Calibration of Pressure Measurements on Research Aircraft using a Laser Air Motion Sensor

A recently developed laser air motion sensor has been deployed on two research aircraft operated by the National Center for Atmospheric Research. The single-beam version of the sensor measures the airspeed of the aircraft by sensing the Doppler shift in light back-scattered by particles located ahead of the aircraft. The laser beam is focused 15 to 30 m ahead of the aircraft so that the airspeed is sensed well outside the region where the aircraft causes distortion of the airflow. With an accurate measurement of airspeed, it is possible to calculate the expected dynamic pressure, and comparing this to the conventional measurement provides a calibration of that conventional measurement. It is argued that this provides a calibration also of the pressure measurement because the total pressure (dynamic plus ambient) is measured accurately by a pitot tube. The result is calibration applicable in high-speed flight (>200 m/s) with an estimated standard error of less than 0.3 hPa and precision of 0.1 hPa under normal flight conditions.

Temperature measurements can also be calibrated using the pressure obtained in this way because the precision is sufficient to permit comparisons of the measurements to predictions from the hydrostatic equation. This is made possible by accurate measurements of altitude available from modern Global Positioning System receivers. The result is calibration of temperature measurements with an estimated standard error of about 0.2 degrees C.

The result has been reduction in uncertainties in airspeed, pressure, and temperature by factors of about five.