A multiple wavelength, pulsed CO(2) lidar system operating at a pulse repetition frequency of 200 Hz and permitting the random selection of CO(2) laser wavelengths for each laser pulse is presented. This system was employed to measure target reflectance and atmospheric transmission by using laser pulse bursts consisting of groups with as many as 16 different wavelengths at a repetition rate of 12 Hz. The wavelength tuning mechanism of the transversely excited atmospheric laser consists of a stationary grating and a flat mirror controlled by a galvanometer. Multiple wavelength, differential absorption lidar (DIAL) measurements reduce the effects of differential target reflectance and molecular absorption interference. Examples of multiwavelength DIAL detection for ammonia and water vapor show the dynamic interaction between these two trace gases. Target reflectance measurements for maple trees in winter and autumn are presented.

The effects of flight geometry, signal averaging and time-lag correlation coefficient on airborne CO2 dial lidar measurements are shown in simulations and field measurements. These factors have implications for multi-vapor measurements and also for measuring a single vapor with a wide absorption spectra for which one would like to make DIAL measurements at many wavelengths across the absorption spectra of the gas. Thus it is of interest to know how many wavelengths and how many groups of wavelengths can be used effectively in DIAL measurements. Our data indicate that for our lidar about 80 wavelengths (i.e., about 4 groups of wavelengths, each contains 20 wavelengths, transmitted at a group repetition frequency of 2-4 Hz) can be used for DIAL measurements of a stationary vapor. The lidar signal is composed of fluctuations with three time scales: a very short time scale due to system noise which is faster than the data acquisition sampling rate (MHz) of the receiver, a medium time scale (KHz) due to atmospheric turbulence, and a long time scale (Hz) due to slow atmospheric transmission drift from aerosol inhomogeneities. The decorrelation time scale of fluctuations for airborne lidar measurements depends on the flight geometry. INTRODUCTIONThe effects of signal averaging and time-lag correlation on differential-absorption lidar (DIAL) systems were studied by Menyuk et al.'3 for a horizontal path of a medium range (2.7 km) from a diffuse target (flame-sprayed aluminum). Menyuk et al.showed that the signal averaging does not reduce the standard deviation of the dial measurements by the expected n112 , where n is the number of pulses that were averaged (running mean), because of the presence of a long-term temporal fluctuations in the atmospheric transmission. In our work the effect of flight geometry, signal averaging, and time-lag correlation coefficient on airborne dial measurements is presented. The lidar signal P measured at time t, (i.e., from a laser pulse transmitted at time t) from a topographical target with reflectivity 3 (i.e. the surface albedo ofthe target) is given by P(r,t1) =Kflcos(8) exp(-2ar) I r2 where r is the distance to the target along the lidar line of sight (L.O.S), e is the angle between the L.O.S and the normal to the target, a is the atmospheric volume extinction coefficient such that (a r) is the optical depth between the lidar and the target, and K is a constant for the lidar system which includes the area A of the lidar receiving telescope. In DIAL measurements the concentration of a trace gas of interest is deduced from the time series measurements which is given by the vector = 1n[ I 7] which is the log ofthe ratio of a lidar measurements x(t1 ) = P,2 , (r, t. ) measured at wavelengths 2. at a time t, and the lidar measurement y(t1 + At) = P2 (r, t. + At) measured at wavelengths 22 at time t. + At where At, depends on how fast the lidar can switch between wavelengths. The vector is proportional to the trace gas concentration as a function oftime, and an average concentration for the ...

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To determine the presence of a pollutant cloud in the atmosphere, France and the United States have collaborated on the development of a DIAL (Differential Absorption LIDAR) and DISC (Differential Scattering) LIDAR. This system called MIRELA, is financed by the DGA (France) and ERDEC (USA). It was developed in cooperation with the CILAS company (France) and uses a frequency agile CO2 laser designed and manufactured by the Hughes Aircraft Company (USA).Before using a LIDAR for the remote detection of atmospheric pollutants, the optical characteristics of the products to be detected must be known. This basic characterization is used to define the parameters of the system and select the detection technologies and algorithms.A simulation with the HITRAN data base provides a set of expected measurements. Comparison with the real results is excellent.The tests were run on realistic clouds. The backscattered signal received from the aerosols at the front of the cloud was detected as well as the return from a target placed beyond the cloud thus a transmission measurement was taken simultaneously with the backscattering measurement. These measurements show that the backscattering signals are characteristic of the cloud and will be used to detect and identify the products.