In the late 60s USSR was in need of a new and more efficient AWACS, to replace Tu-126 Moss.
In 1972 two DRLO projects were considered : The Tupolev Tu-156 (a new design based on a lengthened Tu-154 fuselage with new wings and four D-30KP engines) and a modified Tu-154B.
The radar was the new Schmel (and it's upgraded version Schmel-M) to replace the unsuccessful Liana complex of the Tu-126.
Finally the Beriev design bureau, offered a quicker solution : A Schmel-M mounted on a slightly modified Il-76.
The Tu-154B DRLO :