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How many years before the E-3A did the Tu-126 "Moss" enter service?
Short answer
The Tu-126 "Moss" entered service in 1965, while the USAF's first E-3 was delivered in March 1977, its introduction date. The Soviet prototype had flown on 23 January 1962, followed by eight production aircraft built between 1965 and 1968.

An airborne radar for the northern coast
In 1958, growing concern over the possibility of a US nuclear attack on the USSR from the north led to a new requirement for airborne radar, a solution that avoided the problems and expense of trying to build a network of ground radars capable of covering the entire vast Soviet northern coast. For the system, it was decided to use the wider-fuselage Tupolev Tu-114. The first Tu-126 prototype flew on 23 January 1962. The prototype was followed by eight production aircraft, built between 1965 and 1968.
In service since 1965
The Tu-126 entered service in 1965, although Western intelligence did not identify it until 1968, when it appeared in a Soviet documentary.
The E-3 Sentry, delivered in 1977
Engineering, test and evaluation work on the first E-3 Sentry began in October 1975. The USAF's first E-3 was delivered in March 1977, the type's introduction date. Between 1977 and 1992, a total of 68 E-3s were built.
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