Mill Valley Site Description - Radar Operation Area
The operations area is the smaller of the two parts of the station, but it is the most visible from a distance. The white, spherical domes or "radomes" of the two radar towers can be seen for many miles over a wide area and were the heart and soul of the station. The Radomes were the means of detecting a military threat aimed at the west coast. Adjacent to the radome towers is the original Operations Building where Air Force personnel staffed the radar scanners 24 hours a day, in search of such threats, and the Operations Building Annex, added to accommodate automation of the system. These structures are currently administered by the FAA as a Joint Use Site (JUS) for purposes of monitoring domestic air flights, as well as playing a diminished military surveillance function.
In addition to these four structures, three other buildings remain today from the original radar station, all identified by facility numbers. There is also a helipad built by the FAA after the Air Force station closed. Numerous features have been demolished, mostly of secondary importance both visually and functionally. The most conspicuous of these were several steel radio towers and the exterior overhead steam line system.
In plan, the operations area is much simpler and more compact than the cantonment area. The two domed radar towers, and the operations building and its annex are all close together on the principal terrace of the operations area, enclosed by a chain link fence.
A pair of water tanks is located just below the principal terrace at its southwest comer within a second chain link fence; the radio building is located southwest of the tanks within a separately fenced and administered area; and the gatehouse is located on the road leading back to the cantonment. Except for the facilities on the principal terrace, each of these is sited according to topographical constraints. Most of the facilities on the principal terrace were originally built with a near north-south orientation, each oriented to the others along parallel axes or cross-axes. When the second domed tower was built, it was oriented diagonally to the original buildings.
The principal buildings of the operations area are of fire proof construction. The two radar towers are built with steel frames, the operations building, its annex, and the radio building are all reinforced concrete buildings. The electronic radar and radio equipment which these buildings housed has been almost completely replaced since the period of significance. Only one radar scope, from the original circa 1953 system, an AN GPA-127 Planned Position Indicator (PPI), built by the Bendix Corporation, remains (photos).
The principal alterations to the operations area are the demolition of several buildings and structures and the removal of nearly all significant electronic equipment used in the principal mission of the station. Most of the buildings and structures which have been demolished or removed had a supportive rather than primary relation to the mission of the station. The loss of fuel tanks, a pump house, a heating plant and overhead steam lines, a supply building, a laboratory, and a training building all diminish the integrity of design, materials, workmanship, and association in a minor way. More important was the removal of several 90-foot-high radio towers which had a direct relation to one aspect of the station's mission, radio communication with friendly aircraft sent to check on unidentified aircraft. This change has a more serious effect on integrity of design, materials, workmanship, feeling, and association. Most important of all is the removal of the principal electronic equipment from the radomes and operations buildings used in performance of the station's mission. Only the one radar scope mentioned above, currently broken, is left in the operations building from the period of significance. The radar scope is approximately two feet wide and three feet tall.