Interferometer Slide Rule, U.S. Navy Space Surveillance Fence

Display Status:

This object is not on display at the National Air and Space Museum. It is either on loan or in storage.

Collection Item Summary:

This custom-built slide rule was used to determine the orbital elements of objects which the U.S. Navy Space Surveillance Fence detected. The system's three transmitter stations located across the southern United States emitted a continous wave of radio energy in a narrow fan-shaped pattern. Together the beams extended about 5,000 miles from west to east and about 15,000 miles into space. Any object passing through a beam was illuminated and the reflected signals acquired by one or more or the six receiving stations along the same latitude as the transmitter stations.

These slide rules were used to determine the orbital elements of detected objects (from 1958-1963 (the first years the Space Surveillance Fence was operational). A different slide rule was required for each of the six receiver stations. In 1963, computers replaced the slide rules. Unclassified and classified catalogs of all known objects in orbit were prepared from data from the Space Surveillance Fence and other sensors.

The Air Force assumed operation of the Space Surveillance Fence from the Navy in 2004. It was decommissioned in 2013, to be replaced by a space-based system.

The manufacturer of the slide rule is unknown. The Air Force transferred it to the Museum in 2014.