If we posit, then, that there was an air defense system sitting somewhere on this missile development base, what would they have seen? Using the ADS-B data, we can simulate the path of the aircraft from the perspective of a notional missile complex defending the facility.

For the sake of this analysis, we have selected a revetment to simulate the missile battery’s position; however, the facility is small enough that the perspective with respect to the trajectory of PS752 is relatively constant. From this position, looking towards Imam Khomenini International Airport, the IRGC ASF Tor system would have had a view like the following, where the trajectory (reported by ADS-B) of PS752 is shown in green.

From this angle, we can start to see their perspective. They likely had recently been moved to this location to defend it from an urgent American threat; a threat that would tend to mask behind terrain using stealth aircraft and suddenly pop over it. In this scenario, they would be presented with a target that must be engaged as soon as possible. From their angle, they could not see PS752’s takeoff — all they could see was an aircraft that could be a threat to them and the facility they were protecting.

Now, let us consider the timeline. Comparing our reference position to the ADS-B track, we can determine that the PS752 became visible to the Tor battery between 2:44:13 UTC and 2:44:24 UTC, with sightline vectors shown in blue and green on the below map.

The blue line, for 2:44:13 UTC, intersects with a hill. By 2:44:24 UTC, the Tor can clearly see PS752 along the green line, suggesting that this was probably the first time the operators saw the aircraft, coming towards them at hundreds of kilometers per hour at a range of less than 13km. We will refer to times with respect to this 2:44:24 time of likely detection; in this system, the last ADS-B message received by FlightRadar24 was at T+34s, when the aircraft was 8.46km away from our notional launch site.

If we suppose that the ADS-B transponder was disabled immediately after this message was sent, what would that engagement look like?

To determine how long the missile would take to arrive, we can refer to this helpful chart, from the technical information on the system:

This graph depicts the missile’s velocity versus time. To determine position versus time, we can simply graphically integrate the area under the curve. PS754 was not maneuvering, so we take the upper line. Each square in the chart represents a distance of 400 meters, depicted as an area of 4284px. Thus, to cover the 8.42km to the airliner (reduced as the airliner is approaching the missile site), we subtend this area:

Thus, it would take around 12 seconds for the first missile to reach the target from the notional launch site. This means that the missile would have had to have been fired at T+22 seconds; just below the green line from the crew’s point of view and when the target was around 10km away.

It takes a Tor system some time to respond once the decision has been made to engage a threat; according to the aforementioned specifications, this is at minimum around 7 seconds. However, this time was derived using the very best operators, against a target whose time of arrival they likely knew, working with perfectly functional equipment. In real operation under fighting conditions where none of this is true, this time is most likely closer to 10 or as much as 15 seconds. Using 10 seconds as our baseline, the decision to engage would have had to have been made at T+12 seconds after first detection.

We now can see into the minds of the Tor’s crew, at least to some extent. They saw an unknown — or at least unidentified — aircraft appear over a ridgeline without warning, coming straight towards their position. If it was an American bomber, they would have only seconds to react: they would have to get their missiles off before it would have a chance to release its bombs. In this scenario, it appears that they took around 10 seconds to make this choice; 10 seconds later, the first missile was away, and 12 seconds after that, PS752 had a large number of holes in its bottom.

PS752 getting hit. Courtesy of Bellingcat and Nariman Ghraeb.

That explains what the first noise that Mr. Ghraeb heard was. However, what was the missile that he captured? His video is clearly several seconds after the first intercept — if we assume that the missile was what stopped ADS-B transmissions — so what did he see? His video depicts PS752 getting hit approximately 3.6km away from the last transponder position. This distance would have taken 25 seconds for the aircraft to travel, assuming it did not slow down. What happened?

The second missile would have taken about 8 seconds to arrive from the notional launch site, suggesting that it was fired around 17 seconds after the first missile impacted. To explain this, we need to describe typical SAM engagement doctrine.

The principle objective of an air defense system is not to destroy its target. Rather, its purpose is to defend the air. As a result, their goal is to divert an incoming threat, be it by destroying them or forcing them to turn away. They will continue engaging a target until it has either been destroyed or is no longer a threat. To do so, they may need to fire more than one missile.

A single surface to air missile can have a relatively low probability of actually destroying its target. For example, against an F-15, the stated probability of kill (pK) is around 45% with a single Tor missile under optimal conditions (source: the above manual). This is unacceptably low under many scenarios, so a second missile is needed, raising the pK to an aggregate 69%. However, the timing between the two missiles can vary.

We will contrast two engagement doctrines: shoot-shoot-look and shoot-look-shoot. Under shoot-shoot-look doctrine, the system will fire two missiles (shoot-shoot) at a predetermined interval and guide both to the same target. Finally, the crew will observe the target (look) to determine if it has been destroyed or diverted. Under shoot-look-shoot, the operators will fire one missile, then determine if the target has been diverted or destroyed, and finally re-engage if the target has not been neutralized.

Two-missile impact geometry. Purple is first missile, turqoise is second.

The Iranian crew was likely using a shoot-look-shoot doctrine. They waited for the first missile to impact and then observed the target post impact. After being hit (end of the green line, missile along magenta line), PS752 continued on its course towards the missile development facility as indicated by its position in Mr. Ghraeb’s video: thus it was still potentially a threat and should be engaged a second time. After observing for 15 to 17 seconds (as the aircraft flies along the red line in the graphic), the crew decided to fire again. This second shot would take 7 to 8 seconds to arrive (dark turquoise line), and is shown in the video.

Likely, the only reason why the aircraft was not engaged again is because it begun its turn to the right. While we do not have a detailed track of the aircraft after the ADS-B data stops, it ended up substantially to the northeast of the intercept position. Thus, the aircraft no longer presented a threat and the engagement would have stopped.