GROUND OBSERVER CORPS FILTER CENTERS: United States. 1951-1958. The key facility and nerve center of the Ground Observer Corps reporting system was the Filter Center. Filter Centers were combined military and civilian organizations.  For the Air Force an Observer Detachment Commander supervised training, administration, and support of the Filter Center and its Posts. On the civilian side, a Supervisor was appointed by the Area Supervisor or the District or County Coordinator for Ground Observer Corps activities (appointed by the State Director of Civil Defense).  The actual operations of the Filter Center were performed by a volunteer, civilian staff.  Pictures show that this staff was not uniformed and was predominantly female.

Individual Ground Observer Posts detected aircraft visually, composed a standard Aircraft Flash Message report, and used the general telephone system to make their reports.  Where possible telephone lines to Posts were private lines, directly wired in clusters of Posts to the Filter Center, although in many cases the Post shared a party line.  The Observer would preface his or her telephone call with the words "Aircraft Flash" to key all users of the telephone system to the emergency nature of the call.  The Filter Center staff member taking the call would answer "Air Defense, Go Ahead" and copy the contents of the Aircraft Flash Message.

Each aircraft reported was plotted, normally on a horizontal plotting table using vertical tree markers that depicted the aircraft's location, speed, height, and direction of travel.  Communications jacks were installed on the sides of the plotting table.  This system was essentially the same as that developed by the Royal Observer Corps in the United Kingdom early in World War II.  The key role of the Filter Center was to filter reports and make sense of the resulting air picture.  Low flying aircraft might be detected by a single post and then some time later by another post; filter center staff had to determine if the two reports were the same aircraft and estimate its continued track.  On the other hand, aircraft at medium or high altitudes could be seen and reported by several posts; the filter problem was to eliminate duplicate reports and provide the best estimate of the aircraft's location based on triangulation.  Photographs of Filter Centers in operation show staffs of approximately ten people receiving Aircraft Flash Messages and plotting tracks.

Once an aircraft track had been reported by Observer Posts and verified through the filtering process at the Filter Center, the Filter Center staff forwarded it to the Air Force aircraft control and warning system where reports were compared with radar track data. Ground Control Intercept stations then vectored air defense interceptors to intercept, identify, and, in wartime, destroy the aircraft if it was hostile.  

Comparison of surviving pictures of World War II Filter Centers operated by the Army's Aircraft Warning System and the Women's Air Raid Defense with those of 1950s Filter Centers show that the basic physical facilities, plotting tables, communications, etc. were generally similar to World War II practice.

Documented filter centers include:

Sources: Chenoweth, Candace A. and A. Kam Napier, Shuffleboard Pilots: The History of the Women's Air Raid Defense in Hawaii, 1941-1945, Honolulu, Hawaii, United States of America, Arizona Memorial Museum Association, 1991.  Cohen, Stan, V for Victory: America's Home Front During World War II, Missoula, Montana, United States of America, Pictorial Histories Publishing Co., 1991.  United States, Department of the Air Force, Aircraft Recognition for the Ground Observer, AF Manual 355-10, St. Louis, Missouri, United States of America, Von Hoffmann Press, 1955.  United States, Department of the Air Force, Ground Observers' Guide, AF Manual 50-12, St. Louis, Missouri, United States of America, Universal Printing Company, 1951.  Wood, Derek, Attack Warning Red: The Royal Observer Corps and the Defence of Britain 1925-1975, London, United Kingdom, MacDonald and Jane's, 1975. Cromwell, Bob, "Cold War Watchtower Delta Lima 3 - Green," location http://www.cromwell-intl.com/dl3g/Index.html, 2003.  Taylor, Eric, "Scotts Valley's Past: Skywatch," location http://www.cyber-times.com/newspapers/archives/00000227.shtml, 2002.

Entry 0127 - updated 8 October 2002