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Flat Twin / RSN-225 radar of complex 5K17

This system was a component of the ABM-2/S-225 road-mobile anti-missile and anti-aircraft system developed by Chief Designer A.A.Raspletin of TSKB Almaz between 1965 and 1978. The S-225 included the Flat Twin phased array radar for target tracking and interceptor guidance, the Pawn Shop station for transmission of commands, anti-missile interceptors with nuclear warheads, and the command homing posts 5Ya26 (designed by the OKB Novator) and 5Ya27 (designed by the Fakel bureau). The 5K17 "Azov" (former S-225 radar) was later installed on Kamchatka.

RSN-225 guidance radar was the first combat radar in the USSR with a passive phased array. The radar was supposed to be accompanied simultaneously by a ballistic target moving at a speed of about 7 km / s and a missile, which, after entering the tracking sector, had to be quickly detected. The only possible solution here was to use an antenna system with a phased array antenna. Only the headlamp could provide fast beam redirection when tracking several objects. Raspletin Design Bureau did not have experience in designing such antennas, but when did such "little things" stop Alexander Andreevich and his associates? In his activity, the “coefficient of novelty” had always been extremely possible, but no more than necessary.

For radar transmitters (but not only for them) specific powerful electrovacuum devices are used - klystrons or magnetrons. Although they were invented even before World War II, the active development of powerful models for mass production began only in the second half of the 1950s. At the time of work on the RSN-225, they had not yet learned how to generate large power in a single device. There were problems with the waveguides. They often experienced electrical breakdowns when transmitting high power. An original solution was proposed for the first prototype of the radar - the addition of the energy of 24 modular transmitters on klystrons on a common 6x6m mirror antenna with klystrons of maximum achievable power at that time, each operating on 1 phase-controlled irradiator (24 in total). Such a simplified solution did not allow the formation of a high-quality radiation pattern, but this was partially offset by the quality of the radiation pattern of the receiving antenna, which contained a significantly larger number of receiving elements. That is, the transmitting antenna was a passive reflective phased array. The receiving antenna was also VFAR.

In the next version, which became the base, they refused a large mirror. The transmitting elements were deployed towards targets and missiles, and now the addition of the energy of the transmitters took place directly in space. The sector of electronic scanning of the beam increased from 4x5 degrees to 20x20 on the target, and on the radio beacon of the rocket right up to 90x90. The design was significantly optimized, and the tracking accuracy was improved. The radar has the potential to select false targets.

The tremendous speed of attacking warheads required an impossible speed for a person to analyze the situation and actions. The cost of human operator error could be incredible. This is the very human factor that journalists mention out of place and out of place. The restriction imposed on the human-machine system by the psychophysical capabilities of man. Designers of any equipment are required to take it into account. And they took into account. The solution was the assignment of the entire operational management of the S-225 computer complex. Only one button “Start allowed” was “assigned” to a person in battle. In addition, the computer was engaged in the analysis of goals and the allocation of true against the backdrop of false.

The first prototype was a computer 5E65, the second 5E95 and the third 5E261 designed by V. Burtsev. Subsequently, he became the developer of the famous Elbrus-2.

The development of the second prototype S-225 was almost parallel with the development of the S-300 air defense system. It was logical to use the same computer in both systems. 5E261 is a three-processor computing complex in which one processor executes a combat program and controls some functions, while the other two continuously monitor it, they consider it to be parallel. This is called a hot reserve. A special device compares the results and, when discrepancies are detected, it selects by “voting” what “most” processors give, immediately replacing a failed one that disagrees with them.

Flat Twin was a transportable, multifunction, phased-array radar in two versions, similar in general appearance. The prototype had a a hybrid antenna system that used a conventional, line-fed, reflector-type transmit antenna and a phased-array receive antenna. The operational Flat Twin radars, had both transmit and receive phased-array antenna systems indicating an improved transmitter capability from the prototype Flat Twin radar.

Western of all-source data indicates that the Flat Twin radar consisted of an electronically steered, phased-array antenna mounted on a pedestal that allows mechanical steering in both azimuth and elevation. Once mechanically directed, the Flat Twin electronically scans in a limited search area for multiple targets usin handover data from long-range surveillance assets. This electronic scanning feature gives the radar the capability to cover a spatial sector more quickly than could mechanical scanning alone. Theoretically, a Flat Twin—equipped missile battery could engage targets arriving from any direction, although not simultaneously.

Flat Twin consisted of a number of modules that can be transported by truck, rail, or aircraft. For example, the Flat Twin's size and mass are such that a single disassembled radar antenna can be carried by an An-l24 Condor, a large Soviet aircraft similar to the US C-SA Galaxy. The Flat Twin radar is supported by 20 electronics vans providing power, cooling and computer support. Relocation time, excluding checkout, could be reduced to fewer than four months.

The Soviets used the Flat Twin engagement radar in diverse ways throughout its history. The Flat Twin radar was initially an integral part of an ABM program called the ABM-X-3, which CIA believed was under development for nationwide deployment before the signing of the ABM Treaty in 1972. Despite the demise of that program, the Flat Twin continued.

CIA believed that the role of the Flat Twin radar in the Soviet ABM program had changed. Because the Flat Twin was transportable and had been associated with an ABM system intended to be widely deployed, CIA considered the possibility of the Soviets breaking out of the ABM Treaty with a fast-paced, nationwide ABM system based on the Flat Twin radar and the Gazelle interceptor. CIA explored the limitations of the Flat Twin radar and the advantage to the Soviets of developing a new ABM radar for a widespread ballistic missile defense

The Flat Twin radar was designed and was almost certainly intended to be widely deployed as part of the ABM-X-3 system. Before the signing of the ABM Treaty in 1972, the Soviets had begunn preparations for ABM testing. What the Soviets intended to gain from ABM-X-3 testing after signing the ABM Treaty is less clear.

However, the demise of the ABM-X-3 system occured by the late 1970s with the abandonment of the associated interceptor program. CIA believed that Flat Twin testing conducted after 1972 probably stemmed from a requirement to continue to investigate a rapidly deployable system as a hedge against a breakdown of the treaty.

In 1972, the USSR and the USA signed an agreement to limit missile defense systems, after which the fate of the S-225 was a foregone conclusion: they had to destroy it. At the test site, it remained possible to test anti-missiles with the number of launchers no more than 15, but this resource was reoriented to the development of the A-135 system. Anti-missiles of the near endoatmospheric interception 5Y26 (serial model under the index 53T6 or PRS-1) inherited from her from C-225. And RSN-225 began to work as a control and measuring complex (KIK) 5K17, designed to work out a complex of means to overcome missile defense, that is, it “switched sides” in the interception game.

The prototypes were tested and brought to the Sary-Shagan test site for more than 10 years, when the very concept of a “simple and cheap" missile defense system was already forgotten, but the potential for target selection and accurate tracking of ballistic trajectories turned out to be very valuable. In 1977, the experimental S-225 intercepted with a 5Y27 missile a complex ballistic target, the warhead of a R-29 submarine ballistic missile. The story of RSN-225 did not end there - she was destined not only to become the basis of the control and measuring complex at the Kura training ground, but also to participate in a real space drama.

In 1975, this complex — portable, modular — was transported to the Kura test site, where the KSP (complex of overcoming facilities) missile defense systems were tested on real samples of domestic ICBMs. The complex itself was continuously improved, it worked out new algorithms for the formation of selective signals, combat programs, tracking algorithms and the allocation of false targets in a complex jamming environment. RSN-225 began to play the role of a potential enemy missile defense, which our ICBMs experimentally broke through. At the same time, the computer complex was modernized: 5E65 and 5E95 in the first polygon samples and 5E261 as part of the KIK 5K17. The developments in computer systems were used in the development of 5E261 combat programs — the foundations of the digital computers of the S-300 complex of the first modifications — in full, that is, together with work programs for ballistic targets.

Flat Twin antenna's face comprises 692 subarrays of about 16 different sizes arranged in a pseudorandom pattern and a core array, which is probably for beacon tracking of ABM interceptors. The total area of the antenna would allow a maximum of l7,000 phase shifters (elements) if spaced one-half wavelength apart. However, the volume of space available behind the face, in addition to one hole per subarray for coaxial wiring, suggests only one phase shifter per subarray. This reduced number of phase shifters to 692 — a 96-percent reduction of the theoretical maximum. The reduced number of phase shifters also minimizes the production cost of the radar.

The elements within each subarray are all phased uniformly and only phase of the individual subarrays — rather than each element — are changed to steer the beam. In the late 1960s. when the Flat Twin was being designed, the Soviets were having difficulties producing phase shifters.

The Flat Twin's scanning capability is limited by the occurrence of secondary antenna pattern maxima (grating lobes) when steered off-bore-sight. US research on a phased-array antenna with a 96-percent reduction in phase shifters showed that such a radar would have a scan angle of 10 degrees with side lobes more than 30 decibels (dB) below the main beam and a grating lobe more than 25 dB below the main beam.

The effort involved in redesigning the Flat Twin to achieve a significant increase in off-boresight scanning capability probably would be tantamount to designing a new phased-array radar. Because of Flat Twin's unique antenna subarray design, any modification in the number of phase shifters to increase the off-boresight scanning capability would be difficult to achieve.

The reduced off-boresight scanning capability of the Flat Twin increases Flat Twin deployment estimates by about 30 percent. CIA had determined that defense of a selected set of 125 high-priority deployment areas would require some 600 to 720 Flat Twin radars to defend against an attack by US missile forces. This analysis also indicated that deployment at Moscow and only 40 of the most important deployment areas would require 380 to 450 Flat Twins. These calculations assumed that the Flat Twin radar would have a 45-degree off-boresight capability.

A radar similar to the Horse Leg radar, the engagement radar prototype for the ABM-4 system's Pill Box battle management radar, could be developed. But a new mobile radar would be more appropriate than the Flat Twin for a widespread ABM defense system. The mobility alone of a new system would increase its survivability requirements indicates that a new radar could reduce the required number of radars by as much as 60 percent. This reduction in the required number of radars could increase the attractiveness of a nation-wide ABM defense system to the Soviets.

Though the ABM radar on Kamchatka was not strategically significant, the US decided to raise the issue in order to set the record straight on whether Kamchatka is an ABM test range and to protect the principle that additional ABM test ranges require prior mutual agreement. The US SCC Commissioner engaged the Soviet SCC Commissioner but an ambiguous response was received. The Issue was again raised at SCC-VIII to establish that Kamchatka is was an ARM test range and that Sary Shagan and Kamchatka were, as of now, the only ABM test ranges in the USSR. Tho Soviets replied that the radar instrumentation complex was on the Kamchatka on the date of signature of the ABM Treaty and that if it would help, the USSR would be ready to consider the range to be a current test range withln the meaning of the Treaty.

Discussions of topics related to the ABM Treaty during SCC-IX included references to the Kamchatka radar but the US did not reopen the subject of compliance during that session. The Kamchatka radar issue remained formally open for future discussion. In SCC-X, the Issue, while not raised specifically, was discussed in the process of defining what constitutes an ABM test range. The Soviets had not formally agreed to identify the Kamchatka area as an ABM test range. They, however, privately stated that they will do so contingent upon satisfactory resolution of the issue of the procedures for establishing new test ranges.

In 1985, faced with the threat of loss of the Salyut-7 space station, we also faced the need for accurate wiring of the space “target” and high-precision determination of its orbital parameters in uncontrolled mode simultaneously with Soyuz T-13 spacecraft tracking. To solve this problem, the same RSN-225 from the equipment of the Kura complex was used, although these are mere trifles for the radar, created and licked to work on targets with a negligible EPR. Nevertheless, this demonstrated its potential in solving the problems of anti-satellite defense and once again confirmed the effectiveness of the selected solutions. Complex 5K17 continued its work at the training ground until 2006.