In 1974, Turkey invaded Cyprus in response to an attempted coup against Archbishop Makarios, the President of the Cypriot Republic. Cyprus was home to Britain's Sovereign Base Areas which amounted to some ninety-nine square miles of military facilities and these were now threatened. The most important British assets on the island were sigint collection stations, including 9 Signals Regiment at Ayios Nikolaos near Famagusta. This station was in fact a mixed unit, employing personnel from all three services and GCHQ civilians. There was also an American NSA site at Yarallakos near Nicosia and elaborate British and American elint radars and warning systems elsewhere. Invading Turkish forces captured the city of Famagusta and their tanks made their way to the edge of the British sigint base at Ayios Nikolaos, firing several shells into the base area. The commander, Colonel Hugh Johnstone, walked out and calmly asked the Turks to direct their armoured vehicles elsewhere.
As well as the traditional HF interception sites Cyprus was also important for elint stations and over the horizon radar that watched Soviet aircraft and missile testing. Treasury files in the TNA show that Britain developed an highly successful elint system called Project Zinnia in 1959 and then launched an operational version called Project Sandra on Cyprus in 1961. Project Sandra was gradually eclipsed by a more powerful elint system called "Cobra Shoe", run jointly by the British and the Americans on Cyprus which had the additional function of providing war warning.
The invasion of 1974 had wider implications for sigint in the region. Washington placed an arms embargo on Turkey as a result of the invasion and in reponse Ankara closed many of the NSA's large and expensive sigint bases in Turkey, and so for a period the alternative sigint bases on Cyprus became correspondingly more valuable. Their significance was further increased in 1979 when the overthrow of the Shah of Iran displaced other British and American listening stations. Turkey relented in 1980 and permitted many of the NSA listening stations to reopen.
The relevant TNA files include:
Turkish advance on the sigint bases: AIR 8/2628, DEFE 24/578
Projects Zinnia, Sandra and Cobra Shoe: AVIA 6/17569, DEFE 44/93, T 225/2198
Turkish closure of US bases: AIR 8/2747