(According to the views of foreign military experts)
The militarist circles of the United States do not abandon attempts to achieve military superiority over the Soviet Union, making a special emphasis on new types of technology and weapons. Under current conditions, when the USSR and the United States concluded the Treaty on the Elimination of Medium and Short Range Missiles, negotiations are in progress on 50-percent reduction of strategic offensive weapons, in the plans of Pentagon strategists more and more attention is paid to low-profile aircraft (LA). Since 1983, the American program "Stelc", aimed at developing the technology of low-profile aircraft, is less well covered in the Western press. The strategic defense initiative came out on top in popularity. Nevertheless, the implementation of the stealth program continues at a rather high rate. According to foreign military experts, the results obtained in the course of its implementation will have a strong influence on the appearance of promising aircraft. It is believed that the reduction in visibility will become the leading trend in military aircraft 90-s. This is confirmed by the programs for developing the highest priority aircrafts of various classes that have the property of low visibility. Such aircraft include the B-2 bomber, a promising tactical ATF fighter, and the AFM cruise missile.
Reducing the visibility of the aircraft is carried out in different parts of the electromagnetic spectrum: radar, optical, infrared and acoustic. The greatest attention is paid to reducing the radar visibility, since at present the main means of detecting aircraft in air defense systems are radar stations. There are also known technical ways of reducing the radar visibility of aircraft: improving aerodynamic forms, using new construction materials and radio absorbing coatings, reducing the number of antennas, etc. Judging by foreign press reports, modern technologies created by the Stealth program make it possible to reduce the effective area of dispersion (EPR) aircraft almost 70 percent. in comparison with the aircraft of traditional schemes. At the same time, the detection range of such an inconspicuous aircraft will be reduced by a third, since the detection range is proportional to the fourth root of the EPR values.
Predicting the massive introduction of low-profile aircraft into service in 90's, foreign military departments are deploying a wide range of research into the problems of countering such aircraft. Special attention is paid to the problems of increasing the range of radar detection of inconspicuous airplanes, believing that the implementation of the results will largely determine the appearance of 90's radar facilities.
Current R & D is conditionally divided into two groups. The first group of studies is conducted within the framework of the traditional approach to solving the problem of increasing the range of radar target detection. In particular, the possibilities of increasing the radar power potential and increasing the sensitivity of radar receivers are being studied. A characteristic feature of these works is that in the course of their work, the specifics of Stealth-type aircraft as radar targets are practically not taken into account. The results of the work will be used mainly in the modernization of existing radars.
The second group of R & D is characterized by a large variety of ideas and research directions. It presents both completely new approaches and ideas known in theoretical radar, which for various reasons have not been implemented previously. Common is the desire of researchers to use signs (for example, characteristic forms) that are specific to low-profile aircraft to increase the detection range. As a result of these R & D, as a rule, the necessity of creating fundamentally new systems and tools is justified.
The problem of detecting subtle LAs is related to the effective scattering area, the magnitude of which depends on many factors: the size, shape, spatial position of the LA, the material from which it is made, frequency, polarization, and the shape of the irradiating signal. Moreover, even a slight change in any of these factors can lead to a significant (an order of magnitude or more) change in the ESR value. Therefore, when specifying the EPR values of specific aircraft, the conditions under which they are obtained should be precisely determined. However, in foreign publications devoted to inconspicuous aircraft, this rule is often neglected. So, speaking of the magnitude of the ESR of the low-profile aircraft, its value is usually given when the apparatus is irradiated in the forward hemisphere, although the average indicator is the average ESR of the aircraft when irradiated from all directions. Due to such "little tricks" in the western publications devoted to low-profile aircraft, their ESR value appears, equal to 10-2 м2.
Foreign military experts point out that most of the authors of publications on low-profile aircraft are directly related to their development. Therefore, in these articles, as a rule, the advantages of low-profile aircraft are emphasized, and there is no mention of shortcomings or controversial issues. Common in calculating the detection range of low-profile aircraft is the use of the characteristics of existing radar defense. Opportunities to improve the radar, as well as changes in parameters affecting the EPR of the target, are usually not considered, although experts in the field of radar based on an objective analysis of the features of low-profile aircraft and their dependence on the characteristics of the radar have already identified promising ways to increase the range of targets of this type.
Traditional methods of increasing the detection range are based on increasing the energy potential of the radar and improving the quality of signal processing. The first can be increased by increasing the transmitter power and the directional coefficient of the radar antenna. In the future, the expected appearance of generator devices, which will increase the power of the radar transmitters in 2 — 3 times.
Increasing the directional coefficient, as a rule, is associated with an increase in the geometric dimensions of the antennas. The possibility of creating conformal antennas based on phased antenna arrays for long-range radar detection aircraft is being investigated. Antennas of this type will form part of the skin of the aircraft, which will allow them to be placed, for example, along the entire fuselage or front edge of the wing. Thus, it is possible to increase the geometric dimensions of the antenna to the limits determined by the size of the aircraft carrier. However, calculations show that even increasing the antenna size to the limit values will increase the detection range only by 60 — 70 percent, which will compensate for the decrease in the EPR of the target by 10 dB. In this regard, foreign experts pay attention to the fact that the role of ground-based radar systems is increasing again, the antennas of which have virtually no restrictions on geometric dimensions.
Improving the quality of operation of radar receiving devices is planned to be achieved primarily through the analysis of the fine structure of signals based on the implementation of digital filtering algorithms on computers. In this regard, high hopes are pinned on the introduction of ultra-high-speed integrated circuits and monolithic integrated circuits of the superhigh-frequency and millimeter-wave ranges. To perform certain signal processing operations, charge-coupled devices are created, as well as using surface acoustic waves.
In order to increase the detection range of unobtrusive targets, the United States Air Force plans to modernize the AWACS E-90 radars of the AWACS system (see color inset) in the first half of the 3-s (see color inset), that is, to improve the quality of digital signal processing using a computer. It is believed that after upgrading the target detection range will increase significantly due to an increase in the level of signals on 10 — 13 dB, as well as the reliability and radar noise immunity will increase. The improvement will also affect other electronic equipment of the aircraft E-3. It is planned, in particular, to install direct radio intelligence systems for passive detection of enemy aircraft, aperature satellite navigation system NAVSTAR and terminals of the 2 class of the combined tactical information distribution system JITIDS.
A known way to increase the detection range is to increase the coherent accumulation time of the echo signals. On the basis of this principle, a method of inverse synthesis of aperture has been developed. It uses algorithms that are inverse to those used in the modes of synthesizing the radar aperture and allow obtaining detailed images of ground objects based on the analysis of the Doppler signal frequency shifts. A distinctive feature of this method is that the signal accumulation occurs due to the movement of the target, and not the radar antenna, as in conventional aperture synthesizing.
The method of inverse synthesis of aperture was tested in ground-based measurement systems (radar signatures of space objects were obtained using radar on Kwajalein Island), and was also implemented in on-board radar that underwent flight tests at the beginning of 80-s. The first serial on-board station in which this method was applied was the AN / APS-137 radar, designed to perform recognition and classification tasks for marine objects. It is installed on the deck of the Viking S-3B anti-submarine aircraft and the Orion base patrol R-3. The disadvantage of this method is the need to know the distance to the target and the speed of its movement. Errors in the determination of these parameters lead to a deterioration of the accuracy characteristics of the radar in the mode of operation using the method of inverse synthesis of the aperture.
The conventional methods of increasing the detection range of low-profile aircrafts conditionally include those based on the choice of the optimal operating frequency range of the radar. Currently known means of reducing observability are effective only in a limited frequency range. It is believed that the lower limit of this range is 1 GHz, and the upper limit is 20 GHz. Moreover, the reduction of visibility in the entire specified range can be achieved only through the integrated use of various methods and means. Separate funds even more narrow-band. The range 1 — 20 GHz is not chosen by chance. First, most of the existing radar systems of air defense work in it, so the designers seek to reduce the aircraft visibility in this particular range. Secondly, there are a number of fundamental physical restrictions on ways to reduce the visibility of LA outside this range.
The choice of the optimal operating frequency range of the radar is based on the dependence of the EPR of the aircraft on the frequency of the irradiating signal. For example, the EPR of fighters of traditional schemes with a decrease in the frequency (increase in wavelength) of the probing signal increases according to a law close to linear. For subtle LAs, a similar dependence is even more pronounced — the EPR is proportional to the square of the probing signal's wavelength. Calculations show that the detection range in the free space of an inconspicuous aircraft in the 1 — 2 GHz band is 1,75 times as large as in the 2 — 4 GHz band, and 2,2 is more than the 4 — 8 GHz band. In this regard, foreign experts note the increased interest in the radar meter and decimeter ranges. For several decades, one of the leading trends in radiolocation was the assimilation of more and more high-frequency bands, which was due to the possibility of obtaining higher resolution. The appearance of subtle LA again attracted the attention of specialists to the meter and decimeter ranges.
An important direction in reducing the visibility of aircraft is the use of radar absorbing coatings. It is believed that if radar systems of various ranges are used in air defense systems, it will be practically impossible to create an effective radar absorbing coating for an aircraft. Ferrite-absorbing materials are relatively narrow-band. Thus, materials known as ekosorb, with a thickness of 5-8 mm, provide an absorption of 99 percent. incident wave energy in the band of approximately 300 MHz. It is noted that to reduce the visibility of the aircraft in a wider range, it is necessary to apply multilayer coatings. But taking into account the fact that the specific gravity of a modern ferrite coating is almost twice as large as that of aluminum, this is hardly feasible. Coatings based on dielectrics have a lower mass, but their thickness is directly dependent on the frequency of the absorbed waves. For example, to counter the probing signals of a radar operating at a frequency of 1 GHz, it is necessary that the coating thickness be approximately 300 mm, which, of course, is unacceptable for aviation.
If the probing signal's wavelength is commensurate with the size of the target, then the reflection will be of a resonant nature, due to the interaction of the direct reflected wave and the waves around the target. This phenomenon contributes to the formation of strong echoes. The phenomenon of resonance can also occur on the elements of a target's structure For example, stabilizers and wingtips fall into the resonance region of the radar system of the DRLO E-2 “Hokai” aircraft operating at frequencies around 400 MHz (wavelength 0,75 m). The command of the US Navy plans to leave the aircraft "Hokai" in service after the next modernization of equipment.
The possibility of using two ranges and changing the frequency of the probing signal in accordance with the shape of the target is the main idea in creating a promising ASTARA (Aircraft Radar Aircraft Atmospheric Surveillance Technology) aircraft, which is specifically designed to detect subtle aircraft. It is assumed that it will complement the aircraft E-3 system AWACS. Flight tests are scheduled for 1991 year.
The creation of over-the-horizon radars in the United States began long before the organization of work to counter the inconspicuous aircraft. However, the fact that such stations operate in the meter wavelength range, now gives grounds for American experts to consider them as one of the important means of detecting subtle aircraft. Therefore, further development and testing of over-the-radar radars are carried out in view of their fulfillment of the new function. The development of over-the-horizon radars for reciprocating sensing, the US Air Force specialists have been doing since 1975. It is planned to build four radars, which should ensure the detection of targets approaching the North American continent from any direction, with the exception of the north. The latter cannot be covered due to the unstable nature of the propagation of short-wave signals in high geographic latitudes.
In 1988, the United States Air Force conducted the first tests of over-the-horizon radar to detect small targets that simulated cruise missiles. Its ability to detect targets in the airspace between f. Puerto Rico and Bermuda Islands. Radar operates in the range 5 — 28 MHz. Due to the influence of the ionosphere in the daytime, higher frequencies of this range were used, and lower frequencies at night. Cruise missiles were simulated by unmanned aerial vehicles AQM-34M, which were launched from the carrier NC-130. Their flight was carried out at various altitudes (150, 4500, 7500 m) at a speed of 650 — 750 km / h. As stated by the representative of the US Air Force, tests confirmed the possibility of detecting small targets of over-the-horizon radar at a distance of up to 2800 km. Based on their results, a decision was made to increase the size of the radar receiving antenna being built on the US West Coast from 1500 to 2400 m, which will double the sensitivity of the radar receiver. It is planned to complete the deployment of a system of four over-the-horizon radars in the 90s.
The US Navy is developing a transportable over-the-horizon ROTHR radar, the main advantage of which is the possibility of transferring it in a relatively short time to previously prepared positions. This station detects airplanes at a distance of 925 — 2700 km in the sector 60 °. Its electronic equipment is housed in 30 vans. In potential combat areas, antenna fields are being created, where, in the event of a crisis, vans with equipment will be transported. According to the representative of the company "Raytheon", a prototype radar is already placed on the position in Virginia, in the future it is planned to relocate to the Aleutian Islands. No other positions have yet been selected for the radar, however, it is planned to deploy at least nine radars primarily in marine (ocean) theaters, where they will be used in conjunction with the E-2 “Hokai” and E-3 “Sentry” airplanes.
In order to improve the quality of operation of over-the-horizon radar, US Air Force experts are exploring the possibility of creating an artificial ionospheric mirror. In their opinion, it will contribute to a more focused reflection of probing signals, which will increase the resolution and will allow to detect targets at distances less than 500 km.
Even the most ardent supporters of over-the-horizon radar systems recognize their inherent serious flaws: low resolution and low noise immunity. Nevertheless, according to foreign experts, over-the-horizon radar systems are the only type of systems that can be put into service by a number of Western countries in the future and provide detection of low-profile aircraft. All other types of systems, whatever benefits they may have, are in earlier stages of development.
The above approach to the optimal choice of range was focused on increasing the wavelength of the probing signals compared to those used in modern air defense radars. In the foreign press, an alternative way is also discussed, which consists in switching to the range of millimeter waves. Since it is believed that at present there are no radio absorbing materials that are most effective in the millimeter range, therefore radars operating in the range of millimeter waves can become an important element of advanced air defense systems. Mastering the millimeter range is high. The element base and principles of construction of systems operating at frequencies 30 — 40 and 85 — 95 GHz have already been worked out, as well as samples are created with operating frequencies close to 140 GHz.
Non-traditional ways of increasing the detection range of aircrafts with small EPRs are based on new approaches to solving the problem - time-frequency and spatial. Within the framework of the time-frequency approach, the methods of forming and processing new complex radar signals are investigated.
The use of probing signals, consistent with the shape of the target, can significantly enhance the echo signals. This method is similar to the matched filtering method used in modern radars. The formation of probing signals is based on the impulse response of the target, depending on its configuration, spatial position and movement dynamics. In practice, nanosecond pulses are required to align signals with a target. A special case of such pulses are non-sinusoidal signals, the important properties of which include ultra-wideband. In the foreign literature, signals that occupy the 0,5 — 10 GHz band and have a duration of 0,1 — 1 ms are considered as an example. Their use provides range resolution within 0,15 — 0,015 m. At the same time, reflections from the target are a set of echo signals from several point reflectors distributed over the target surface, which makes it possible to build a model of reflections from a particular aircraft, with which the shape of the sounding signals . Calculations show that ferromagnetic materials weakly absorb the energy of radar non-sinusoidal signals.
Since information on the configuration of the aircraft can be used to increase the detection range of aircraft with small ESR, foreign military experts are considering possible measures to conceal it. They include the following: placement of aircraft in shelters; rational choice of locations and restriction of training flights in the daytime in order to reduce the likelihood of receiving photographs of aircraft by various reconnaissance vehicles; improving training complexes and transferring the center of gravity of flight personnel training to simulators; equipping low-profile aircraft with devices that increase and distort the aircraft's EPR, since during training flights in the radar range of a civil aviation air traffic control systems, a potential enemy can receive information about real EPR.
The use of radar with multi-frequency signals also applies to time-frequency detection of low-profile aircraft. In this case, the target is irradiated simultaneously with several continuous signals at different frequencies. Reception and processing of the echo signals are performed using a multichannel receiver, in each channel of which pairs of signals are formed at close frequencies, and then multiplied and integrated or Doppler filtering is performed. The advantage of multi-frequency radar is the ability to select a set of frequencies that provide the maximum detection range. As in the previous method, the defining parameter is the configuration of the target.
To increase the detection range of aircraft with small ESR, the possibilities of using the “nonlinear radar” effect are also being investigated. This effect is that the objects of technology during irradiation not only reflect the incident waves, but also generate re-radiation on harmonics. Sometimes this phenomenon is called the “rusty bolt” effect, since the source of generation on harmonics is, in particular, the combination of metal elements. However, semiconductors have a similar property. The latter circumstance is of interest to researchers in connection with the equipment of LA with multifunctional active phased antenna arrays in which it is planned to use elements on gallium arsenide. The radiation level with an increase in the harmonic number decreases sharply. That is why only the second and third harmonics are of practical interest.
Judging by Western press reports, all methods of the time-frequency group are still in the early stages of theoretical and experimental research and development, and therefore their implementation will be possible only in the long term.
As part of a spatial approach to increasing the detection range of low-profile aircraft, methods and tools are being developed based on the dependence of the EPR of an aircraft on the direction of irradiation. As a rule, the designers of such devices can reduce the value of the EPR mainly when irradiated in the forward hemisphere.
In recent years, the interest of specialists to the so-called multi-position radar, which represent a system of several interacting transmitters and receivers spaced apart, has increased. The simplest multi-station radar, consisting of one transmitter and one receiver, is called bistatic. The principles of building multi-station radars were known at the dawn of radar, but some technical problems, such as providing data transmission for synchronizing transmitters and receivers, did not find a satisfactory solution in those years. Therefore, the further development of radar has gone the way of improving single-station systems.
An important parameter of bistatic radar is the angle between the directions from the target to the transmitting and receiving positions - the so-called bistatic angle. Special attention is paid to studies of radars with a bistatic angle equal to 180 °, that is, when the detectable aircraft is on the straight line connecting the transmitter and receiver. In this case, the EPR of the aircraft strongly (by tens of decibels) increases as a result of the effect known as “forward scattering”. In the first approximation, the “front scattering” EPR is equal to the ratio of the square of the irradiated area of the aircraft to the square of the radar transmitter wavelength multiplied by a factor equal to 12. Since the EPR of “front scattering” does not depend on the material from which the aircraft is made, the effect of using composite materials and radar absorbing coatings in the low-profile aircraft will be neutralized. The “front scattering” ESR value decreases with decreasing bistatic angle, but even at 165 ° angle it is still significantly larger than that of single-position radar.
In the foreign press, various options are proposed for constructing multi-position radars, differing mainly in the method of organizing irradiation of targets. Radar of early warning systems and reconnaissance-strike complexes, space-based radars or even television broadcast stations can be used as transmitting stations. The possibility of introducing a multi-position regime into existing radars and creating radar networks on their basis is also being considered.
The use of space based radar. This will allow LA to be irradiated from In this case, the EPR of the aircraft will increase due to an increase in the irradiated area. Currently, specialists from the United States, Great Britain and Canada are implementing a joint program for creating space-based radars designed for detecting and early warning of a raid by bomber and cruise missiles. At the same time, the requirements imposed by each of the countries on the space system have their own characteristics.
British experts believe that space-based radars should also provide tracking and tracking of land and sea targets, including on the battlefield. According to their estimates, tracking of marine objects does not represent serious technical difficulties, but to realize the possibility of tracking targets on the battlefield, a large amount of research will be required. The most suitable type of stations for placement on a space carrier is considered to be synthetic aperture radar.
Canada is participating in a number of joint projects with the United States to provide air defense systems for the North American continent, including upgrading the network of ground-based radars, creating over-the-horizon radars, and expanding areas controlled by E-3 aircraft. However, representatives of the Ministry of Defense of Canada consider the space-based radar to be the only means that can provide tracking of the entire territory of the country with the adjacent airspace and maritime waters. In addition to solving the main task, such a station, in their opinion, should perform the functions of search and rescue systems, navigation and air traffic control. Initial plans include launching four to ten AESs equipped with radar into low polar orbits. In order to increase the survivability of the system, US Air Force experts are considering the possibility of creating a space-based distributed radar. The joint functioning of the “constellation” of the satellite makes it possible to realize an extremely large common aperture of the system. Proposals for placing radars on dirigibles or aerostats that provide payloads of up to 1 tonnes to altitude up to 25 km are also put forward as intermediate ones.
In parallel with the development of a radar station in the USA, an experiment is being prepared to place an infrared telescope into orbit as a detection tool with a passive mode of operation and a higher resolution. It was planned to deliver the telescope to orbit in March 1986 of the year with the help of the Space Shuttle space shuttle, however, the Challenger crash delayed the experiment for several years.
Assessing the problem of increasing the detection range of low-profile aircrafts in general, foreign experts note that intensive theoretical and experimental work is being carried out in all possible directions. Separate results can be realized in the near future after receiving reliable information about what methods and means of reducing observability will be put into practice on 90's aircraft. Radar experts are optimistic, because история The development of technology shows that radar always had advantages over countermeasures, and this situation will continue, obviously, in the foreseeable future.
As for the problem of combating an aircraft of the stealth type, it is of less concern to foreign military specialists. It is believed that, with reliable detection and tracking, they can be destroyed with a given probability both by existing anti-aircraft missile systems and by promising ones.
Foreign Military Review №7 1989 С.37-42
PS: Please pay attention to the date of publication - 1989.
Even then, for many specialists, it was clear that the powerfully promoted conception of invisibility could not become a panacea for “invulnerability”. And time has confirmed this - all Ф117, created by the technology of stealth to the detriment of aerodynamics, hastily, before the end of the service life, were removed from service.
The same is true with respect to the following products of marketers - F22.
And it is commendable that our designers, when creating the T-50, did not follow this disastrous path ...