The flight of a constant-level balloon, the trajectory of which is determined by ground tracking equipment. Thus, it is a form of upper-air, quasi-horizontal “sounding. ” The most usual observations are of successive positions of the balloon located by radio direction-finding or radio-navigation equipment, giving trajectory, wind speed, and wind direction. Instrumentation can also be added to the balloon to sense and transmit pressure, temperature, relative humidity, and other meteorological elements.