's team.
When these reflected electromagnetic waves are received by the multiple radar antennas that carry them, they undergo signal processing to decipher information such as direction, speed and altitude, so that the target that was originally invisible to the radar becomes visible.
This solution is mainly the first problem of passive radar, namely the stability of the signal source; after all, the number of satellites in chain orbit has now reached more than 7,000, it is unlikely--Chinese military analysts say--that the United States will prevent China's new radar from using “Starlink” technology and shut down the entire constellation of signal transmission to the ground.
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