Range Resolved Optical Remote Sensing Using a Continuous Wave, Pseudo-Random Modulated, Carbon Dioxide Heterodyne LIDAR Backscattered from Aerosols.
Optical remote sensing has been a useful technique used to measure the atmospheric wind velocity. In the late 1960's and early 1970's, researchers were using double -ended laser systems to study the effects of the turbulent atmosphere on laser beam propagation, and to determine the crosswinds. While the double-ended systems were useful in verifying theory, their applications were limited to those in which both ends of the propagation path were accessible. Toward the latter part of the 1970's, however, the push for single -ended laser systems became more desirable to locate the laser transmitter and receiver at the same end of the propagation path. The early single-ended systems used a hard target to backscatter the laser radiation to estimate the path averaged crosswind speed between the transmitter/receiver and the hard target. Since the end of the 1980's, interest has shifted to obtaining range resolved wind estimates, where the desired wind measurements can be limited to remote regions, called range bins, instead of an average wind estimate over the entire laser propagation path. Instead of using a metallic hard target as the laser scatterer, air particles, or aerosols, act as a distributed target to backscatter the laser radiation. Since the aerosols move with the wind, the laser signal will be Doppler shifted by the component of the vector wind along the line of sight from the laser to the scattering volume. Therefore, if the frequency drift from a known modulation frequency due to the Doppler effect can be measured, the wind along the line of sight from laser to aerosols can be determined. This gives the possibility to measure the 3-D wind velocity, where the crosswinds are estimated through the techniques adapted from the path-averaged hard target scenario, and the line-of-sight winds are obtained by detecting the Doppler shift. Although the range resolved 3-D wind speed laser systems have more flexibility than the path averaged systems, the absence of a hard target introduces several new problems. The first difficulty is that the signal-to-noise ratio (SNR) is very poor (about -20 to - 30 dB) because aerosols backscatter much less laser radiation than a hard target. To combat this problem, a processing scheme was developed to detect the amplitude and frequency of a cosine buried in noise specific with the sampling rate and maximum coherent sampling time. Another problem arose because the laser operates cw (continuous wave) rather than pulsed. In the pulsed systems, the receiver can be range gated to look at different parts of the path. But a cw system needs a scheme to discriminate the scattering returns from different parts of the propagation path. This was solved by pseudo-random-code (PRC) modulating the outgoing laser beam phase. The backscattered laser signal can be demodulated with a time delayed version of the PRC to produce an intensity versus range profile. The theoretical analysis of the time delayed statistics of the received intensity was completed for the case of a cw laser backscattered from aerosols. Additional analysis was devoted to the effect that a finite aperture has on the time delayed statistics. Finally, the system was constructed to be able to propagate over horizontal and vertical paths to study such atmospheric properties as the backscattering coefficient and strength of turbulence in addition to the range resolved wind velocity.
- Pub Date:
- January 1995
- Physics: Optics; Remote Sensing