Historic California Posts, CampStations and Airfield
Gualala Radar Site B-75/J-75
by SGM Dan Sebby, Post Historian, Camp San Luis Obispo
In 1942, the War Department authorized the acquisition of land on the Ohlson Ranch between the south fork of the Gualala River and the Pacific Ocean for use as an air defense radar site. Due to the secrecy surrounding radar sites, the Site was constructed to resemble farm buildings common to the coastal regions of California to hide its identity. This secrecy continued throughout World War II. The Site was not listed in the War Department Owned, Leased and Sponsored Facilities, 31 December 1945, or any of the Army of the United States Station Lists published by the Adjutant General of the U.S. Army as late as 7 May 1946.

The Site was garrisoned by elements of the San Francisco Control Group of the Fourth Air Force. The mission of this unit was to detect and identify all aircraft in the unit's area of responsibility and to direct fighter interceptors to targets deemed to be possible threats (San Francisco Control Group 1944). A building list, compiled from a hand-drawn map in Susan Clark's The Del Mar Ranch: From the German Rancho to The Sea Ranch, California, 1845-1964 and conversation with Russell Ohlson, for the Gualala Radar Site is presented below.
The 75-mm antiaircraft gun indicated above was more than likely a .M2 50 caliber Machine Gun or an M1 40mm Antiaircraft Gun
The site was equipped with the SCR-588 radar set, This was a Canadian-built copy of British long-wave CHL/GCI system (Type 7 height finder). The Site was initially garrisoned by the 656th Signal Aircraft Warning Company and was replaced a detachment from the 653rd Signal Aircraft Company. Both units were subordinate to the 411th Army Air Force Base Unit (San Francisco Control Group).
In July 1944, the Site transitioned from a tactical operation in which it operated 24 hours per day, seven days per week, to an semi-active or "on guard" status. In February 1945, the Site was re-designated an "Identification Friend or Foe" or "IFF" station. IFF Stations would electronically challenge and identify aircraft entering or operating in the station's area of responsibility. Aircraft that did not respond properly were intercepted by fighter aircraft from supporting U.S. Army airfields.

Beginning in the Spring of 1945, the Site returned to full tactical status to detect incoming Japanese rice paper balloons that were armed with incendiary and high explosive bombs. The Site was responsible for the detection and resulting interception of several of the over 9,000 balloons launched. The Site remained on full tactical status until 8 February 1946 when it was returned to an "on guard" status (411th Army Air Forces Base Unit 1945).

The U.S. Army began the process of deactivating the Site on 5 August 1946 when the Headquarters of the San Francisco Control Group removed all classified materials from the Site. The Fourth Air Force reported the Site as being surplus to their needs on 24 July 1947. The Site was dismantled and formally transferred to the District Engineer, USACE for disposal on 28 March 1948 (505th Aircraft Control and Warning Group 1948).
SCR-588 Long Wave Aircraft Detector and Ground Control Intercept Radar

Description: Fixed medium-range, long wave aircraft detector and GCI. Provides azimuth, range, and altitude data. Type HR and PPI scopes.
Uses: To give continuous plan position and accurate relative height of enemy plane and friendly fighter plane for GCI. Set can also be used for early warning and to give increased coverage at low angles of search. IFF equipment RC-188 is used. When operating as GCI, VHF equipment SCR-624 is required.
Performance and Siting: Maximum range on a medium bomber, with set on flat sea level site:
 Altitude, feet




 Range*, miles




*Range of PPI limits GCI operation to about 45 miles.
When operating as GCI, set must be sited so that a flat unobstructed surface extends at least 1/4th mile in the height-finding sector. Good GCI sites are extremely rare. For early warning, sets should be sited between 100 and 1,000 feet above an unobstructed surface.
Transportability: Set is packaged for shipment in 55 units, weighing a total of 54,000 lbs. Largest unit measures 15.3' x 3.8' x 1.8'. Total shipping space is 3500 cu. ft.
Installation: Requires a weatherproof building approximately 20' x 40' for housing radar components and a building approximately 20' x 20' for power units and switchboard. A 25' tower on concrete footing is required for support of antenna. Buildings and tower can be built by Engineers in about 3 weeks. Radar can be installed by 5 men in 2 weeks.
Personnel: 8 men are operating crew. For 24-hour operation about 54 men are required to run radar, communication radio, and camp.
Power: 4 KW at 230 volts and 1 KW at 115 volts, from three PE-198 electric diesel units, supplied with set. (Two units operate simultaneously). Fuel consumption is 21/2 gals. of Diesel fuel oil per hour.

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Updated 8 February 2016