The AIM-9L was an incomplete product when it entered service. It was intended to contain flare rejection logic, be fully digital, and have high sensitivity, but these requirements would have to wait until 9M. The Brits would take it upon themselves to modify 9L to increase range.
How did they accomplish that? By removing the "chirp tone" that provided the pilot with an audible positive confirmation of lock-on. The threshold for the chirp tone generation was far higher than the detection threshold. This left the only indication of a lock as hud symbology.
It is my understanding that this chirp tone is generated by circuitry in the launching aircraft, much like the "chirp tone" of later rear aspect Sidewinders, which drove the seeker off-center during radar-slaved lock to ensure the seeker was tracking a target and not a cloud.
Given what I know about 9L's seeker design, I don't believe that this was the method used to generate the chirp, but I suspect the mechanism was similar.
However, given what I can find, this indicates that the dechirped 9L was relegated to being a radar-slaved IR missile.
This removal of the chirp tone was absolutely horrendous for the purposes of 9L as a dogfight missile.
However, I think I have an explanation for this somewhat questionable choice. This comes back to the mission planning and intended targets of the Tornado Air Defense Variant.
The Tornado ADV was intended to intercept low-flying Soviet bombers, and the usage of Skyflash meant that only one target could be intercepted at a time. Skyflash was a semi-active radar-homing missile, and therefore the launching aircraft needed to acquire a single-target-track.
This meant that the Tornado was expected to have to intercept targets with its IR AAMs as well as its Radar AAMs. At lower altitudes, the lower seeker sensitivity of 9L(and therefore shorter lock range) hurt its range performance especially compared to 9M.
You'll notice in this chart the significant differences between lock ranges on cruise power versus afterburner power, and this was at high altitudes! Bombers would also likely not put out as much heat as an F-16, and therefore a more faint IR signature (180° appears to be front).
As the first diagram in this thread shows, de-chirping did solve the problem of low-altitude performance as intended.
I still think it was a terrible idea and a bodge job to cover for the ADV being a questionable interceptor design, but that's a conversation for another time.
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