The union between these colonies began to take shape on 12 September 1922, when Ascension Island became a dependency of Saint Helena by letters patent. Lightly populated Tristan da Cunha, even today little more than an outpost with a population of less than three hundred, followed suit on 12 January 1938. The three island groups shared this constitutional relationship until 1 September 2009, when the dependencies were raised to equal status with Saint Helena and the territory changed its name from "Saint Helena and Dependencies" to "Saint Helena, Ascension and Tristan da Cunha".During the years of the Battle of the Atlantic of World War II and the following several years of struggle against the u-boat menace in the Atlantic, both St. Helena and Ascension Island were used to base patrolling anti-surface-commerce-raider and anti-submarine warfare (ASW) forces by the United Nations (allies) against the axis powers' naval units. Initially long range naval patrol flying boats were used in the effort, and later in the war during the struggle to improve air coverage over the commercially important sea lanes (SLOC), air strips were built to support land based aircraft which supplied, augmented and complemented the PBY Catalina patrol planes in the vitally important ASW mission. The United States and Great Britain still jointly operate the airfield on Ascension Island, which also serves as a space-based communications and navigation nexus and hub (Ground station). One of only five Global Positioning Satellite (GPS) system ground stations is located there.