SIGINT Activity Designator A SIGINT Activity Designator ( or SIGAD ) identifies a signals intelligence ( SIGINT ) line of collection activity associated with a signals collection station , such as a base or a ship . For example , the SIGAD for Menwith Hill in the UK is USD1000 . SIGADs are used by the signals intelligence agencies of the United States , the United Kingdom , Canada , Australia and New Zealand . There are several thousand SIGADs including the substation SIGADs denoted with a trailing alpha character . Several dozen of these are significant . The leaked Boundless Informant reporting screenshot showed that it summarized 504 active SIGADs during a 30 day period in March 2013 . General Format . A SIGAD consists of five to eight case insensitive alphanumeric characters . It takes the general form of an alphanumeric designator normally composed of a two - or three - letter prefix followed by one to three numbers . Often a dash is used to separate the alphabetic and numeric characters in the primary part of the designator , but less frequently a space is used as a separator or the alphabetic and numeric characters are concatenated together . An additional alphabetic character can be added to denote a sub - designator for a subset of the primary unit , such as a detachment . Lastly , a numeric character can be added after the aforementioned alphabetic to provide for a sub - sub - designator . In the examples below an X represents an alphabetic character and an N represents a numeric character that are part of the primary designator . Likewise , an x represents an alphabetic character and an N represents a numeric character that are part of a sub - designator . Here are valid generalized examples of SIGADs : The first two characters show which country operates the particular SIGINT facility , which can be US for the United States , UK for the United Kingdom , CA for Canada , AU for Australia and NZ for New Zealand . A third letter shows what sort of staff runs the station . SIGADs beginning with US without a third letter are used for intercept facilities run by the NSA . PRISM SIGAD . One prominent SIGAD as of April 2013 is US - 984XN , with an unclassified codename of PRISM . It is " the number one source of raw intelligence used for NSA analytic reports " according to National Security Agency sources in a document leaked by Edward Snowden . The President ' s Daily Brief , an all - source intelligence product , cited SIGAD US - 984XN as a source in 1 , 477 items in 2012 . The U . S . government operates the PRISM electronic surveillance collection program through NSA ' s Special Source Operations , an alliance with trusted telecommunications providers . SIGADs for spy ships . The declassifed SIGAD for the USS Liberty was USN - 855 . The USS Liberty incident occurred on on 8 June 1967 , during the Six - Day War , when Israeli Air Force jet fighter aircraft and Israeli Navy motor torpedo boats attacked the USS Liberty in international waters . The USS Pueblo was a technical research ship , which was boarded and captured by North Korean forces on 23 January 1968 , in what is known as the Pueblo incident . The declassified SIGAD for the NSA Direct Support Unit ( DSU ) from the Naval Security Group ( NSG ) on the USS Pueblo patrol involved in the incident was USN - 467Y . The USS Pueblo , which officially remains a commissioned vessel of the United States Navy , is the only ship of the U . S . Navy currently being held captive . Vietnam War SIGADs . The following are the Vietnam War era declassified SIGADs from inside South Vietnam during the period of 1969 to 1975 : Some locations have multiple SIGADs due to different types of collection activities and / or collection at different times during the period . The SIGADs beginning with USA were operated by the United States Air Force ' s United States Air Force Security Service ( USAFSS ) . The SIGADs beginning with USM were operated by the United States Army ' s Army Security Agency ( ASA ) . Lastly , the SIGADs beginning with USN were operated by the United States Navy ' s Naval Security Group ( NAVSECGRU ) . All three of these units have been merged into other units or inactivated . The above list consists of the higher echelon SIGINT designator address . It does not include the numerous miscellaneous and temporary detachments , or Direction Finding stations belonging to major units or sites unless that detachment or site was the only one stationed in South Vietnam . Many of the " dets " were short - lived , often formed to support ongoing MACV operations or forward deployments of combat operational or maneuver units . These detachments usually were designated by a letter suffix attached to the higher echelon SIGINT address , such as " USM - 633J , " which was a detachment of the 372d Radio Research Company , USM - 633 , supporting the United States Army ' s 25th Infantry Division . Supporting Southeast Asia SIGADs . The following declassified SIGADs were highly relevant to the Vietnam Campaign , but were located in areas outside of South Vietnam in Southeast Asia . Again , detachments are not listed separately . In the case of the USS Maddox , naval Direct Support Units ( DSUs ) used the SIGAD USN - 467 as a generic designator for their missions . Each specific patrol received a letter suffix for its duration . The subsequent mission would receive the next letter in an alphabetic sequence . Thus , SIGAD USN - 467N specifically designates the USS Maddox patrol involved with the Gulf of Tonkin incident . Joint Base SIGADs . In November 2005 , the US Congress performed a fifth round of Base Realignment and Closure . This 2005 law also created twelve joint base s by merging adjacent installations belonging to different services in an effort to reduce costs and improve efficiencies . Joint bases with a primarily SIGINT mission have SIGADs that begin with USJ . A joint base would have a primary SIGAD in the general form of USJ - NNN , where NNN are numeric characters . An actual example is not given , since these units are currently active .