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- Author or Editor: R. L. Schwiesow x
- Journal of Atmospheric and Oceanic Technology x
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Abstract
Misalignment of pyranometers used for airborne measurements can lead to serious errors in the determination of downwelling radiation flux. The magnitude of these errors depends strongly on the elevation angle of the sun. This note presents an iterative numerical procedure for determining the angles of misalignment of upward-facing pyranometers. Deviations in pitch and roll of the instrument with respect to the aircraft's inertial navigation system (INS) must be added to the pitch and roll angles measured by the INS before the radiometric data are corrected for the attitude of the aircraft. For successful determination of the two angles of deviation, a calibration flight must be performed in which the aircraft flies in at least three directions at the same altitude under clear skies and above any haze.
Abstract
Misalignment of pyranometers used for airborne measurements can lead to serious errors in the determination of downwelling radiation flux. The magnitude of these errors depends strongly on the elevation angle of the sun. This note presents an iterative numerical procedure for determining the angles of misalignment of upward-facing pyranometers. Deviations in pitch and roll of the instrument with respect to the aircraft's inertial navigation system (INS) must be added to the pitch and roll angles measured by the INS before the radiometric data are corrected for the attitude of the aircraft. For successful determination of the two angles of deviation, a calibration flight must be performed in which the aircraft flies in at least three directions at the same altitude under clear skies and above any haze.
Abstract
The National Center for Atmospheric Research Airborne Infrared Lidar System is being developed for Doppler wind measurements using heterodyne detection. Its design is based on a pulsed CO2 laser transmitter and a single continuous-wave CO2 laser as local oscillator and injection seed for the pulsed laser. Research during the system development has shown the workability of the two-laser concept, the need for optical isolation between the lasers, and advantages of averaging complex autocorrelation functions rather than velocity estimates. The authors describe the system and show calculated performance for wind measurements.
Abstract
The National Center for Atmospheric Research Airborne Infrared Lidar System is being developed for Doppler wind measurements using heterodyne detection. Its design is based on a pulsed CO2 laser transmitter and a single continuous-wave CO2 laser as local oscillator and injection seed for the pulsed laser. Research during the system development has shown the workability of the two-laser concept, the need for optical isolation between the lasers, and advantages of averaging complex autocorrelation functions rather than velocity estimates. The authors describe the system and show calculated performance for wind measurements.
Abstract
Continuous-wave (CW) Doppler lidar measurements of wind magnitude and direction that are based on radial velocity data on only a part of a full azimuth circle compare favorably with measurements based on a full circle. Winds were measured over an altitude range of 750 m. For example, the rms difference between 76 wind data pairs at various altitudes, taken from a full circle and from a ¼-circle sector is 0.43 m s−1 in magnitude (correlation coefficient 0.98) and 4.2° in direction, even when only 12 s of measurement time is used for the ¼-circle sector. Increased integration time leads to an even closer comparison. Useful velocity estimates can be obtained from sector scans as small as ⅙ of a circle when a weighted least-squares fitting program is used to analyze the radial velocity versus azimuth data. Results from a two-point scan technique compare less favorably with the full-scan results than do results from a sector-scan technique. A scan employing a π/2, two-point azimuth difference results in an rms difference of 0.78 m s−1 (correlation coefficient 0.95) for 2 s of measurement time when compared with a full circle scan. We conclude that even if data are available or of interest over only part of an azimuth circle, good wind estimates are still possible.
Abstract
Continuous-wave (CW) Doppler lidar measurements of wind magnitude and direction that are based on radial velocity data on only a part of a full azimuth circle compare favorably with measurements based on a full circle. Winds were measured over an altitude range of 750 m. For example, the rms difference between 76 wind data pairs at various altitudes, taken from a full circle and from a ¼-circle sector is 0.43 m s−1 in magnitude (correlation coefficient 0.98) and 4.2° in direction, even when only 12 s of measurement time is used for the ¼-circle sector. Increased integration time leads to an even closer comparison. Useful velocity estimates can be obtained from sector scans as small as ⅙ of a circle when a weighted least-squares fitting program is used to analyze the radial velocity versus azimuth data. Results from a two-point scan technique compare less favorably with the full-scan results than do results from a sector-scan technique. A scan employing a π/2, two-point azimuth difference results in an rms difference of 0.78 m s−1 (correlation coefficient 0.95) for 2 s of measurement time when compared with a full circle scan. We conclude that even if data are available or of interest over only part of an azimuth circle, good wind estimates are still possible.
Abstract
Measurement of air motion relative to an aircraft by a conically scanned optical Doppler technique has advantages over measurements with conventional gust probes for many applications. Advantages of the laser air motion sensing technique described here include calibration based on physical constants rather than experiment for an accurate measurement of mean wind, freedom from flow distortion effects on turbulence measurements, all-weather performance, reduction in error from mechanical vibrations and ability to measure vertical wind shear. An experiment comparing a single-component laser velocimeter and a differential pressure gust probe shows that the optical approach measures the turbulence spectrum accurately at frequencies up to 10 Hz and that the signal-to-noise ratio is not a limiting factor. In addition, we have observed the effect of spectral skewing caused by airflow distortion in cloud.
Abstract
Measurement of air motion relative to an aircraft by a conically scanned optical Doppler technique has advantages over measurements with conventional gust probes for many applications. Advantages of the laser air motion sensing technique described here include calibration based on physical constants rather than experiment for an accurate measurement of mean wind, freedom from flow distortion effects on turbulence measurements, all-weather performance, reduction in error from mechanical vibrations and ability to measure vertical wind shear. An experiment comparing a single-component laser velocimeter and a differential pressure gust probe shows that the optical approach measures the turbulence spectrum accurately at frequencies up to 10 Hz and that the signal-to-noise ratio is not a limiting factor. In addition, we have observed the effect of spectral skewing caused by airflow distortion in cloud.
