The Oboe blind-bombing system probably provides the most accurate radar range-measuring system to date. The system has been used to measure the velocity of propagation of centimetre waves over sea and land paths at low level, in addition to paths between ground and aircraft at 10,000, 20,000 and 30,000 ft. The paper describes the experiments over a sea path between two shore-based stations each situated at a height of about 400 ft. The results give a value of 186,218±15 miles/sec for 9-cm waves over a 40-mile path. A subsidiary check over a 30-mile path gave a figure of 186,216.5±17 miles/sec. It is concluded that the final result is 186,217±16 miles/sec. Using a conversion factor of 1 m = 3.280843 ft, this is equivalent to 299,687±25 km/sec.