THE Marine Department is prosecuting a Hong Kong master for speeding at sea in the first case to be brought using evidence from a radar gun. The Attorney-General's Office cleared the use of the British-made device in September after deciding the data gathered using microwaves was admissible in court. The master, who has not been named, was travelling near Tuen Mun when Marine officers used the $20,000 unit, the only one in use, to measure the boat's speed. Marine officer Eric Lau Wing-win said: 'We will see how the evidence is processed in the courts and consider whether to purchase more or not.' Mr Lau refused to disclose the range of the radar device, but said trials involved it being used both from land and at sea. News of the prosecution came as the Marine Department announced its latest campaign, beginning tomorrow, aimed at clamping down on mariners who break the harbour rules. The initiative will concentrate on the Northern Fairway, one of the busiest areas of the harbour for barges, container vessels and high-speed ferries. It runs from a point near Yau Ma Tei to Kwai Chung container terminal.