Historic California Posts, Camps, Stations and Airfields
Mill Valley Air Force Station
(Mount Tamalpais Radar Site B-78, San Francisco Defense Area Site SF-90DC)
by Daniel M. Sebby,Military Historian, California Military Department

From 1942 to 1950, the War Department leased three pieces of land totaling 276.96 acres from the Marin Municipal Water District, the State of California and private landowners. The three sites were:

1) A lower operational area for Mount Tamalpais Radar Site

2) An administrative and housing area or “Camp Site” for Mount Tamalpais Radar Site

3) An additional, upper operational site located atop Mount Tamalpais which became Mill Valley Air Force Station (AFS), a Cold War-era air defense radar station. Due to the geography allowing for 360º coverage, this area was the site for Homing Beacon 4H (also known as “Homer Site 4H”) and Forward Relay Site 4M, as well as radar during World War II.

The War Department’s report, Owned, Leased and Sponsored Facilities, dated 31 December 1945 shows that the former Mount Tamalpais Radar Site (also known as Site B-78) was under the jurisdiction of the Army Ground Forces. It is suspected that this information may been outdated since air defense radar functions were transferred from the Army Ground Forces to the Army Air Forces when the 4th Anti-Aircraft Command was transferred from the Western Defense Command and Fourth Army to the Fourth Air Force on 1 May 1944.

The “Camp Site” was situated at the Mountain Theater, a Civilian Conservation Corps built outdoor amphitheater constructed of cut rock. It consisted 24.4 acres permitted from the California State Parks Commission in 1942. Temporary buildings were built that housed 94 enlisted men and five officers assigned to a Signal Aircraft Warning Company. The U.S. Army maintained control of this area until the permit was terminated on 30 November 1946

The initial operational site was located to the west of the Pan Toll Ranger Station on the then privately owned Ridgecrest Avenue. It consisted of two parcels leased in 1942 from the Marin Municipal Water District (49.2 acres) and Douglas A. Nye (13.46 acres). These leases were terminated in the fall of 1946.

The second operational area, which became Mill Valley AFS , consisted of 106.4 acres leased from the Marin Municipal Water District in 1942 and held by the Federal government until the final lease expired on 30 June 2005, at which time it reverted back to the Marin Municipal Water District.

From 1944, the Mount Tamalpais Radar Site was under the control of the 411th Army Air Forces Base Unit (AAFBU) (San Francisco Control Group). This unit was superseded by Squadron B, 412th AAFBU (Western Aircraft Control and Warning Group) on 28 February 1947. On 21 May 1947, Squadron B became the 636th Aircraft Control and Warning (AC&W) Squadron at Hamilton Field. Hamilton Field was described as the squadron’s temporary duty station for the “duration of time necessary to inactivate former radar sites of the old San Francisco Control Group, at which time it will move to its permanent station at McChord Field, Washington”

Soon after the above referenced reorganization the site was closed and its equipment was transferred to Berkeley while the Forward Relay site remained operational. The leases for the operational and housing sites were terminated by 1948. The Army Corps of Engineers retained control of the parcel on top of Mount Tamalpais.

Construction of the former Mill Valley AFS began in 1950 and finished in 1952; the Station was dedicated in 1953. Mill Valley AFS was referred to as “Weather Station” to disguise its true function. The station was divided into an operations area and cantonment (administrative and housing) area. Improvements included 62 buildings, roads, perimeter fencing and guard posts, communications facilities, and utilities infrastructure. Radar dome (“radome”) tower buildings housed SLBM detection and warning and AC&W search and height finder radar sets.

Recreation facilities included a tennis court, pool and bathhouse, theater, gymnasium, and bowling alley.

In addition to the normal garrisoning unit, the 666th AC&W Squadron (later redesignated 666th Radar Squadron), the station also hosted a Submarine Launched Ballistic Missile (SLBM) detection site from July of 1967 until April 1980. This site was operated by Detachment 3, Fourteenth Air Force (later redesignated as Detachment 3, 14th Missile Warning Squadron).

Unique to California radar stations, Mill Valley AFS was integrated into the Army’s antiaircraft defenses and was known as San Francisco Defense Area Site SF-90DC. As such, the station hosted the Army Air Defense Command Post for the 40th Artillery Brigade from 1959 until June 1971, and the 13th Air Defense Artillery Group from July 1971 to August 1974. This command post controlled all of the Nike-Ajax and Nike-Hercules missile batteries in both the San Francisco and Travis AFB Defense Areas. It was equipped initially with the AN/FSG-1 “Missile Master” command and control system which was upgraded to the AN/TSQ-51 “Missile Mentor” in the mid-1960’s.

Also distinctive to Mill Valley AFS was the fact that the Ground-Air Transmitter-Receiver (GATR) site was located over 100 miles away at Beale Air Force Base and was operated by the 666th Radar Squadron’s Detachment 1 from 1963 until the squadron stood down in 1980. The 666th Radar Squadron also maintained an unmanned gap filler annex at the Army’s Fort Ord.

Although under the operational control of the Federal Aviation Administration (FAA) since 1 October 1980 when the 666th Radar Squadron was inactivated, Mill Valley AFS was declared excess to the needs of the Department of Defense in 1982 and transferred through the General Services Administration to the National Park Service and the FFA in 1983 under the provisions of the Golden Gate National Recreation Area Act and other Federal laws and regulations. In June of 2005, the leases between the Marin Municipal Water District expired and the land was returned to their control.


Site Map

Mill Valley AFS 1979 (Click for a larger view)

Extract, War Department Inventory of Owned, Sponsored and Leased Facilities, 1945

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Updated 9 April 2016