Over the past years, the main method of ensuring low visibility of aircraft for enemy radar stations has been a special configuration of the outer contours. Stealth aircraft are designed so that the radio signal sent by the station is reflected anywhere, but not in the direction of the source. In this way, the power of the reflected signal arriving at the radar is significantly reduced, which makes it difficult to detect an aircraft or other object made using a similar technology. Special radio-absorbing coatings also enjoy some popularity, but in most cases they only help from radar stations operating in a certain frequency range. Since the efficiency of radiation absorption primarily depends on the ratio of coating thickness to wavelength, most of these paints protect the aircraft only from millimeter waves. A thicker paint coat, while effective against longer wavelengths, simply prevents an aircraft or helicopter from taking off.
The development of technologies for reducing radio signature has led to the emergence of countermeasures. For example, first theory, and then practice showed that the detection of stealth aircraft can be carried out, including with the help of fairly old radar stations. Thus, the Lockheed Martin F-117A aircraft shot down in 1999 over Yugoslavia was detected using the standard C-125 anti-aircraft missile system radar. Thus, even for decimeter waves, a special coating does not become a difficult obstacle. Of course, an increase in the wavelength affects the accuracy of determining the coordinates of the target, however, in some cases, such a price for detecting an unobtrusive aircraft can be considered acceptable. Nevertheless, radio waves, regardless of their length, are subject to reflection and scattering, which leaves the issue of specific forms of stealth aircraft relevant. However, this problem can be solved. In September this year, a new tool was presented, the authors of which promised to solve the issue of scattering of radio waves of the radar.
At the Berlin exhibition ILA-2012, held in the first half of September, the European aerospace concern EADS presented its new development, which, according to the authors, can turn all ideas about the stealth of aircraft and means of combating them. Cassidian, part of the concern, has offered its own version of the radar "passive radar" version. The essence of such a radar station lies in the absence of any radiation. In fact, a passive radar is a receiving antenna with appropriate hardware and calculation algorithms. The whole complex can be installed on any suitable chassis. For example, in the advertising materials of the EADS concern, a two-axle minibus appears, in the cabin of which all the necessary electronics are mounted, and on the roof there is a telescopic rod with a block of receiving antennas.
At first glance, the principle of operation of a passive radar is very simple. Unlike conventional radars, it does not emit any signals, but only receives radio waves from other sources. The equipment of the complex is designed to receive and process radio signals emitted by other sources, such as traditional radars, television and radio stations, as well as communication facilities using a radio channel. It is understood that a third-party radio wave source is located at some distance from the passive radar receiver, due to which its signal, hitting the stealth aircraft, can be reflected towards the latter. Thus, the main task of a passive radar is to collect all radio signals and process them correctly in order to isolate that part of them that is reflected from the desired aircraft.
In fact, this idea is not new. The first proposals to use passive radar appeared a long time ago. However, until recently, such a method for detecting targets was simply impossible: there was no equipment that would allow to select from all received signals exactly the one that was reflected by the desired object. It was only in the late nineties that the first full-fledged developments began to appear that could provide the isolation and processing of the required signal, for example, the American project Silent Sentry by Lockheed Martin. Employees of the EADS concern, too, as they claim, managed to create the necessary set of electronic equipment and the corresponding software, which can, by some signs, "recognize" the reflected signal and calculate such parameters as the elevation angle and range to the target. More accurate and detailed information, of course, was not reported. But representatives of EADS talked about the possibility of a passive radar to monitor the entire space around the antenna. In this case, the information on the operator's display is updated every half a second. It was also reported that the passive radar so far only operates in three radio bands: VHF, DAB (digital radio) and DVB-T (digital television). The target detection error, according to official data, does not exceed ten meters.
From the design of the antenna unit of the passive radar, it can be seen that the complex can determine the direction to the target and the elevation angle. However, the question of determining the distance to the detected object remains open. Since there is no official data on this score, you will have to make do with the available information on passive radars. EADS officials say their radar works with the signals used by both radio and television broadcasts. It is quite obvious that their sources have a fixed location, which, moreover, is known in advance. A passive radar can simultaneously receive a direct signal from a television or radio station, as well as search for it in a reflected and attenuated form. Knowing its own coordinates and the coordinates of the transmitter, the electronics of the passive radar, by comparing the direct and reflected signals, their power, azimuths and elevation angles, can calculate the approximate range to the target. Judging by the declared accuracy, European engineers managed to create not only viable, but also promising technology.
It is also worth noting that the new passive radar clearly confirms the fundamental possibility of the practical use of radars of this class. Perhaps other countries will be interested in the new European development and will also begin their work in this direction or accelerate the existing ones. So, the United States can resume serious work on the Silent Sentry project. In addition, the French company Thale and the English Roke Manor Research had certain developments on this topic. Much attention to the topic of passive radars may eventually lead to their widespread use. In this case, already now it is necessary to roughly imagine what consequences such a technique will have for the appearance of modern war. The most obvious consequence is to minimize the benefits of stealth aircraft. Passive radars will be able to determine their location, ignoring both technologies of reducing the signature. Also, passive radar can render anti-radar missiles useless. The new radars are capable of using the signal of any radio transmitter of the appropriate range and power. Accordingly, the enemy aircraft will not be able to detect the radar by its radiation and attack with anti-radar ammunition. Destruction of all large radio wave emitters, in turn, is too difficult and expensive. In the end, passive radar can theoretically work with transmitters of the simplest design, which are much cheaper than countermeasures in terms of cost. The second problem for countering passive radars concerns electronic warfare. To effectively suppress such a radar, it is required to "jam" a sufficiently large frequency range. At the same time, the proper effectiveness of electronic warfare means is not ensured: in the presence of a signal that did not fall into the suppressed range, a passive radar station can switch to its use.
Undoubtedly, the widespread use of passive radar stations will lead to the emergence of methods and means of countering them. However, at present, the development of Cassidian and EADS has almost no competitors and analogues, which still allows it to remain quite promising. Representatives of the developer concern claim that by 2015 the experimental complex will become a full-fledged means of target detection and tracking. For the time remaining before this event, the designers and the military of other countries should, if not develop their analogues, then at least form their own opinion on the topic and come up with at least general methods of counteraction. First of all, the new passive radar can strike at the combat potential of the US Air Force. It is the United States that pays the most attention to aircraft stealth and creates new designs with the greatest possible use of stealth technology. If passive radars prove their ability to detect aircraft that are inconspicuous for conventional radars, the appearance of promising American aircraft may undergo major changes. As for other countries, they do not yet put stealth at the forefront and this will, to a certain extent, reduce possible unpleasant consequences.
