Sightems: Mt. Tam Golfball

As we were walking on the west ridge of the park, a friend asked me what was that observatory up on Mt Tam across the bay, just a bit south of the lookout tower at the top of the peak. I had no clue. I’d seen it on clear days, looking like a golfball, and just assumed, as did my friend, that it was an astronomy station. My curiosity aroused, I then went to Google and searched for “observatory” near Mt. Tamalpais, and more broadly, in Marin County. Nothing in the way of astronomy outposts. Searching for “planetarium” got the same null results. Now intrigued, I tracked the roads atop Mt. Tam looking for the item. I finally spotted it, see this map:

Google Maps image of golfball and nearby structures

But was no wiser. There was a clue, however, in the name of the road shown in the upper left corner of the picture: “Airforce Trwy.” I went back to Google and searched for “Air Force Trwy” in Marin County. Bingo! A raft of websites with an overload of information, most of it obsolete. Scouring a dozen sources, I managed to extract the following.

It’s not an observatory or a planetarium, and never was. The golfball is the survivor of two identical inflated nylon/rubber domes, kept taut by air pressure, that sheltered military radar antennas.

The facility with outbuildings for staff was built in 1951 during the Korean War at the height of the McCarthy era as part of a Cold War system designed to detect imagined incoming Russian ICBMs or bombers. The radar had a 360-degree range of 200 miles. Operators watching the radar screen, if they could not immediately identify incoming aircraft, would telephone Hamilton Field near San Rafael, and seconds later, fighter jets would scramble to intercept the arrivals. This happened about ten times each day on the average, at a cost of millions, but no hostile arrivals were ever intercepted.

The Air Force advertised the facility when built as “weather radar,” but SF Chronicle reporters were allowed up to and inside the site in 1953, and published sensational front page reportage revealing its military mission with photos, available online here.

Regardless of historical shifts, the base stayed active until 1983. The Air Force then unloaded most of the property to the Marin Water District and the National Park Service, dumping on them the costly task of demolishing 40 buildings and cleaning up a major asbestos problem. Nothing was done until 1996, when many of the structures were taken down. It’s unclear whether asbestos cleanup was ever achieved.

In 1989, the Dalai Lama visited the derelict site and held a private mountaintop ceremony, covered by the Chronicle, intending to convert the abandoned Air Force radar station into a Tibetan temple of peace.

Much of the site remained dilapidated past 1996. A local photographer was able to drive right up to and inside the complex in the early 2000s, and has published numerous photos, including this one of the dome:


The site had a military history going back to World War II. The War Department leased three sites totaling 277 acres atop Mt. Tamalpais in 1942, and installed radar there, with a staff of nearly 100 Army personnel. Some of these leases expired in 1946, but one persisted until 2005. From 1959 to 1974, the base hosted the command and control system for launching Nike-Ajax and Nike-Hercules missile batteries in the Bay Area. Source. It was bounced between various commands and had various military designations, including Ground Equipment Facility J-33. Its most recent designation appears to be Mill Valley FAA Radar Site, and the golfball is known as ARSR4 Radar Tower.

Mill Valley FAA Radar Site ARSR4 Radar Tower, Photo by John Stanton 3 Sep 2017. Source.

The dome and its neighbor building remain intact and operational. According to a military history source, the FAA (Federal Aviation Authority) took it over after 1980, and continues to operate it. As of 2018:

The radar site data is now available to the USAF/NORAD Battle Control System-Fixed (BCS-F) operations centers (EADS & WADS) as well as the FAA Oakland ARTCC (ZOA) and adjacent ARTCCs. Other federal agencies have access to the data under the Homeland Security umbrella.

So there you have it. Probably more information than you ever wanted. Next time someone asks you about that golfball on Mt. Tam, just answer, “Oh yes, that’s ARSR4.” Enough said.

ARSR4 as seen from Cesar Chavez Park, June 2020
Red line shows direct line distance from Cesar Chavez Park to Mt. Tam golfball: 15.32 miles

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