Historic California Posts, Camps, Stations and Airfields
Janesville Gap Filler Annex SM-157A
 
"During the late 1950s another area of progress was the development and deployment of AN/FPS-14 and AN/FPS-18 gap-filler radars. Having a range of around sixty-five miles, these radars were placed in areas where it was thought enemy aircraft could fly low to avoid detection by the longer-range radars of the permanent and mobile radar networks. Gap-filler radar deployment peaked in December 1960 at 131 sites throughout the continental United States. Because the introduction of gap-filler radars alleviated the need for civilians to scan the skies for enemy bombers, the ADC disestablished the Ground Observer Corps on January 31, 1959."
 
Searching The Skies
USAF Air Combat Command
June, 1997
 
 
 

The Site was utilized by the US Air Force as an Aircraft Control & Warning (AC&W) site from 1958 to 1970. This AC&W Site was part of the Semi-Automatic Ground Environment (SAGE) radar system. The Former Janesville GFA utilized an AN/FPS-18 model gap filler radar, where "AN/FPS" is an acronym for Army-Navy Fixed, Pulsed radar, Search electronic device (Radomes, Inc., 2004). Figure 3-1 shows an aerial view of the Site. Radomes, Inc., a non-profit veterans society dedicated to radar military history and technology, summarizes the function of these facilities:

"During the late 1950s another area of progress was the development and deployment of AN/FPS-14 and AN/FPS-18 gap-filler radars. Having a range of around sixty-five miles, these radars were placed in areas where it was thought enemy aircraft could fly low to avoid detection by the longer-range radars of the permanent and mobile radar networks. Gap-filler radar deployment peaked in December 1960 at 131 sites throughout the continental United States." (Radomes, Inc., 2004).

The Site consisted of two parcels totaling 37.42 acres: 37.12 acres of USFS unimproved road 2 miles more or less (Thompson Peak Tower Access Road, "unimproved jeep trail); and a 0.3 acre building site (130 feet x 100 feet). The AN/FPS facility is further described:

"When most AC&W radar veterans think of a 'gap filler,' they usually think of an unmanned radar facility designed to fill the low-altitude gaps between manned long-range radar stations. Gaps in coverage existed due to the curvature of the earth, mountains, hills, valleys, rivers, and so forth. The typical unmanned gap-filler radar annex was comprised of a small L-shaped cinder block building, with the radar equipment and the data-transmission equipment in one section and one or more diesel generators in the other section. These unmanned gap-filler sites generally had a three-legged radar tower about 85 feet tall. A couple of gap-filler radar towers were four-legged, as they also hosted a forest-fire lookout cabin below the radar-antenna deck. Also, at least one GFA building was not L-shaped, but most were. Unmanned gap-filler facilities in the continental United States (CONUS) used either an AN/FPS-14 or an AN/FPS-18 short range search radar having an effective range of 60 to 65 nautical miles. Both models were built by Bendix, and both operated in the S-band at a frequency between 2700 and 2900 MHz" (Radomes, 2004).

 

US Army Corps of Engineers History

On 21 July 1958, the U.S. Government acquired 37.12 public domain (pal.) acres and on 5 August 1958, they acquired 0.30 pal. acres from the United States Department of Agriculture, Forest Service (USFS). Total acreage acquired for the subject site was 37.42

The site, located in Plumas County, 13 miles southwest of Susanville, California, was known as Janesville Gap Filler Annex (Z 157A), also known as Janesville Gap Filler Annex (SM 157A), Gap Filler Radar Site SM 157A, Thompson Peak Mountain, and Janesville Gap Filler Annex (Z157A)LTMJ, CA. The property was used by the Air Defense Command as an unmanned radar gap filler station supporting Red Bluff Air Force Station. Improvements to the site consisted of one concrete block equipment and generator building, two 15,000 gallon fuel storage tanks, a 70 foot high steel antenna tower, a woven wire fence, and a gravel surfaced road.

On 28 March 1969, 37.42 public domain acres were transferred back to the USFS.
 
 
US Army Corps of Engineers Maps
 

 

Source, Sacramento District, US Army Corps of Engineers
 
 
 
 
 
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Updated 8 February 2016