How can the concept of "intercept geometry" be explained in simple terms?
Intercept geometry is the bubble of space in which the SA-2 missile can be effective against a specific aircraft. Crucial here is the detection range of the radar system guiding the missile, given here as 40 nm.

Depending on the speed of the target, the missile has only a limited time to ascend to the altitude of the target and to close in. The combination of speed and altitude of the SR-71 (1854 kt), moving at more than 30 nm per minute, will leave only a very small intercept geometry, and it has to overfly the missile launcher almost directly to be vulnerable. Once the target flies above 90.000 ft and Mach 3.5 (or employs effective countermeasures), it will be unassailable by the SA-2.

Contrast that with the fate of an aircraft flying only Mach 1 at 60.000 ft (which equals 573.6 kt): Moving at less than 10 nm per minute, its course can go past the launcher with a 25 nm offset, but it will still be in range of the Fan Song - SA-2 combination.