C. Laurence Korb

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

The theory of the double-edge technique is described by a generalized formulation that substantially extends the capabilities of the edge technique. It uses two edges with opposite slopes located about the laser frequency. This doubles the signal change for a given Doppler shift and yields a factor of 1.6 improvement in the measurement accuracy compared with the single-edge technique. Use of two high-resolution edge filters reduces the effects of Rayleigh scattering on the measurement by as much as an order of magnitude and allows the signal-to-noise ratio to be substantially improved in areas of low aerosol backscatter. We describe a method that allows the Rayleigh and aerosol components of the signal to be independently determined. The effects of Rayleigh scattering are then subtracted from the measurement, and we show that the correction process does not significantly increase the measurement noise for Rayleigh-to-aerosol ratios as high as 10. We show that for small Doppler shifts a measurement accuracy of 0.4 m/s can be obtained for 5000 detected photons, 1.2 m/s for 1000 detected photons, and 3.7 m/s for 50 detected photons for a Rayleigh-to-aerosol ratio of 5. Methods for increasing the dynamic range to more than +/-100 m/s are given.

Gated photomultiplier response characterization for DIAL measurements

The characteristics of various detector responses are studied to understand the cause of various systematic biases and to minimize these undesirable effects in measurements of transient signals with large dynamic range. We quantitatively evaluated signal induced bias, gain variation, and the linearity of commonly used gated photomultipliers in the current integrating mode. Analysis of the results indicates that impurity ions inside the photomultiplier tube are the source of the signal induced bias and gain variation. Two different photomultiplier tubes used in this study show significant differences in the magnitude and decay behavior of signal induced bias. We found it can be minimized by using an external amplifier to reduce PMT gain, and by applying a low potential between the cathode and first dynode. The linearity of a photomultiplier tube is also studied over a large dynamic range of input intensities employing a new technique which does not require an absolute calibration. The result of this study shows that the photomultiplier response is linear only for a limited input intensity range below a certain anode current.

A lidar system for measuring atmospheric pressure and temperature profiles

The design and operation of a differential absorption lidar (LIght Detection And Ranging) system capable of remotely measuring the vertical structure of tropospheric pressure and temperature are described. The measurements are based on the absorption by atmospheric oxygen of the spectrally narrowband output of two pulsed alexandrite lasers. Detailed laser output spectral characteristics, which are critical to successful lidar measurements, are presented. Spectral linewidths of 0.026 and 0.018 cm−1 for the lasers were measured with over 99.99% of the energy contained in three longitudinal modes.

Effective frequency technique for finite spectral bandwidth effects.

A technique that uses a single effective frequency to represent the effects of finite spectral bandwidth for active and passive measurements centered on an absorption line, a trough region, or a slowly varying spectral feature is described. For Gaussian and rectangular instrumental line shapes, the effective frequency is shown to have a simple form that depends only on the instrumental line shape and bandwidth and not on the absorption line profile. The technique is applicable to a large class of active and passive measurements and simulations in both the laboratory and the atmosphere. Simulations show that the technique yields accuracies better than 0.1% for bandwidths less than 0.2 times the atmospheric linewidth for a rectangular line shape or better than 0.2% for a Gaussian.

Atmospheric Pressure and Temperature Profiling Using Near IR Differential Absorption Lidar

This paper describes differential absorption lidar techniques for remotely measuring the atmospheric temperature and pressure profile, surface pressure, and cloud top pressure-height. The approach to the pressure measurements1 utilizes a high resolution measurement of absorption in the wings of lines in the oxygen A band where the absorption is highly pressure sensitive throug] the mechanism of collisional line broadening. The approach for temperature2 uses a measurement of the absorption at the center of a selected line in the oxygen A band which originates from a quantum state with high ground state energy. The population of the state depends strongly on temperature through the Boltzmann term which produces a highly sensitive temperature determination. Oxygen is used for these measurements since it is uniformly mixed in the atmosphere, which greatly simplifies the measurement approach, and has lines with appropriate strength and energy levels. Also, it is located in a spectral region (760 nm) easily accessible using tunable solid state and dye lasers and efficient detectors.

High Accuracy Atmospheric Wind Field Measurements with an Edge Technique Lidar

We have developed a ground based lidar system using the edge technique and demonstrated wind measurements. Wind profiles with a vertical resolution of 22 m have a standard deviation of 0.4 m/s for a 10 shot average.

Double-Edge Molecular Technique for Doppler Lidar Wind Measurement

The double-edge lidar technique for measuring the wind based upon using molecular backscatter is described. The technique uses two high spectral resolution edge filters which are located in the wings of the Rayleigh-Brillouin profile. This doubles the signal change per unit Doppler shift, the sensitivity, and gives nearly a factor of two improvement in measurement accuracy relative to the single edge technique. The use of a crossover region is described where the sensitivity of a molecular and aerosol-based measurement are equal. This desensitizes the molecular measurement to the effects of aerosol scattering over a frequency range of plus or minus 100 m/s. We give methods for correcting for short- term, shot to shot, frequency jitter and drift using a laser reference frequency measurement and methods for long-term frequency correction using 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 to 15 km for a 1 km vertical resolution, a satellite altitude of 400 km and a 200 km X 200 km spatial resolution. Results for recent wind measurements, which show an accuracy of 1 m/s up to an altitude of 10 km, are given.