My wife discovered George Washington University's National Security Archive. The Intelligence section is interesting as it contains a declassified copy of United States Signals Intelligence Directives, specifically USSID 18. From the description of the documents:
"The version of USSID 18 currently in force was issued in July 1993 and "'prescribes policies and procedures and assigns responsibilities to ensure that the missions and functions of the United States SIGINT System (USSS) are conducted in a manner that safeguards the constitutional rights of U.S. persons.' Section 4 (Collection, pp.2-6) specifies the circumstances under which U.S. SIGINT activities may intercept communications of or about U.S. persons, as well as the authorities of the Foreign Intelligence Surveillance Court, the Attorney General, and the Director of NSA to approve the collection of such information."
When I was a lieutenant at the Air Intelligence Agency, we used USSIDs to strictly guide our collection efforts.