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 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.

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.

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.