Effect of the turbulent atmosphere on the autocovariance function for a speckle field generated by a laser beam with random pointing error

Possibility of Hard-Target Lidar Detection of a Biogenic Volatile Organic Compound, nu-Pinene Gas, Over Forest Areas

The absorption spectrum of alpha-pinene gas, a biogenic volatile organic compound, was directly measured with a pulsed mid-infrared laser. The maximum absorption wavelength was found to be ~3.42 mum, and an absorption cross section of 4.8 x 10(-23) m(2) molec(-1) was obtained. A simple theoretical calculation with the measured spectral data showed that several hundreds of parts in 10(12) (ppt) of alpha-pinene gas in forest-mountain areas over a range of ~10 km were detectable by a long-path-averaged hard-target absorption lidar. Requirements for system development were also discussed.

Error reduction in laser remote sensing: combined effects of cross correlation and signal averaging

A systematic analysis is presented of the extent to which the accuracy of a differential-absorption lidar (DIAL) measurement may be improved by using the combined effects of signal averaging and temporal cross correlation. Previous studies which considered these effects separately are extended by incorporating both effects into a single analytical framework. In addition, experimental results involving lidar returns from a diffusely reflecting target using a dual-CO2 laser DIAL system with both heterodyne and direct detection are presented. These results are shown to be in good agreement with the theoretical analysis and help establish the limits of accuracy achievable under various experimental conditions.

Dual-wavelength correlation measurements with an airborne pulsed carbon dioxide lidar system

Dual-wavelength correlation data are presented for ground-level and airborne measurements of topographic and ocean backscatter near the 9.5-microm wavelength. Data were acquired with a pulsed carbon dioxide, dual-laser, dualwavelength lidar system. The best system performance gave a dual-wavelength uncorrelated noise component of 2.8% and 9% standard deviation for ground-based and airborne data, respectively. This sets one limit on lidar measurement accuracy with a single pulse pair.

Boresight error in the conical scan method of autoboresighting a laser beam on a specular point-target

While autoboresighting a laser beam on a specular point-target, the automatic tracking is set to the highest degree of precision by accurately estimating the beam position on the target. A serious degradation in the prediction of miss-distance in such autoboresightings (for example, by conical scan tracking) would arise if boresight error is encountered due to various sources of statistical fluctuations in the system. Typically, beam-intensity fluctuations (resulting from beam propagation through a turbulent atmospheric medium), imperfections in the trans-receive optical scanners, random variations in the nutation-drive signal, etc., are the main sources of boresight errors in the tracker system. Presently, for a conical scan tracker which uses a laser beam on a specular point-target, expressions for the mean and variance of boresight error are derived by considering various possible random parameters in the system concerned.

Limitations of signal averaging due to temporal correlation in laser remote-sensing measurements

Laser remote sensing involves the measurement of laser-beam transmission through the atmosphere and is subject to uncertainties caused by strong fluctuations due primarily to speckle, glint, and atmospheric-turbulence effects. These uncertainties are generally reduced by taking average values of increasing numbers of measurements. An experiment was carried out to directly measure the effect of signal averaging on back-scattered laser return signals from a diffusely reflecting target using a direct-detection differential-absorption lidar (DIAL) system. The improvement in accuracy obtained by averaging over increasing numbers of data points was found to be smaller than that predicted for independent measurements. The experimental results are shown to be in excellent agreement with a theoretical analysis which considers the effect of temporal correlation. The analysis indicates that small but long-term temporal correlation severely limits the improvement available through signal averaging.