In the 1950's, seven batteries of Nike Ajax anti-aircraft missiles were located around Baltimore to protect the city, Friendship Airport, various industrial sites, and the Aberdeen ordnance and Edgewood chemical facilities from Russian bombers. Various Air Force and Navy air-warning net systems would alert the Army Nike site to hostile approaching aircraft. The Nike-Ajax was the first operational American surface-to-air missile, SAM-A-7, replacing 90mm and 120 mm anti-aircraft guns of WWII. Its target tracking radar used the MPA-4 antenna, and the missile subsequently became a radar beam rider that could counter target evasive action. By 1956, eighty Nike-Ajax sites were operating in the U.S.
