By Sharon Watkins Lang, USASMDC/ARSTRAT Command HistorianJanuary 18, 2017

The Perimeter Acquisition Radar was an electronically steerable phased array UHF radar and an integral component of the Safeguard ballistic missile defense system. The radar was constructed in North Dakota to provide early attack warning and long ran... (Photo Credit: U.S. Army) VIEW ORIGINAL

The Perimeter Acquisition Radar, or PAR, was an electronically steerable phased array UHF radar and an integral component of the Safeguard ballistic missile defense system. A one of a kind structure, it was constructed in North Dakota to provide early attack warning and long range intercept planning.

The 128-foot structure was "designed to perform its mission in an environment that may consist of false targets (aircraft, satellites, aurora, and meteors) and nuclear weapons effects." The system's capabilities were well recognized and even as it achieved full operational capability in April 1975, the PAR was under consideration to expand its missions further.

Given the PAR's demonstrated ability to detect and track objects in space, the air defense community began to assess the PAR's potential to provide attack assessments and support satellite tracking missions. A series of meetings were held in March and April of 1975 with the North American Air Defense, or NORAD, and the Air Defense Command, or ADCOM, to explore these options.

Bell Telephone Laboratories, one of the lead contractors for the Safeguard program, was tasked to study these issues and report on the PAR operational concept for attack assessment.

ADCOM presented the assembled data to the Joint Chiefs of Staff and the world-wide military command and control support group. Ultimately by September 1975, it was decided that while the PAR missions would be expanded, the expansion was limited to attack assessment.

Specifically the PAR or the new PAR Attack Characterization System, or PACS, would serve as an external sensor to the NORAD/ADCOM attack assessment mission -- locating, identifying and tracking reentry vehicles and providing impact data thereby supplementing the existing Ballistic Missile Early Warning System. Unlike the other radars, the nuclear hardened PAR could distinguish reentry vehicles and provide impact data in a congested environment.

As the Safeguard program came to an end, the PAR continued to move forward. The process to convert the Safeguard PAR to the PACS PAR began in fiscal year 1976 with a 15-month software development program but only minimal hardware modifications.

In June 1976, the ADCOM requested a new experiment be conducted to assess the radar's ability to tracking objects that are not in the Space Defense Catalogue. A review of data from a 12-hour period of the Unknown Satellite Track Experiment found that the PAR successfully tracked 6,931 objects which appeared in the catalogue as well as an additional 1,494 which did not.

Science Applications Inc., or SAI, researchers concluded that the PAR collected low altitude and low-cross section data that was not available elsewhere. They also determined that this material could be used to augment the Space Defense Catalogue and would benefit both spacecraft collision studies and PAR system studies.

The SAI report featured "some unique PAR characteristics" which enhanced its space tracking capabilities. The radar's "large continuous search volume" gave the sensor multiple opportunities to detect and observe objects crossing the scan area. This factor also allowed the sensor to view objects several times and from various angles thereby reducing the probability of non-detection.

In addition, the fully automated PAR can accurately detect and track multiple objects simultaneously.

With these findings, ADCOM renewed their request to adopt the Space Tracking and Detection, or SPADATS, mission with the PAR. The deputy secretary of defense returned a decision months later on Jan. 19, 1977. The Space Detection and Tracking System (SPADATS) modifications for the PAR were approved, adding a space track role to the PACS mission.

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