The Guidance Methodology of the S-75 Surface-to-Air Missile System aka SA-2 Guideline

Note: This is just a basic description. If you wish to see the math and explanations, please click the link at the end of the thread to view the full article on my FREE Substack. Introduction to the S-75 Guidance Logic

The S-75 missiles (known as V-750, 755, 759, or 760 depending on version) guide to target via “Command Guidance”. This means the missile maneuvers in relation to the target solely through commands issued from the guidance station (RSN-75) similar to how a remote controlled drone functions. Therefore, the missile can be extremely simple in construction only requiring a way to receive command signals from the guidance station. This is unlike most modern missiles which have some sort of component for onboard computation in tandem with a radar or electro-optical seeker to intercept the target either on its own or with aid from an off-board command unit. The simplicity of the guidance method is further understood when we note that the S-75 only requires three measurements to generate an intercept solution. These are the slant range from the RSN-75 to the target and missile, the elevation angle (denoted as Epsilon) of the RSN-75 antenna wrt the horizon, and the azimuth angle (denoted as Beta) of the RSN-75 wrt true north. Using these measurements with respect to time, the RSN-75 can calculate everything necessary from target velocity, altitude, and very importantly the “cross-range” distance which is the minimum distance the target will be if it remains on its current course.

The primary issue that stems from the use of command guidance is the inaccuracy that is generated by the system as the missile attempts to intercept the target at greater range. The reason this occurs is almost entirely due to the difficulty in calculating the missile’s kinetic trajectory after the rocket motor runs out of fuel. Because of this, the S-75 only had an accurate interception range of roughly 36 km upon introduction. The S-75 could intercept targets much further after the introduction of the V-755 missile and its derivatives thanks to a new automatic throttleable engine. This increased range to about 43 km against supersonic targets, and 56 km against cooperative subsonic targets.

The S-75’s guidance loop is simple. First, the RSN-75 detects and tracks the target. The missile location is either estimated and/or directly observed in flight by the RSN-75. This information is passed to the “Coordinate Determination System” (SOK) that calculates where in space the target and missile are in relation to the RSN-75. These values are then sent to the “Command Generation System” (SVK) to calculate the error between where the missile is and where it should be so corrective commands can be generated and sent to the missile via the P-16 command signal antenna. The target continues to be observed, the missile corrections are observed by the RSN-75, and the cycle repeats until either a successful or failed missile intercept occurs. Up to three missiles may be commanded on a single target at any given time.

One last note is that for the first 5-6 seconds after launch, the missile flies to the target completely uncontrolled. The purpose of this is two fold. First, this ensures that enough altitude is achieved such that large corrections early in the flight do not cause the missile to strike the ground and thus terminate the intercept. Second, the large plume of the initial rocket booster stage and ignition of the primary missile motor essentially jam radio frequency transmission, so this ensures that when guidance begins the signals are not disrupted thus causing errors in missile control.

This new Chinese jet has really thrown people for a loop. A lot are pushing this as a “6th Gen Fighter” b/c it shares a lot of vague commonalities with the NGAD artistic depictions. I don’t believe this to be the case, but there’s still a lot to be inferred from the pics so far— Let’s talk shape since it’s what has blown people away the most. The tailless, compound delta wing design is extremely reminiscent of NGAD depictions. Do they fulfill the same role? Idk, I’m not on either design team. But this is still a clear indication of intent—

When people talk about stealth aircraft performing air combat, it’s often stuck in the mindset of 4th Gen fights or simply fighting an inferior enemy. People love to taut on about how great stealth is, but always fail to think about what happens when the enemy is also stealthy. For ex., people that don’t know much outside of Wikipedia articles will tell you things like thrust vectoring don’t matter when “le AIM-120D has 100 mile range!”. This of course doesn’t account for even the slightest bit of EW denying shots at that range let alone against stealth

I think there is a very interesting doctrinal contrast to be made with Soviet vs US air superiority missions and how that lead to the ultimate design of the systems within the aircraft, not simply the airframes themselves. Let's contrast the Su-27 and F-15 of the 80s.-- Now, my interest in this was renewed by a casual, tangential convo with @Combination_K earlier today on this very subject. The Su-27 and F-15 are often times seen as practically identical in purpose from the outside: large, long-range, high endurance air-superiority fighters.--

The year is 202X. Through the fog of war, thickened by electronic warfare, you’ve accidentally merged w/ the enemy. It’s a clean head-on just like the movies. As you pass, the only question you ask yourself is: one circle or two? ITT: dogfighting in the age of the HOBS missile. But what’s a HOBS missile? HOBS = High Off Boresight. Meaning the missile is able to launch at a target far off the direction of the launching aircraft’s nose. Now, this technology isn’t brand new. As far back as the AIM-9D(1965), the ability to shoot HOBS was made.