C. L. Korb

Theory of the double-edge molecular technique for Doppler lidar wind measurement

The theory of the double-edge lidar technique for measuring the wind with molecular backscatter is described. Two high-spectral-resolution edge filters are located in the wings of the Rayleigh-Brillouin profile. This doubles the signal change per unit Doppler shift, the sensitivity, and improves measurement accuracy relative to the single-edge technique by nearly a factor of 2. The use of a crossover region where the sensitivity of a molecular- and an aerosol-based measurement is equal is described. Use of this region desensitizes the molecular measurement to the effects of aerosol scattering over a velocity range of +/-100 m/s. We give methods for correcting short-term, shot-to-shot, frequency jitter and drift with a laser reference frequency measurement and methods for long-term frequency correction with a servo control system. The effects of Rayleigh-Brillouin scattering on the measurement are shown to be significant and are included in the analysis. Simulations for a conical scanning satellite-based lidar at 355 nm show an accuracy of 2-3 m/s for altitudes of 2-15 km for a 1-km vertical resolution, a satellite altitude of 400 km, and a 200 km x 200 km spatial resolution.

Edge technique Doppler lidar wind measurements with high vertical resolution

We have developed a Doppler lidar system using the edge technique and have made atmospheric lidar wind measurements. Line-of-sight wind profiles with a vertical resolution of 22 m have a standard deviation of 0.40 m /s for a ten-shot average. Day and night lidar measurements of the vector wind have been made for altitudes from 200 to 2000 m. We validated the lidar measurements by comparing them with independent rawinsonde and pilot balloon measurements of wind speed and direction. Good agreement was obtained. The instrumental noise for these data is 0.11 m /s for a 500-shot average, which is in good agreement with the observed minimum value of the standard deviation for the atmospheric measurements. The average standard deviation over 30 mins varied from 1.16 to 0.25 m /s for day and night, respectively. High spatial and temporal resolution lidar profiles of line-of-sight winds clearly show wind shear and turbulent features at the 1 -2-m /s level with a high signal-to-noise ratio and demonstrate the potential of the edge-technique lidar for studying turbulent processes and atmospheric dynamics.

Edge technique for high-accuracy Doppler velocimetry

The edge technique has been used in simple laboratory experiments to demonstrate velocity measurements with an experimental error, standard deviation, as small as 12 cm/s, which represents a Doppler-shift measurement accuracy of 8 parts in 10(10) of the laser frequency. An edge filter with a spectral width 140 times larger than the measurement accuracy achieved is used. The measurements are made in the presence of short-term frequency drifts equivalent to velocities of 5 to 10 m/s, which are eliminated by the differential frequency measurement used in the edge technique. Long-term frequency drifts are compensated for by servo locking the edge to the laser frequency. High accuracy is achieved for a range of locations on the edge from 0.33 to 4.5 fringe half-widths (half-width at half-maximum), a dynamic range greater than 500 times the measurement accuracy.

Differential absorption lidar technique for measurement of the atmospheric pressure profile

A new two-wavelength lidar technique for remotely measuring the pressure profile using the trough absorption region between two strong lines in the oxygen A band is described. The theory of integrated vertical path, differential ranging, and horizontal path pressure measurements is given with methods to desensitize and correct for temperature effects. The properties of absorption troughs are described and shown to reduce errors due to laser frequency jitter by up to 2 orders of magnitude. A general analysis, including laser bandwidth effects, demonstrates that pressure measurements with an integrated vertical path technique are typically fifty times more accurate than with a differential ranging technique. Simulations show 0.1-0.3% accuracy for ground and Shuttle-based pressure profile and surface pressure experiments.

Frequency-doubled CO2 lidar measurement and diode laser spectroscopy of atmospheric CO2

A lidar instrument based on pulsed frequency-doubled carbon-dioxide lasers has been used at 4.88 μm for remote sensing of atmospheric carbon dioxide. A tunable-diode laser spectrometer provided the high-resolution spectroscopic data on carbon-dioxide line strength and line broadening needed for an accurate differential absorption measurement. Initial field measurements are presented, and instrument improvements necessary for accurate carbon dioxide measurement are discussed.

Laser remote sensing of atmospheric temperature by observing resonant absorption of oxygen

Measurement of atmospheric temperature through the monitoring of laser energy absorption at the center of an O(2) resonant absorption line near 770 nm has been demonstrated using a dual frequency system. The average temperature of a 1-km path can be determined to better than 1.0 degrees C with a noise level of 0.3 degrees C. An iterative algebraic expression for determining temperature from the measured absorption was developed and shown to be applicable in the troposphere. The effects of pressure and humidity on temperature determination are clear from the algorithm and found to be small near the earths surface.

Airborne and ground based lidar measurements of the atmospheric pressure profile

The first high accuracy remote measurements of the atmospheric pressure profile have been made. The measurements were made with a differential absorption lidar system that utilizes tunable alexandrite lasers. The absorption in the trough between two lines in the oxygen A-band near 760 nm was used for probing the atmosphere. Measurements of the 2-D structure of the pressure field were made in the troposphere from an aircraft looking down. Also, measurements of the 1-D structure were made from the ground looking up. Typical pressure accuracies for the aircraft measurements were 1.5-2 mbar with a 30-m vertical resolution and a 100-shot average (20 s), which corresponds to a 2-km horizontal resolution. Typical accuracies for the upward viewing ground based measurements were 2.0 mbar for a 30-m resolution and a 100-shot average.

Spaceborne lidar wind measurement with the edge technique

The edge technique can provide high accuracy spaceborne wind measurements as well as high spatial resolution, high accuracy ground and airborne measurements. Global wind measurements can be made with the edge technique from space with an accuracy of 1 m/s and a vertical resolution as high as 150 m in the boundary layer and 1 km through the troposphere. The edge technique can also be used for ground and airborne measurements with a spatial resolution and accuracy as high as 15 m and 20 cm/s. We have recently demonstrated this capability and present these measurements in this paper.

Infrared lidar windshear detection for commercial aircraft and the edge technique, a new method for atmospheric wind measurement

National attention has focused on the critical problem of detecting and avoiding windshear since the crash on August 2, 1985, of a Lockheed L-1011 at Dallas/Fort Worth International Airport. As part of The NASA/FAA National Integrated Windshear Program, we have defined a measurable windshear hazard index that can be remotely sensed from an aircraft, to give the pilot information about the wind conditions he will experience at some later time if he continues along the present flight path. Our technology analysis and end-to-end performance simulation, which measured signal-to-noise ratios and resulting wind velocity errors for competing coherent lidar systems, showed that a Ho:YAG lidar at a wavelength of 2.1 μm and a CO2 lidar at 10.6 m can give the pilot information about the line-of-sight component of a windshear threat in a region extending from his present position to 2 to 4 km in front of the aircraft. This constitutes a warning time of 20 to 40 s, even under conditions of moderately heavy precipitation. Using these results, a Coherent Lidar Airborne Shear Sensor (CLASS), using a Q-switched CO2 laser at 10.6 μm, is being designed and developed for flight evaluation in early 1992. The edge technique is a powerful new method for the measurement of small frequency shifts which allows high accuracy measurement of atmospheric winds (0.2 to 1 m/sec) with high vertical resolution (10 meters) using currently available technology.

Remote Sensing with a Tunable Alexandrite Laser Transmitter

In this paper we describe a high resolution tunable Alexandrite laser system which we have used to make differential absorption lidar measurements of the atmospheric pressure profile. We also report on measurements of the spectral purity and line shape of the laser emission.