Proceedings of Topical Meeting on Optoinformatics (pdf-format, 1.21 ...

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56 OPTOINFORMATICS’05 LIDAR SIGNAL PROCESSING G. J. Ciuciu, D.N. Nicolae, C. Talianu, M. Ciobanu, V. Babin National Institute <strong>of</strong> R&D for Optoelectronics, 1 Atomistilor Str., Bucharest – Magurele, P.O. Box MG-5, RO-077125, Romania Tel/Fax: 40-21-457 45 22, E-mail: jeni@inoe.inoe.ro, URL: http://inoe.inoe.ro This paper presents LiSA method for Lidar signal processing. It is a combined method, based on the Fernald-Klett algorithm and optical atmospheric model. The purpose is to provide some atmospheric parameters: backscattering and extinction coefficients. LIDAR technique is an active remote sensing method and is based on the emission <strong>of</strong> short laser pulses (ns or fs duration) to the atmosphere under study. The laser photons backscattered by the atmospheric volume under study are collected by a receiving optical telescope. The signals are acquired and digitized in the analog and/or the photon counting mode by fast transient recorders and subsequently transferred to a personal computer for further analysis and storing. Recently, at the National Institute <strong>of</strong> R&D for Optoelectronics – Romania, a compact backscatter lidar was installed. LiSA system can work separately or simultaneous on two wavelengths. It is made to detect from distance (max. 10 Km) micronic aerosols, with a spatial resolution <strong>of</strong> 6 m, using as sounding radiation the beam <strong>of</strong> a Nd:YAG laser with second harmonic. To analyze the return signal in laser remote sensing means to find solutions for the equation which relates the characteristics <strong>of</strong> the received and emitted signal, and the propagation medium. The form <strong>of</strong> the equation depends <strong>of</strong> the interaction type [1] . For those applications which involves scattering (elastic or inelastic), the form <strong>of</strong> equation is quite simple: Z β ( Z ) ⋅ exp[ − 2∫ α( z) dz] Z0 S ( Z ) = C ( Z ) ⋅ + S (1) S Z 2 bg where Z is the distance to the scattering point, S(Z) is the Lidar signal (power), C S (Z) is the so-called system function, β(Z) is the backscattering atmospheric coefficient at distance Z, α(Z) is the extinction atmospheric coefficient at distance Z and S bg is the background signal (power). In writing this equation, the multiple scattering was neglected. The main problem is that we have two unknown parameters - β(Z) and α(Z) – and one equation, so that is necessary to postulate a relation between the two. For this, the Lidar ratio S a ( Z ) = α( Z ) β ( Z ) is used. This parameter is dependent <strong>of</strong> the scatterer dimension and, if is known, the equation can be solved. To know Sa values over entire investigation distance is mostly impossible. In 1972, Fred Fernald [2] realised that Lidar equation is a Bernoulli equation on first rang and obtained its solution in ‘forward’ form, choosing for calibration the closest point Z 0 in the investigation interval. This method works well if the backscattering coefficient in Z 0 can be provided by complementary measurements. In 1981, Klett [3] proved that this solution becomes unstable if atmospheric extinction is important and in that case it diverges with increases <strong>of</strong> the distance. He suggested an ‘inversion’ <strong>of</strong> solution, that means to choose the references point Z ∞ at the end <strong>of</strong> the investigation interval. Rearrangement <strong>of</strong>
SAINT-PETERSBURG, October 17 – 20, 2005 57 integration limits and changing the divisor sign stabilize the solution, but difficult to obtain data in far field is requested. LiSA system signal processing method is based on Fernald-Klett combined, instead <strong>of</strong> complementary measurements data, with optical model <strong>of</strong> the atmosphere developed by Russel and all. in 1979 [4] . The new parameter used in this case is the altitude pr<strong>of</strong>ile <strong>of</strong> Lidar ratio for aerosols scattering θ a ( Z ) = 1 S a ( Z ). The altitude pr<strong>of</strong>ile <strong>of</strong> molecular extinction coefficient α m ( h) is presumed known. In this case, Fernald-Klett solution <strong>of</strong> Lidar equation can be written: F( Z) 2 2 β a ( z) = −α m ( Z) θ m + β ( Z ∞ ) T m ( Z, Z ∞ ) T a ( Z, Z F( Z ) ∞ (2) where T m ( Z, Z ∞ ) is the molecular transmission, and T a ( Z, Z ∞ ) is the aerosol transmission corrected with atmospheric model. From eq. (2) we find that Fernald-Klett solution allows 2 to include the contribution <strong>of</strong> aerosols extinction through T a ( Z, Z ∞ ) factor, this being the unique solution <strong>of</strong> Lidar equation. Lidar systems can be very useful in environmental investigations, especially <strong>of</strong> the atmosphere, due to the large covered area and the real time response. The accuracy <strong>of</strong> obtained information is dependent <strong>of</strong> technical performances <strong>of</strong> the device and <strong>of</strong> sensibility <strong>of</strong> data processing method, which can be critical in some cases. Presentation <strong>of</strong> this paper was partially supported by ICO travel-grant program. ∞ ) 1. R.M. Measures, Laser Remote Sensing. Fundamentals and Applications, Krieger Publishing Company, Malabar, Florida, 1992, p. 237. 2. F.G. Fernald, B.M. Herman, and J.A. Reagan, Determination Of Aerosol Height Distribution By Lidar, J. Appl. Meteorol. 11(1972)482. 3. J.D. Klett, Stable Analytical Inversion Solution For Processing Lidar Returns, Appl. Opt. 20(1981)211. 4. P.B. Russell, T. J. Swissler, and M. P. McCormick, Methodology for error analysis and simulation <strong>of</strong> lidar aerosol measurements, Appl.Opt., 18(1979)3783.
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