R. M. Hardesty

Characteristics of coherent lidar returns from calibration targets and aerosols

A CO(2) heterodyne lidar system and high speed digitizer were used to examine properties of returns from disk and belt-type calibration targets and atmospheric aerosols. Amplitude statistics of the returns from the targets examined corresponded to those of the Rayleigh phasor predicted by theory. Returns from a belt sander fluctuated at a much slower rate than those from the disks or aerosols, requiring longer averaging times for accurate power measurement. At very close focal lengths returns from single large particles often domier nated the backscattered aerosol signal.

Coherent DIAL measurement of range-resolved water vapor concentration

A pulsed coherent CO2 lidar was employed to measure water vapor profiles by the differential absorption lidar (DIAL) technique. Measurements were obtained to ranges of 10 km along horizontal paths and 6 km when the lidar beam angle was elevated. Comparisons with nearby rawinsonde soundings showed fairly good agreement, although a tendency for the lidar to overestimate relative to the sonde was observed. Uncertainties in the individual measurements were attributable primarily to speckle, quantum noise, and atmospheric nonstationarities. The DIAL data set was also used to obtain radial wind velocity measurements at ranges beyond the maximum range of the concentration measurement.

Receiving efficiency of monostatic pulsed coherent lidars. 1: Theory

The receiving efficiency eta as a function of range z is investigated for pulsed coherent lidars using a theory that relates eta(z) to the transmitted laser intensity and the point-source receiving efficiency eta(s)(r,z). The latter can be calculated either by a forward method, or by a backward method that employs the back-propagated local oscillator (BPLO) approach. The BPLO method is efficient and accurate provided that cascaded diffraction effects inside the lidar system are properly taken into account. The theory is applied to the ideal case to examine the optimization of the system when both transmitted and BPLO fields at the antenna are Gaussian, including optimum telescope aperture.

Detection techniques for validating Doppler estimates in heterodyne lidar

We investigate the ability of detection techniques based on the likelihood ratio to discriminate between heterodyne lidar Doppler estimates at low signal levels using examples generated by simulation. The distinction between estimates that are regarded as acceptable and as spurious is based on the Cramer-Rao lower bound. The conditional false alarm probability, which ordinarily describes recording detection of a signal when none is present, is then found to be an approximate upper bound on the probability of selection of a spurious estimate. The method is superior theoretically to similar techniques based on detection functions other than the likelihood ratio. The likelihood ratio also provides a basis for reprocessing rejected data in the light of contextual information provided by those estimates that are accepted.

Calibration of coherent lidar targets

We determine the backscatter reflectance (more properly the bidirectional reflectance distribution function at a 45 degrees angle of incidence and observation) for circularly polarized radiation of lambda = 10.6 microm for 120-grit aluminum oxide sandcloth (1.5 x 10(-2) sr(-1)) and 400-grit silicon carbide sandpaper (1.1 x 10(-2) sr(-1)) with respect to sublimed flowers of sulfur (1.8 x 10(-1) sr(-1)). The effect of range and the atmospheric effect of turbulence-induced beam wander are discussed for both rotating disks and linearly translated belts. The advantages of large slowly rotating disks for field calibrations are presented.

Technique for correcting effects of long CO 2 laser pulses in aerosol-backscattered coherent lidar returns

The convolution effect in aerosol backscattered CO(2) coherent lidar returns caused by the long tail of the laser pulse is analyzed by modifying the original lidar equation and introducing a correction function C(r)(R). The characteristics of the correction function and its effect on differential absorption lidar water vapor measurements are investigated for coherent lidars. A deconvolution technique is developed consisting of a leastsquares fitting and iteration procedure for retrieval of the mean value of the atmospheric backscattering coefficient beta and a reverse filtering procedure for estimating the fluctuation components of the beta profile. Data obtained with the WPL coherent CO(2) lidar are reanalyzed using the above method, giving improved estimates of the structures and the mean water vapor content.

Receiving efficiency of monostatic pulsed coherent lidars. 2: Applications

Using the theory developed in Part 1, the receiving efficiency as a function of range, eta(z), is calculated under different conditions for the NOAA/ERL/Wave Propagation Laboratory CO(2) Doppler lidar. Theoretical analyses, numerical calculations, and experimental measurements are carried out to quantify the sensitivity of eta(z) to transmitted laser beam quality, telescope focal setting, telescope power, scanner astigmatism, LO beam divergence, and system misalignment. These results bring insight to the design of practical coherent lidar systems.

Refractive-index structure parameter in the planetary boundary layer: comparison of measurements taken with a 10.6-µm coherent lidar, a 0.9-µm scintillometer, and in situ sensors

An optical technique is described that determines the path-averaged value of a refractive-index structure parameter at 10.6 microm by use of a pulsed coherent CO(2) lidar in direct detection and hard-target returns. The lidar measurements are compared with measurements taken by a 0.9-microm scintillometer and temperature probe (with humidity corrections). The experimental results show good agreement for C(n)(2) >or= (-14) m(-2/3). With respect to practical applications the new technique permits C(n)(2) lidar measurements in a neutral meteorological situation to an unstably stratified convective boundary layer over long ranges (1 km or more).

Nonlinear Kalman filtering techniques for incoherent backscatter lidar: return power and log power estimation.

Recursive estimation of nonlinear functions of the return power in a lidar system entails use of a nonlinear filter. This also permits processing of returns in the presence of multiplicative noise (speckle). The use of the extended Kalman filter is assessed here for estimation of return power, log power, and speckle noise (which is regarded as a system rather than a measurement component), using coherent lidar returns and tested with simulated data. Reiterative processing of data samples using system models comprising a random walk signal together with an uncorrelated speckle term leads to self-consistent estimation of the parameters.

Time series identification and Kalman filtering techniques for Doppler lidar velocity estimation

The use of recursive techniques based on Kalman filter algorithms for identification of time series system models for Doppler lidar returns and the subsequent filtering and smoothing of measured data is explored. The form of possible stochastic system models is reviewed, and reiterative maximum likelihood and innovation spectral tests are used for identification. It is found that a random walk model is adequate for the returns here, and possible explanations for this are considered. Examples are given to illustrate the extension of our method to real-time applications and on-line outlier rejection.