The accuracy of winds measured in flight using a doppler navigation system has been investigated by comparison with simultaneous measurements from a Meteorological Office radar wind-finding station and also by flying different tracks using a modified multiple-drift technique. The errors of wind measurement arise from errors in the ground-speed vector as measured by the doppler navigator and also from errors in the true airspeed vector as obtained from the readings of the airspeed indicator, altimeter, outside air thermometer and compass. The trials, which were made at various altitudes to 48,000 ft. in a Canberra aircraft of the Meteorological Research Flight, showed that, providing suitable in-flight calibrations are carried out, the error in the vector wind can be reduced to about 5 kt. but that without these calibrations it may be 10 kt. or more.