The AIM9L sidewinder was fitted with an audible tone that would “growl” when the IR reading was of a certain level. This however greatly reduced lock range, and in fact the 9L can successfully acquire a heat signature from much further away at a lower level. The British seeing the potential to greatly increase the range of the sidewinder decided to remove this tone and use the radar/HUD display to determine if the missile had the energy to hit the target.
This would allow you to make near 20km long front aspect shoots against fully afterburning targets (Lock ranges would depend on heating of aircraft skin, engines heat and many other things.)
I got Smin to nudge it a month or so back. So if we get lucky. They might action it at long last. But so far, they’ve fixed very little for a lot of aircraft
The RAF modified the rails to get the dechirped ranges.
Flame's Explanation from the old forums
Over in the Tornado thread I’ve alluded a couple of times to a previously unknown British modification to the AIM-9L. Now that I’ve got a bit of time I thought I’d write a proper explanation for what it was.
As a bit of background: the AIM-9L seeker produces an acquisition audio tone whenever an IR source irradiates the detector cell. The pilot can then fire the missile in boresight mode (where the missile will just fire and try to lock onto whatever is in front of it), or press a button which will cause the seeker to attempt to the lock onto the the target and then uncage before launch (like we have in game), the latter being the normal mode of operation. In the case of the Tornado F.3 the button the pilot pressed to lock the seeker on was known as the “Target Acquisition Enable” (TAE) button.
The absolute minimum IR intensity the AIM-9L can detect is 15pw/cm -2 , but it needs about 35 pw/cm -2 in order to track a target reliably. It seems that when the AIM-9L was in development the Americans were concerned that the pilot couldn’t easily tell from the audio tone what the IR intensity of the target was; so the pilot may end up firing the missile without a strong enough return for it to track properly. They therefore implemented the “chirp” system into the missile (so called because it made the missile make a chirping sound when locked on). Basically (I’m simplifying a little) when the pilot attempted to lock the missile onto the target before launch the seeker would be repeatedly driven off-centre from the target, meaning that the target needed to have an IR intensity of about 70 cm/pw -2 before the missile could successfully lock on it, as the seeker wouldn’t be looking straight at the target. This would ensure that if the seeker had managed to obtained a lock it would easily be able to track the target after launch (because the IR intensity required for lock was much higher than that required for tracking).
The British decided that the chirp system “constitutes a very conservative confidence factor”, and that it wasn’t even needed because the pilot could use the sidewinder seeker symbol on the aircraft’s HUD to determine if the missile was tracking properly before launch. They therefore set about developing a way to remove the chirp system from the AIM-9L so that they could lock and fire it at greater range.
This is where the Tornado F.3 STF 113 de-chirping modification comes in (a proper British bodge job). They worked out that by modifying the wiring inside the LAU-7 missile launcher they could trick the AIM-9L seeker into thinking that the missile had already been launched (even though it was still attached to the aircraft) meaning the seeker could be made to lock-on to targets without the chirp system coming into play (as chirp was disabled as soon as the trigger was pulled). This modification to the launchers enabled the Tornado F.3 to lock onto targets with the AIM-9L at much greater ranges than other AIM-9L equipped aircraft could. According to the Tornado F.3 tactics manual the lock on range of the AIM-9L was essentially doubled under some conditions (which makes some sense as it now only needed half of the IR intensity it previously did in order to lock on).
I’m aware that this sounds like bit of a wild story, so here is the proof to back it up.
It would allow them to be locked and fired from greater ranges. The 9M report comes from the RAF comparing Dechirped Aim-9Ls to Aim-9Ms