Make the sentence simpler: It began being used in 1971 and eventually replaced all other anti-aircraft weapons in the British Army. <SEP> It replaced both guns for targets that were flying low down, and the English Electric Thunderbird which was used against targets that were higher up and further away.
Entering service in 1972, it eventually replaced all other anti-aircraft weapons in Army service; both the Bofors guns used against low-altitude targets and the Thunderbird missile used against longer-range and higher-altitude targets.