The Topsfield Gap Filler Annex is located on top of Musquash Mountain in Topsfield, ME
During the fifties and sixties, several Aircraft Control & Warning (AC&W) "gap filler" sites were constructed along the borders of the United States and across many of the "lower 48" states and Canada. These sites filled in the low altitude gaps between the long-range radar stations.
Sites built before 1958 would have been part of a manual radar defense network. After 1958, new sites would have been a part of the Semi-Automatic Ground Environment (SAGE) radar network and existing sites would have been upgraded. The last gap filler site in Maine was closed in 1968 and their phase-out continued across the country until approximately 1970. Topsfield was one of two gap filler sites associated with Charleston AFS in Charleston, ME, which is now a youth correctional facility. The other gap filler site was Sedgwick. Topsfield was activated at the end of 1958 and was inactivated at the end of 1967.
The SAGE continental air defense system was developed by the Massachusetts Institute of Technology's (MIT) Digital Computer Lab, the Air Force Cambridge Research Lab and MIT's Lincoln Lab between 1950 and 1957. SAGE was intended to identify and track Soviet bombers coming over the north pole. By the early 1960's, there were 23 SAGE "block houses" at various Air Force stations across the country. Each block house was a hardened, windowless structure that was home to an enormous 200+ ton computer. Each computer would speak to the other SAGE computers, ground radar, ship-board radar, and weather stations in order to keep tabs on legit air traffic and keep a constant eye out for Soviet traffic. The Ground to Air Transmitter/Receiver sites were used for communication between these fellow SAGE sites.
To reach the Topsfield site from Bangor, take I-95 north to Lincoln. Get off at the Route 2/Route 6 exit and head east into town. Stay on Route 6 through Lee and Carroll and cross into Washington County. After you cross into Washington County, it's only a few quick miles to East Musquash Lake, which will be on your left. At the far east end of the lake, you should see Tower Road on your right. The old yellow Air Force gate should still be there and there are power lines running up along the road as well.