At least from what ive heard it essentially some microphones tuned to pick up the direction of a helicopter and its approx. Range, and a camera to detect its location. Theres conflicting reports on whether it offered any irst functionality. But seeming as it was an early 70s design id wager to bet its unlikely but plausible.
There is nothing directly, except it fitting way too well.
There is also this further information about the ammunition (note the APFSDS missing, its development likely wasnt finished when the Brochure was published (11/'77))
All that we know about the heli thingie is from the respective patents, which are somewhat … theoretical.
Its developed by Zeiss and probably Eltro Gesellschaft für Strahlentechnic, although I never found the exact link. Eltro seems to have been a partial subsidiary of Zeiss, but it has been fully absorbed in the 1990s, so information is rare.
Its based around a thermal sensor working in a wavelength somewhere around 10 μm. The biggest differention to ADAD is the way of rejecting false targets:
The Begleits system watches for signatures that change their intensity, like it is caused by the rotor of a helicopter being more or less visible to the observer depending on how its rotated.
The microphone thing is a possible variation using unidirectional microphones for identification, although the Bagel itself wasnt planned with it
Spoiler
microphones were the numbers 18 to 20, note that the Begleit doesnt have that separate device
But ALL patents (there are like 5 of them) mention the same
“calculated azimuth and elevation angles are automatically fed to the fire control computer. Simultaneously the weapon as well as targeting and observation devices are being trained on the targets position” or
“In further and advantageous forms of construction of the invention the signals which come from the individual channels and contain information on the direction and, if required, also on the modulating frequency and, consequently, the type of helicopter located can be analyzed by a digital computer and signals fed to a fire control computer and be used to direct a suitable sighting and/or defensive means.” (thats ONE sentence btw)
To me that sounds alot like IRST
It also was reported on old forums, but we were told to write a suggestion instead. And those were all purged in the great forum swap
Found a reddit thread from close to a year ago talking about a possible BR for the Bagel if it gets it’s missing HWS and APFSDS. On the forums, a user by the name of Sugarstudd paid Thyssen-Henchel archives for the missing pages of the pamphlet/brochure. He got all of the missing pages but not the last page containing the ammunition types because it contained info that’s classified.