This International Standard describes the establishment and operation of an ultrasonic (transit-time) gauging station for the continuous measurement of discharge in a river, an open channel or a closed conduit. It also describes the basic principles on which the method is based, the operation and performance of associated instrumentation and procedures for commissioning. It is limited to the transit time of ultrasonic pulses technique, and is not applicable to systems that make use of the Doppler shift or correlation or level-to-flow techniques. This International Standard is not applicable to measurement in rivers with ice.