The Saugatuck Gap Filler Annex (ADC ID: P-67C, NORAD ID: Z-67C, Z-34G) is a decommissioned air defense radar installation previously of the United States Air Force. It served in the vast Cold War era Semi-Automatic Ground Environment (SAGE) air defense system. Of the hundreds of SAGE radars Saugatuck's is the only annex that remains nearly completely intact.
Located immediately across the Kalamazoo River from Saugatuck, Michigan, at the top of Mount Baldhead, a 230-foot dune on the shore of Lake Michigan, the annex was positioned to fill gaps in the coverage of long-range "heavy" radars sited further inland. The heavy radars searched for attacking Soviet bombers but were unable to detect aircraft flying low to the west of the dunes along Lake Michigan. Saugatuck's original AN/FPS-14 radar was commissioned in mid-1958 and operated until it was replaced with a more capable AN/FPS-18 in 1963. The FPS-18 radar served continuously until the site was decommissioned early in 1968. The city of Saugatuck purchased the building, tower, and radar equipment from the Air Force in 1969. Today, the installation appears very much as it did when operational with virtually all of the Cold War-era electronic equipment still in place.
The Saugatuck Gap Filler Annex was listed on the National Register of Historic Places in 2022, and efforts are underway by a group of volunteers to stabilize the site and secure funding for further preservation and restoration.
The Cold War The Strange Alliance The United States, the United Kingdom, and the Union of Soviet Socialist Republics (Soviet Union) joined forces in World War II to become the key members of the Grand Alliance against the Axis Powers. It was always a "Strange Alliance," with a pronounced wariness between the United States, the world's greatest capitalist country, and the Soviet Union, the greatest socialist country. Post-war, with aggravations from both sides, the strain slowly crystallized into hostility approaching open conflict. Spanning decades, this period in history is known as the "Cold War."
Despite increasing Soviet aggression the United States felt its nuclear monopoly was an effective deterrent against attack until mid-1949 when the Soviet Union successfully tested its first atomic bomb, about four years sooner than had been anticipated. Earlier, they had unveiled the totally unexpected TU-4 bomber, a literal piece-by-piece copy of the United States B-29 with a one-way range sufficient to carry an atomic bomb from the Soviet Union to most large cities in the continental United States.
The Lashup System In 1949, the only vestige of a North American continental air defense system was a motley collection of surplus WWII-era radars scattered sparsely across Canada and the northern United States. These were euphemistically named the Lashup System. Previously, lacking a viable threat, there had been little incentive to invest in upgrades to this network. With the sudden advent of the Soviet atom bomb and the TU-4, however, priorities were rapidly reassessed and the US Air Force launched an aggressive program to augment radar coverage of the most likely routes of attack from the north over Canada.
By 1952, the Lashup System had been reinforced with the emerging Permanent System radars arrayed along the border between the United States and Canada, soon supplemented by the Mid-Canada radar fence. The Ground Observer Corps (GOC), disbanded at the end of World War II, had been brought back into service and greatly expanded to assist in visually identifying aircraft and estimating altitude. The GOC's sightings and those detected by radar were reported by telephone to Manual Control Centers (MCCs) where the flight tracks of suspect aircraft and of the fighters dispatched to intercept them were plotted by hand on large plexiglass maps as controllers attempted to guide the fighter pilots by radio. But a number of exercises simulating attacking bombers revealed the MCCs were quickly overwhelmed by even modest incursions. Additionally, numerous blind spots were discovered in the coverage of the heavy radars where their ability to detect low-level aircraft was obstructed by terrain.
SAGE The solution identified to address the issues with manual control was to use massive computers to analyze radar data, assess which defenses were most appropriate for a response, and automatically direct interception of suspected Soviet bombers, all with minimal human involvement. The system that emerged to satisfy this requirement was SAGE, the Semi-Automatic Ground Environment, a continent-wide network of enormous AN/FSQ-7 computers linked to hundreds of radars.
The number of radars in the SAGE network reached a peak in 1962 with nearly 250 heavy and gap-filler radars from the Permanent System, the remnants of the Lashup System, and some semi-mobile radars, all distributed primarily across the northern United States with some in Canada. In addition, there was the Distant Early Warning (DEW) Line radars just north of the Arctic Circle, Canada's Mid-Canada Line radar fence, and the Pine Tree Line radars across southern Canada. To these were added three ill-fated oil-rig-like Texas Tower radar installations built on the continental shelf off New England, a small number of radars carried by early warning aircraft and radar picket ships which operated off both coasts, and a few dirigibles.
To augment the Permanent System of heavy radars and resolve the low-level blind spots, around 240 gap-filler radar sites were planned between 1956 and 1962. Not all of the proposed sites were actually developed and, even if the buildings were constructed, not all received the radar equipment and towers as funds were slowly diverted to address the emerging threat of missile attack, highlighted by the launch of the Soviet Union's Sputnik satellite in 1957. Ultimately, a maximum of 131 gap-fillers were in service at any one time, the Saugatuck Gap Filler Annex among them.
The Saugatuck Gap Filler Annex History The Air Force entered into a lease with the City of Saugatuck on 1 June 1956, for a small site at the top of Mt. Baldhead, the dune upon which the radar installation was constructed. Also included in the lease was an area at the eastern foot of the dune, below and slightly south of the radar site. This area was for ancillary equipment consisting principally of underground diesel fuel tanks and pumps used to move fuel to a storage tank at the top of the dune which supplied a generator in the annex building. There was also an easement connecting the two sites, running parallel to the stairs up the east face of the dune, to accommodate diesel fuel lines, electrical service, and telephone lines.
Construction of the equipment and generator building began on 23 August 1956, with an anticipated completion date in January 1957. Many gap-fillers were being constructed at this time and there was a delay before the radar equipment arrived. After installation and testing, the Bendix AN/FPS-14 radar became operational between 1 April – 30 June 1958, reporting to the 781st Aircraft Control and Warning (AC&W) Squadron at Custer Air Force Station (AFS), the "prime site," near Battle Creek, Michigan, where the heavy long-range and height-finder radars were located. The 781st's prime associated interceptor squadron was the 319th Fighter Interceptor Squadron, stationed at Bunker Hill Air Force Base in Peru, Indiana.
Like the other three gap-fillers under the control of Custer AFS, the Saugatuck Gap Filler Annex was designed to be unmanned and was operated remotely by the prime site. Also, like the other gap-fillers, the Saugatuck annex employed AN/FST-1 Coordinate Data Transmitter to send radar data to the immense AN/FSQ-7 computer housed at SAGE Direction Center DC-06, co-located with the 781st AC&W Squadron at Custer AFS. The AN/FST-1 was a state-of-the-art digital electronic device implemented almost entirely with vacuum tubes. It filtered spurious and fixed radar echoes from those of actual aircraft, converted the analog signals into digital data, then transmitted it at about 1600 bits/second via leased telephone lines using the first modem employed in an actual application outside of research.
As SAGE became fully operational, the squadron title was changed to 781st Radar Squadron (SAGE) on 1 October 1959. Custer AFS was known by Permanent System ID P-67 and Saugatuck, one of four subordinate gap-fillers, held site ID P-67C until 31 July 1963, when all SAGE radar squadron designations were changed to the NORAD prefix Z and Saugatuck became Z-67C.
Custer AFS operated the following Gap Filler sites in addition to its own heavy and height-finder radars: Midland, MI (P-67A/P-20G) Richland Center, IN (P-67B/P-73J) Saugatuck, MI (P-67C/Z-34G) Shelby, MI (P-67D/Z-34H)
The AN/FPS-14 radar was removed in June 1963, to be upgraded by a Bendix AN/FPS-18 radar. The FPS-18 was an improved design having a more refined ability to discriminate between fixed and moving targets and an output power of one megawatt, a little over twice that of the FPS-14. The additional power helped add definition to marginal returns, but effective range remained 48 miles due to limitations of the FST-1. A number of months lapsed between the removal of the old radar and installation of the new equipment. The AN/FPS-18 radar finally became operational on 14 December 1963, and remains in place in the equipment room today. At the same time, a rigid radome was installed to protect the new antenna at the top of the tower. This is the iconic white ball still visible for miles in all directions.
Control of the Saugatuck radar annex was transferred to the 752nd Radar Squadron (SAGE) on 27 January 1965, located at Empire Air Force Station, Z-34, near Empire, Michigan. The Saugatuck Gap Filler Annex was redesignated Z-34G and recommissioned on 2 February 1965. As the missile threat continued to gain priority, the 752nd received notice on 31 March 1968, that operation of all gap-fillers would terminate. The squadron was directed to drain and flush the transmitter coolant systems, tape the doors of the equipment cabinets, and prepare the diesel generators for storage.
On 1 April 1968, the Saugatuck Gap Filler Annex was permanently deactivated. After removing the diesel generator and all spare parts, the Air Force terminated its lease of the annex site effective 31 December 1968. Possession of the land reverted to the city of Saugatuck, which purchased the building, tower, and all the radar equipment for $250 in July 1969.
Apparently, removal of the radar antenna had been scheduled at some point prior to Saugatuck's purchasing the radar station because, very shortly after the sale, an Air Force crew appeared and began to dismantle the radome. The misunderstanding was quickly straightened out and the crew reassembled the dome without removing any equipment.
Recent history In the decades following the city's purchase of the radar, the building and the equipment inside slowly yielded to the passage of the years. Occasional stop-gap work was performed to seal roof leaks and secure the site against souvenir collectors but, high atop the dune, it was basically out of sight, out of mind, and the condition of the annex steadily worsened. At one point, the city of Saugatuck formally set aside funds for the demolition of the aging building, awareness of the site's importance having been lost with the succession of new city councils. A handful of local residents responded to each new threat by alerting council members to the radar's history and rarity. Eventually, appreciation for the importance of the radar station grew. It was the subject of a standing-room-only History Center presentation in August 2016 and became the anchor of the center's museum exhibit for 2017 and 2018 which included a visit by one of the Air Force technicians who had maintained the radar when it was operational.