William D. Jones

Airborne Doppler lidar measurements.

This paper describes recent measurements using coherent Doppler lidars operating at a wavelength of 10.6 microm aboard the NASA Ames Convair 990. The purpose of the measurements was to obtain data on the atmospheric wind fields and the distribution of the backscatter coefficient at 10.6 microm. A description of the instruments is provided detailing the modifications incorporated following the 1981 test flights of the systems. The measurement program is outlined, and preliminary results are discussed.

Signal Processing and Calibration of Continuous-Wave Focused CO2 Doppler Lidars for Atmospheric Backscatter Measurement

Two continuous-wave (CW) focused CO(2) Doppler lidars (9.1 and 10.6 µm) were developed for airborne in situ aerosol backscatter measurements. The complex path of reliably calibrating these systems, with different signal processors, for accurate derivation of atmospheric backscatter coefficients is documented. Lidar calibration for absolute backscatter measurement for both lidars is based on range response over the lidar sample volume, not solely at focus. Both lidars were calibrated with a new technique using well-characterized aerosols as radiometric standard targets and related to conventional hard-target calibration. A digital signal processor (DSP), a surface acoustic wave spectrum analyzer, and manually tuned spectrum analyzer signal analyzers were used. The DSP signals were analyzed with an innovative method of correcting for systematic noise fluctuation; the noise statistics exhibit the chi-square distribution predicted by theory. System parametric studies and detailed calibration improved the accuracy of conversion from the measured signal-to-noise ratio to absolute backscatter. The minimum backscatter sensitivity is ~3 × 10(-12) m(-1) sr(-1) at 9.1 µm and ~9 × 10(-12) m(-1) sr(-1) at 10.6 µm. Sample measurements are shown for a flight over the remote Pacific Ocean in 1990 as part of the NASA Global Backscatter Experiment (GLOBE) survey missions, the first time to our knowledge that 9.1-10.6-µm lidar intercomparisons were made. Measurements at 9.1 µm, a potential wavelength for space-based lidar remote-sensing applications, are to our knowledge the first based on the rare isotope (12)C (18)O(2) gas.

Australian aerosol backscatter survey

This paper describes measurements of the atmospheric backscatter coefficient in and around Australia during May and June 1986. One set of backscatter measurements was made with a CO(2) lidar operating at 10.6 microm; the other set was obtained from calculations using measured aerosol parameters. Despite the two quite different data collection techniques, there is quite good agreement between the two methods. Backscatter values range from near 1 x 10(-8)m(-1)sr(-1) near the surface to 4 - 5 x 10(-11)m(-1)sr(-1) in the free troposphere at 5-7-km altitude. The values in the free troposphere are somewhat lower than those typically measured at the same height in the northern hemisphere.

Coherent focal volume mapping of a cw CO2 Doppler lidar.

In this paper we determine the coherent response to a 100-μm diam spinning wire as a function of its position within the focal volume of a 10.6-μm coherent Doppler lidar and generate SNR contours both transverse to and along the system line of sight. Application of the contours to single-particle scattering is discussed.

Design and calibration of a coherent lidar for measurement of atmospheric backscatter

This paper presents a discussion of the background requirements for, and the design and calibration of a coherent Doppler lidar which is oriented specifically toward the measurement of low values of atmospheric backscatter, β (π), at a wavelength of 10.6μm. The lidar to be described is a compact, continuous wave system which can operate in two modes--multiple particle scattering and single particle scattering. In the multiple scattering mode, the lidar employs an extended focal volume and utilizes a technique similar to Dicke switching to achieve measurement of the volume backscatter. In the single scattering mode, the focal volume is reduced to ensure a high probability of single particle scattering. Measure-ment of the single particle backscatter as a function of time leads to the formation of signal histogram from which the volume backscatter can be inferred. In addition to providing the atmospheric backscatter value, appropriate data processing algorithms and focal volume calibration allow the single particle mode to yield information on the atmospheric aerosol scattering cross-section distribution. The system hardware and signal processing are described in this paper along with the algorithms used to calculate the backscatter, (7). Calibration techniques described include the use of known targets such as spinning disks and wires.

Determination of atmospheric backscatter at 10.6 microns

We present profiles of atmospheric aerosol backscatter coefficient, β(π) at 10.6 μm versus altitude at four different geographical locations. The profiles show the general decrease in backscatter expected at higher altitudes.