Effects of C(2)(n) on a vertically pointing diffraction-limited lidar.

Examples of different C(2)(n) profiles lead to substantially different profiles of lidar image radius in a study of the calculated performance of a diffraction-limited lidar system. The differences in image radii indicate the usefulness of a ground-based lidar for measurement of C(2)(n) profiles used to predict optical propagation phenomena. We conclude that the overall strength of the C(2)(n) profile and its general altitude dependence can be determined from inspection of the image radius profile. Approximate calculations of available and required SNRs show that a lidar with a telescope aperture of 0.5 m and a few pulses of ~1-J total transmitted energy will provide useful image radius data to an altitude of 20 km under daytime conditions. The weighting function for sensitivity of the fractional increase in image radius to changes of C(2)(n) on a logarithmic altitude scale is approximately constant with height.

Offset local oscillator for cw laser Doppler anemometry.

Using two CO(2) lasers with one frequency-locked to the other, we have developed a simple heterodyne cw Doppler backscatter anemometer for remote sensing. The configuration as tested in a lidar application requires no additional optics or detector over a homodyne setup. Spectra of the vertical wind show an example of the results available from a simple heterodyne Doppler lidar with a threshold of 0.25 m sec(-1) and an uncertainty of +/-10% in the velocity estimate.

Temperature profiling by Rayleigh-scattering lidar.

A lidar with high spectral resolution can be used to measure atmospheric temperature profiles according to conservative performance calculations. The technique analyzed relies on determining the temperature-dependent Rayleigh-scattering linewidth with two stabilized Michelson interferometers in parallel. From ratios of four integrated flux values from two photomultipliers, one can determine temperature profiles within a 1 K standard deviation to 5 km with 50-m height resolution in 1(1/4) min using a laser of 1-W average power and a telescope of 30-cm diam.

Calibration of a cw infrared Doppler lidar.

A moving scattering target used as a transfer standard allows absolute calibration of the response of a cw Doppler lidar to an atmospheric target. The lidar in this study operated at a 10.6-microm wavelength. Consideration of the distribution of radiant energy density near the focus of the lidar transceiver permits measurement of a backscatter coefficient from a distributed array of scatterers, such as atmospheric aerosols, based on the diffuse reflectance of the surface of the transfer standard. The minimum detectable signal for our system with a 5-sec averaging time corresponds to a backscatter coefficient of 2.4 x 10(-12) m (-1) sr (-1) +/- 2.5 dB, which is ~ 9 dB greater than the theoretical threshold. Calibration shows that the lidar response is 5+/-1 dB less than the ideal limit for signal powers well above the minimum detectable signal.

Atmospheric refractive effects on coherent lidar performance at 10.6 microm.

The aperture diameter over which a lidar transceiver collects backscattered signal that can be coherently detected depends on refractive index inhomogeneities in the atmosphere. We have studied the change in coherently detected signal with change in aperture for a cw (focusing) lidar operating at 10.6-microm wavelength over a horizontal path 2 m above the ground at target ranges between 0.6 km and 2.6 km. We conclude that for average daytime conditions at a range of 1 km, for example, a transceiver aperture diameter of 27 cm gives approximately 65% of the coherent signal expected from an aperture of twice that diameter.

Coherent differential Doppler measurements of transverse velocity at a remote point.

We consider the analysis and results of a laser remote velocity sensing experiment in which both the longitudinal and transverse velocity components at one location were simultaneously measured from a range of 33 m. Fully coherent homodyne detection was used with a test target that simulates many aspects of distributed atmospheric aerosol targets.