Prospects for using cellular networks to detect air targets
It regularly happens that visitors to the Military Review website suggest topics for my next article. This is exactly what happened this time: in a publication devoted to the problem of early detection of enemy UAVs, one reader wrote the following:
Since 2018, research and development work has been underway on the use of cellular signals reflected from drones, in passive radar systems for UAV detection. No one is stopping the mobilization of cellular communications infrastructure for military purposes, especially since the Ukrainians are already doing so.
Also, an interesting and well-written article was recently published on Military Review Some challenges of drone acoustic location: why sound is not radio .
Given the topicality of the issue, within my area of expertise and drawing on open sources, I will attempt to provide a comprehensive understanding of the suitability of cellular networks for detecting aircraft in general and UAVs in particular. I will also examine the chronology of development work in this area and the outcomes.
Passive, bistatic and multistatic radar
To understand how mobile communications infrastructure of various standards can theoretically be used to detect objects moving in airspace, it's worth understanding the terminology and briefly discussing what radar is in general, and how passive radar differs from bistatic and multistatic radar.
Different methods of radar detection of aircraft
The most common are active radars (Fig. a), which transmit into space and receive a signal reflected from a target. The antenna of such a radar generates a narrow beam (1-6°) in the azimuthal plane with electromagnetic pulses. The beam can be scanned in a circular or sectoral pattern in the vertical or horizontal plane. If an aerial target enters the beam, the receiving part of the station receives the pulses reflected from it, which is used to determine the target's direction. Thus, the azimuth is determined as the beam passes over the target, and the distance to it is calculated based on the time delay of the reflected pulses relative to the probing pulse.
Transponders that transmit response signals after receiving a coded interrogation (Fig. b) are widely used in civil aircraft traffic control. State identification systems operate on the same principle, but they are unable to independently detect aerial targets without processing the response signal.
Passive radars (Fig. c) do not emit electromagnetic waves themselves, but receive signals transmitted by aircraft, for example, by recording the operation of onboard radar, communications equipment, and radio altimeters. A significant advantage of passive radars is that they do not reveal themselves with high-frequency radiation, are not detectable by enemy electronic reconnaissance equipment, and cannot be targeted by anti-radar systems. missiles.
To more accurately determine the target's location, course, and speed, multiple passive receivers are typically deployed, sending their data to a command post where triangulation is used to calculate the parameters of the observed object. Direct measurement of the pulse arrival delay at each receiving station is also used.
Czechoslovakia was a pioneer in the field of passive radar among the Warsaw Pact states, putting the PRP-1 station into trial operation in 1963.
The equipment room of the passive Czechoslovakian radar PRP-1
Three receiving equipment stations on a Praga V3S cargo chassis, located in different locations (one central and two on the sides), made it possible to determine with acceptable accuracy the coordinates and headings of radio signal sources in the frequency bands of 1000–2000 MHz (frequency range D), 4000–8000 MHz (G/H), 8000–20000 MHz (I/J), as well as IFF and TACAN transponders operating at a frequency of 1090 MHz. The operator manually recorded radio signals on the station's screen, and target information was transmitted verbally to a mobile information processing center via a radio network. The target's position and heading were then calculated based on the available data. The PRP-1 system's equipment could simultaneously track six aircraft. The PRP-1 passive search system remained in operation until 1979.
The experience of operating the PRP-1 station was considered successful, after which the Czechoslovakian company Tesla designed and mass-produced the passive radar systems KRTP-81 "Ramona", KRTP-81M "Ramona-M", KRTP-86 "Tamara" and KRTP-91 "Tamara-M".
The Tamara radio system components were mounted on eight Tatra T815 8x8 trucks. The cylindrical passive direction-finding sensors were hydraulically raised to a height of up to 25 meters. At the combat position, the receivers were spaced 10–35 km apart.
Antenna post RTS KRTP-86 "Tamara" with hydraulic lift
The Tamara RTS was capable of operating against tactical (deck) aircraft. aviation, strategic bombers, AWACS aircraft, electronic reconnaissance aircraft, and jammers. The receiving equipment could detect airborne radar emissions, IFF transmitters, TACAN navigation signals, DME rangefinders, JTIDS tactical information exchange systems, and active jammers operating in the 0,82–18 GHz range.
During field trials conducted on the border with Germany, the KRTP-86 passive direction-finding equipment detected an F-16A target at a range of 400 km, and an F-15A at 365 km. Previous-generation fighters such as the F-4E were detected at 395 km, and the F-104G at 425 km. The detection range of jamming aircraft and AWACS was within the radio horizon. The Tamara system's computing system could track up to 72 targets within a 100-degree sector. The upgraded KRTP-91 Tamara-M electronic system featured improved display and processing capabilities and was capable of engaging targets within a 120-degree sector.
Before the collapse of the Warsaw Pact, Tesla built 23 Tamara family electronic intelligence systems. According to Western sources, 15 systems were delivered to the USSR, one to the GDR, and four passive radars were adopted by Czechoslovakia. In 1991, the United States managed to acquire one modernized KRTP-91, purchasing it through Oman.
Work on passive radars was also underway in the USSR. In 1987, the Topaz plant in Donetsk began producing the Kolchuga electronic reconnaissance station. The Kolchuga mobile passive electronic reconnaissance station's equipment was housed in three vans on a KrAZ-260 chassis.
Passive electronic reconnaissance hardware stations "Kolchuga"
The Kolchuga system can accurately determine the coordinates of ground and surface targets within the range of their emitted radio signals, as well as their movement routes. For airborne targets flying at an altitude of 10 km, the detection range reaches 800 km. The station is capable of not only detecting but also recognizing ground and airborne targets. The system's memory stores various patterns of suspected targets.
However, despite all their advantages, passive radars are a supplement to active radars that generate high-frequency radiation, and they are unable to detect an aerial target operating in complete radio silence mode. Passive radars have low accuracy in determining aircraft coordinates, making them unsuitable for direct target designation by air defense systems. They can only be used for initial detection, which requires additional reconnaissance and clarification of the air situation.
In a bistatic radar, the transmitting and receiving antennas are separated by a significant distance, which is sometimes used, for example, in over-the-horizon radars.
The working diagram of a bistatic radar
The probe signal from the transmitter is reflected off the target and reaches the receiving antenna. The receiving equipment analyzes not only the reflected signal but also the direct signal—the one coming directly from the transmitter. The difference in the arrival times of these two signals allows us to calculate the transmitter-to-target and target-to-receiver distances, and therefore the target's coordinates.
A system consisting of multiple spatially distributed bistatic radar components with a common coverage area is called a multistatic radar. It consists of at least three components—for example, one receiver and two transmitters, or two receivers and one transmitter, or multiple receivers and multiple transmitters. One or more receivers process signals reflected from targets transmitted by one or more geographically dispersed transmitters. Thus, a radar network built using cellular transmitters can be considered multistatic.
Projects for the creation of radar systems for detecting air targets based on cellular communication stations
Back in 2002, the multinational aerospace corporation BAE Systems contracted British company Roke Manor Research to develop the Celldar (CELLular raDAR) system for practical use. In 2003, during tests of equipment developed by Roke Manor Research specialists and consisting of a receiver, a remote antenna, and a laptop with specialized software, the ability to detect air and ground objects within the GSM network coverage area was demonstrated to the leadership of the British military. However, during the tests, it became clear that the range of the receiving equipment depended on many changing factors and did not ensure reliable detection of objects across the entire range of altitudes and speeds. Therefore, this project was not pursued further. However, according to available information, the software developments obtained during the development of the Celldar system were used for the "fight against terrorism" to precisely determine the coordinates of mobile devices.
Testing of the dual-use capabilities of signals emitted by cellular base stations was also conducted at the Belarusian State University of Informatics and Radio Engineering, the American corporation Lockheed-Martin, the Israeli company Elta Systems, and the European concern EADS. However, all the developers concluded that this approach was unviable, and the project did not progress beyond the prototype stage.
We have also conducted research in this area. For example, in the fall of 2016, Russian media reported that specialists from the Moscow-based Kant Research and Production Enterprise had developed a specialized radar system for detecting, tracking, and providing target designation for small targets and low-altitude cruise missiles flying at speeds up to 500 m/s and altitudes up to 500 m.
It was reported that the conceptually new product was based on an idea proposed by employees of the Scientific and Technical Center for Electronic Warfare (STC) EW) during the development of the Pole-21 system. The Pole-21 system, with its R-340RP transmitters, is designed to generate noise interference over a large area, impeding the operation of satellite navigation systems such as GPS, GLONASS, Galileo, and BeiDou. Initially, the existing network of cell phone towers was intended to be used for the transmitting antennas, linking the numerous "highly protected" command and control posts of the Pole-21 system with the antennas, receivers, computing system, and communications equipment of the multistatic air target detection radar system.
It's worth noting that the Pole-21 mobile electronic warfare system, with equipment housed in vans based on KamAZ-5350 or KamAZ-6350 all-wheel drive trucks, is in mass production and has demonstrated good performance. According to open sources, a single R-340RP transmitter with a radiated power of up to 1000 W is capable of jamming satellite navigation signals within a radius of 20-25 km. Each system post includes one van containing the equipment and up to three antenna modules.
However, the idea of placing antenna modules of the Pole-21 system on cellular towers, which are also used to detect air targets, was not implemented due to insurmountable organizational and technical problems.
Even without considering how the specialists at the Moscow-based Kant Research and Production Enterprise planned to ensure resilience to the massive use of air attack weapons, the receivers' immunity to jamming, and their electronic compatibility, the reliability and redundancy of communication channels, the provision of autonomous power sources, and the integration with existing air situational awareness systems and warning networks, the proposed multistatic radar system, built on cellular base stations, initially had insurmountable flaws. Cellular networks, as we know, do not scan space, and their antennas have fixed beam patterns of 60–120°. The range and location of a phone are determined by analyzing the time delay when in the coverage area of several stations, and the more stations there are, the higher the measurement accuracy. The phone itself can also transmit a "tag" with real time.
The black spot is the approximate location of the phone, determined using three base stations.
But to accurately determine a phone's location, it must be registered with the network and turned on. The base station sends a signal to a specific subscriber, and only that subscriber must respond. This means that mobile networks, like civilian air traffic control radars, use the principle of active response radar. If an aircraft doesn't have an active device on board that works with a SIM card from that specific operator, it won't be able to be detected. In the past, attack UAVs were sometimes equipped with mobile communication devices with SIM cards from cellular operators operating in the area where the targets were located to connect to the internet, and base stations could theoretically be used to detect such kamikaze drones. However, currently, drones On-board SIM cards have been virtually abandoned. Thus, active response radar is useless for detecting enemy UAVs.
Another problem is that the structure of GSM (2G), UMTS (3G), and LTE (4G) signals contains discrete bit, syllable, and frame components, and they lack a probing pulse as such. Such signals are closer to a quasi-continuous wave signal, but their pulses are unmodulated and unmanipulated, making it impossible to measure the distance to a target.
Theoretically, it is also possible to detect an aerial target using cellular transmitters and a passive receiver using the "shadowing" (shielding) method, which utilizes the effect of interrupting the reception of a high-frequency radio signal by an object located between the transmitting and receiving devices. This effect was discovered in 1897 by A.S. Popov, when a third ship accidentally intervened between two ships exchanging messages via radio telegraph. However, this is not suitable for adequately measuring the coordinates, speed, and identification of aircraft. Achieving satisfactory results using multiple transmitting stations and a single receiver is technically impossible, and constructing a network of passive radars and connecting them to high-performance computing power is too costly and offers no advantages over existing surveillance radars. Defense.
A major obstacle to using cellular networks for multistatic radar detection is the low power emitted by the base station. For example, in a 4G (4th generation) network operating at frequencies of 800–2100 MHz, the signal power is 40–60 watts. In rural areas, base stations operating at 800 MHz are typically used, providing a coverage range of no more than 30 km on flat terrain.
The presence of natural and artificial uneven surfaces, as well as tall trees, greatly impedes the signal transmission.
Here it would be appropriate to compare the characteristics of the 4G base station transmitter with the domestic mobile radar 39N6 "Kasta-2-2", specially designed for the detection of low-altitude aerial targets and is a fairly effective means of detecting aircraft-type UAVs.
Radar 39N6 "Casta-2-2"
The 39N6 Kasta-2-2 dual-axis decimeter-wave radar was developed in the late 1980s, but its mass delivery to the military did not begin until after the collapse of the USSR. According to open sources, the export version, known as the 39N6E Kasta-2E2, has a pulse power of 40 kW. The detection range of a target with a 2-meter-square radar image flying at an altitude of 100 meters using the standard 14-meter-high mast is over 40 km. When the antenna is raised to an altitude of 50 meters, the detection range is 55 km. The ceiling is 6 km. It is capable of simultaneously tracking 40 targets. Information is updated every 5 or 10 seconds (depending on the antenna rotation speed). The beamwidth (directional pattern) of the Kasta-2-2 radar in azimuth is 5,5°. Assuming the base station antenna distributes its energy over 120° (at 60 W), the signal received by the target will be thousands of times weaker than that from a specially designed surveillance radar. The return signal reflected from the target in the receiver of the multistatic radar system will be four to five orders of magnitude weaker than in the receiver of the Kasta-2-2 radar, which will undoubtedly have a very negative impact on detection range.
Based on the above, it can be concluded that the power values, antenna patterns, and base station radiation modes make the design of a radar detection system based on mobile communication networks difficult to implement, if not doomed to failure.
However, this does not mean that multistatic radars relying on civilian high-frequency sources have no practical application potential. In this regard, widespread commercial FM radio stations operating on ultra-short waves in the 87,5 to 108 MHz frequency range appear much more promising than cellular networks. Small radio stations broadcasting to populated areas have a transmitter power of 0,1–1 kW. However, there are also radio stations covering large cities and large rural areas with transmitter power of up to 50 kW. The reliable reception range of such powerful stations, with antennas raised to a height of over 100 m, can exceed 70 km.
Even more suitable for multistatic radar than FM radios are DVB-T2 digital terrestrial television transmitters, which use the decimeter range of 470–860 MHz. To cover blind spots and small towns, transmitters with a power of 10–100 watts are used, while in medium-sized cities and suburbs, stations with a power of 250–1000 watts operate.
Antennas for high-power stations (up to 5 kW) with a broadcast coverage of up to 80 km are installed on towers up to 250 m high. The tallest television tower in Russia (540 m) is the Ostankino Television Tower.
Information has been publicly released that the Israeli company Elta Systems has developed a detection system consisting of multiple small antennas and receiving units.
The echo signal from local television and radio broadcasting stations is simultaneously received by several microwave receivers, undergoes preliminary hardware processing, is digitized and transmitted to the computing center, where the air situation is analyzed.
Similar equipment was developed by the German corporation Hensoldt AG. The radar, designated Twinvis, is designed to detect aircraft using signals reflected from civilian television and radio transmitters.
For distributed signal reception, container-type modules are used, which transmit the received information to a single control center, where the data is processed.
The American corporation Lockheed Martin tested its experimental radar system Silent Sentry.
The American Silent Sentry radar system has been reported to have demonstrated good performance and even observed aircraft built using stealth technology. However, there is no information regarding the adoption or serial production of this radar system. American sources state that the standby radars operating in the US fully meet the requirements of the North American Aerospace Defense Command (NORAD), and that the Silent Sentry radar system is of little use for expeditionary forces. Experts note that systems of this type require illumination from high-frequency radiation sources, the functionality of which is questionable in combat zones. At the same time, such radars can be used to service small civilian airfields that lack other radar equipment.
Information