Towards the end of its service, the Lightning was increasingly outclassed by newer fighters, mainly due to avionics and armament obsolescence. The radar had a limited range and no track while scanning capability, and it could detect targets only in a narrow (40°) arc. While an automatic collision course attack system was developed and successfully demonstrated by English Electric, it was not adopted due to cost concerns. Plans were mooted to supplement or replace the obsolete Red Top and Firestreak missiles with modern AIM-9L Sidewinder missiles to help rectify some of the obsolescence, but these ambitions were not realised due to lack of funding. An alternative to the modernisation of existing aircraft would have been the development of more advanced variants; a proposed variable-sweep wing Lightning would have likely involved the adoption of a new powerplant and radar and was believed by BAC to significantly increase performance, but ultimately was not pursued.