Possibility of spaceborne lidar application to the exploration of circumterrestrial space has been analyzed. The optical characteristics of the principal light scattering constituents including molecules and atomic gases, free electrons, aerosols, and space debris have been evaluated at altitudes of 100 - 500 km above the Earth's surface. The power of lidar return signals has been estimated based on the laser sensing and laser detection and ranging equations. It is shown that spaceborne lidar with modern technical characteristics is capable of reliable reception of signals from disperse components of circumterrestrial space at distances of several hundred meters from a spacecraft. Pieces of space debris can be detected by a lidar at distances from a few kilometers to several tens of kilometers.

In this paper we present preliminary results of measurements carried out with a Russian spaceborne lidar `Balkan' installed onboard the orbiting station `Mir', which is operated here since August 1995. This lidar is the first spaceborne lidar operated on a routine basis. We present here the basic specifications of the lidar and the orbiting station that may affect the lidar measurement results. We also consider some methodological aspects concerning the arrangement of the spaceborne experiments along with the analysis of some results on sounding the ocean surface, cloud fields, and the land obtained in different regions of the Earth and under different illumination of its surface, including sunlight and moonlight.

ATLID is a spaceborne backscattering LIDAR using a solid- state Nd-YAG laser (1.064 micrometers wavelength). The paper describes the instrument's Detection Chain breadboard which has been designed and tested under an ESA contract. The Detection Chain is based on an Avalanche PhotoDiode/transimpedance preamplifier hybrid (Detection Front-End Assembly) included on a Detector Module and a Detection Electronics. Main goal aimed at proving the feasibility of the SNR performance and of the clouds backscattered signal detection and datation. The DFEA characterization and the flight design representativity were also a major purpose of this work.

Differential Absorption Lidar measurements from a satellite platform are more complex and can yield more information than terrestrial DIAL measurements because of the significant Doppler shift associated with high satellite velocity relative to the atmosphere. This Doppler effect can be exploited as an extra degree of freedom in matching laser lines to molecular absorption lines or as a mechanism enabling measurement of species density versus altitude. The molecular absorption cross section depends upon altitude (pressure broadening) and upon the range rate of change to the satellite (Doppler). Thus measurements at many different angles during a satellite pass over a ground point will generate data that can be inverted to solve for molecular species density versus altitude. A demonstration experiment using a CO₂ laser on the ground and corner reflectors on the NASA Clark satellite is described. The characteristics of a satellite-based Doppler enhanced DIAL sensor are presented.

A clear need exists for improved soundings of the atmospheric state for purposes of weather prediction. In particular, lidar offers the promise of making detailed profile measurements from space of temperature, water vapor, and perhaps most importantly, wind speed and direction. In order to make satellite Doppler lidar wind measurements both feasible and practical in the troposphere and lower stratosphere, Doppler measurements using both aerosol and molecular scattered signals will be required. This suggests a lidar system operating in the visible or near ultraviolet to exploit the strong molecular scattering at these wavelengths. Additionally, the hardware constraints imposed by spaceborne large aperture optics make the use of a non- diffraction limited receiving telescope or telescopes very attractive. This effectively discourages the possibility of making this type of measurement from a satellite platform by coherent detection means. A technique for making Doppler wind measurements using low-coherence Doppler lidar will be described and ground-based measurements demonstrating the capability will be shown. The technique utilizes high spectral resolution Fabry-Perot interferometry to measure the small Doppler shift and is equally applicable to aerosol or molecular scattering. In particular, the scalability of this type of lidar system to full spaceborne use will be discussed.

AEROSPATIALE, prime contractor, presents the main results related to the activities performed in order to demonstrate the feasibility of a coherent 2 micrometers lidar instrument capable of measuring water vapor and wind velocity in the planetary boundary layer, and to determine the main subsystem critical items: selected instrument configuration and associated performances, 2 micrometers laser configuration with phase conjugation, coherent receiver chain architecture, and frequency locking and offsetting architecture. The second phase of this study will be dedicated to breadboard the most critical elements of the instrument at 2 micrometers in order to technologically consolidate the feasibility of such an instrument.

An airborne cw Doppler lidar measures the Doppler shift of aerosols in the focal volume which are moving with the wind, but also stronger reflections from the ground below the aircraft can be registered. Now one problem is to distinguish between the ground return and the aerosol signal. One possibility is to use the intensity of the back scattered signal, but the ground return may sometimes be very weak, for example at a higher flight level or over sea. Another way to deal with this problem is to use the statistical properties of the back scattered light also known as speckle effect. The back scattered light of different types of targets like hard target, aerosol, and cloud seem to have different correlation times. At a pulsed Doppler lidar this statistical approach has to be modified due to the short correlation time of the laser pulse itself, but a cross correlation between emitted and received pulse may be a solution. This paper will present an analysis of the correlation time at the practical example of a cw Doppler lidar measurement obtained with the system ADOLAR. The measured wind field will also be discussed.

It is shown that the statistical analysis of the Doppler shift fluctuations of non-Gaussian signal based on the use of the perturbation technique accepted in the Gaussian statistical estimation theory leads to the approximation irregularity. The series of the perturbation technique which satisfies the requirements of the regular approximation are obtained by the renormalization of the average Doppler lidar signal power.

Pulsed coherent solid-state 2 micron laser radar systems have been developed at Coherent Technologies, Inc. for ground- and airborne-based applications. Ground-based measurements of wind profiles and aerosol backscatter have been performed for several years. Examples of wind and aerosol backscatter coefficient measurements will be presented which cover a variety of weather conditions. Airborne measurements of wind profiles below the aircraft have been performed by Wright Laboratories, operating in a VAD measurement mode and will be reviewed. An engineered flight-worthy coherent lidar system is under development at CTI for flight on the SR-71 aircraft, in support of the High Speed Civil Transport program. Flights will be conducted by NASA-Dryden Flight Research Center at altitudes above 60,000 feet for the measurement of atmospheric turbulence ahead of the aircraft. Efforts are also underway at CTI for the development of high power coherent laser radar systems. Extensive detailed physical optics models of diode-pumped solid-state laser performance have been developed to characterize transient thermo-optic aberrations and the overall efficiency of lasers intended for space-based applications. We are currently developing a 2 micron 0.5 J/pulse transmitter with a 10 Hz PRF and a pulse duration of 400 - 500 ns. The status and expected space-based wind measuring performance for this system will be presented.

A Doppler lidar can measure wind profiles in the atmosphere with excellent accuracy. Lidars are proposed for spaceborne application. The backscatter lidar technology experiment in space LITE was successfully tested in 1994. A Doppler lidar in space can give both, wind and backscatter information. Existing spaceborne instruments to measure wind velocity over sea are proposed for a synergetic application together with the planned Doppler lidar.

Lidar sounding of clouds give rise to multiple scattering contributions that contain useful information on cloud parameters. The challenge is to extract and make use of this information for the retrieval of the cloud droplet density and effective size. This paper is on the continuation of our group's effort for the development of a reliable inversion method of lidar returns measured at multiple fields of view. It is proposed to use the multiple scattering information to calibrate each lidar signal and derive from there the extinction coefficient at the lidar wavelength and the effective droplet diameter. The method is applied to two stratus cloud events. The solutions obtained are consistent with independent depolarization measurements and with general knowledge on cloud properties.

The U.S. Air Force Phillips Laboratory is evaluating the feasibility of long-standoff-range remote sensing of gaseous species present in trace amounts in the atmosphere. To date, the Phillips Laboratory program has been concerned with the preliminary design and performance analysis of a commercially available CO₂ laser-based DIAL system operating from mountain-top-observatory and airborne platform and more recently with long-range ground testing using a 21.8 km slant path from 3.05 km ASL to sea level as the initial steps in the design and development of an airborne system capability. Straightforward scaling of the performance of a near-term technology direct-detection LIDAR system with propagation range to a topographic target and with the average atmospheric absorption coefficient along the path has been performed. Results indicate that useful airborne operation of such a system should be possible for slant path ranges between 20 km and 50 km, depending upon atmospheric transmission at the operating wavelengths of the ¹³C¹⁶O₂ source. This paper describes the design of the airborne system which will be deployed on the Phillips Laboratory NC-135 research aircraft for DIAL system performance tests at slant ranges of 20 km to 50 km, scheduled for the near future. Performance simulations for the airborne tests will be presented and related to performance obtained during initial ground-based tests.

The Air Force Phillips Laboratory conducted a series of measurements in February, May and August 1995 at the Air Force Maui Optical Station (AMOS) facility on Maui, Hawaii, to determine system requirements for an airborne long path CO₂ DIAL system. The lidar incorporates a cavity-matched mode-locked 3-J laser with the 60 cm diameter AMOS Beam Director Telescope. The one-way beam propagation path length was 21.3 km, originating at the AMOS facility on Haleakala at an altitude of 3.050 km ASL, and terminating at a target site near sea level. Both heterodyne and direct detection techniques are compared with respect to radiometric performance and signal statistics. Minimum detectable absorption levels for DIAL systems using both detection techniques and a variety of targets are estimated from long- range measurements with controlled absorbers. The signal correlation as a function of interpulse temporal separation was determined for long-range direct detection measurements. Radiometric models including system optical characteristics, beam propagation considerations, target reflectivity characteristics,a nd atmospheric effects have been developed and validated experimentally. A new receiver system is currently being fabricated and the laser transmitter is being upgraded for pulse-to-pulse wavelength agility, prior to incorporation into a C-135E airborne platform for future flight experiments.

Between the different techniques of remote sensing the lidar-dial one has been obtaining more importance in achieving concentration profiles of several pollutants. In this paper we describe the system set up at the University of Calabria based on two CO₂ pulsed lasers. In order to evaluate the amount of the pollutants it is necessary the knowledge of the related absorption coefficients. These have been measuring since most of them are not present in the literature. As example the calibration measurements taken on CFC11 are presented after a brief remark of the theory applied. Beside this work preliminary concentration profiles of water vapor and ozone along an horizontal path obtained with the dial system are showed and discussed.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a high-resolution Fourier-Transform spectrometer which is being developed as one of the ESA payload instruments to be flown on-board the Envisat environmental satellite. MIPAS will be used to measure concentration profiles of atmospheric constituents on a global scale over a period of several years. The first MIPAS instrument model will undergo integration and test during early 1997, prior to which the individual subsystems will be fully tested and characterized. This paper summarizes the critical instrument and subsystem requirements and gives an overview of the MIPAS design to meet those requirements.

In the standard CZCS atmospheric correction method the aerosol radiance is derived assuming that the water-leaving radiance at 670 nm is zero. This assumption, justified for water whose reflectance is determined solely by absorption (Case 1 waters), causes the errors for water whose reflectance is significantly influenced by scattering (Case 2 waters)--for example the Baltic Sea. The values of aerosol radiance are too high and the water-leaving radiance too low in comparison to the experimental ones. The relative error for the aerosol radiances (normalized to their values for 670 nm) decrease with wavelength: from 60% at 443 nm to 13% at 550 nm.

This paper proposes to remotely sense the translational temperature and the densities of important nocturnal radiators, viz., nitric oxide, carbon dioxide, and atomic oxygen, as functions of altitude in the quiescent nighttime terrestrial thermosphere. The densities of the excited states as functions of altitude are obtained by inverting the measured infrared limb emissions from nitric oxide near 5.3 micrometers , from carbon dioxide near 15 micrometers , and the fine structure lines of atomic oxygen at 63.2 micrometers and 145.5 micrometers . A knowledge of the excitation mechanisms then permits the calculation of the ground states densities.

In this paper a physical model that describes the relationship between the optical properties of the atmosphere and the characteristics of an imaging system is suggested. The model describes how different components of the light reaching the imaging system, after passing through the atmosphere, are detected by it. The model includes the effects of the final size of the detector elements of the imaging system and the dynamic range and the final field of view limits of the imager. It is found that for common imaging systems (with resolution of 8 bit or 12 bit) working in general atmosphere conditions (VIS >= 5 km), the processes of atmospheric scattering and absorption hardly contribute to spatial blurring of the recorded images. A field experiment was carried out in order to verify the predictions of the suggested model. The measurements were performed using a scanning point radiometer, while a local meteorological station and a visibility meter measured the properties of the atmosphere. Theoretical predictions, which were accomplished by using a Monte-Carlo simulation of atmospheric scattering effects, are compared with the experimental data acquired in the field tests. A good agreement was obtained between the measured data and the theoretical predictions.

Estimates of the fractional fade time, expected number of fades, and mean duration of fade time associated with a transmitted Gaussian-beam wave are analyzed for both uplink and downlink laser satellite-communication channels and compared with similar results based on spherical wave (uplink) and plane wave (downlink) models. Weak fluctuation theory using the lognormal model is applicable for zenith angles less than 60 degree(s). Because spot size and off-axis scintillations are significant in the Gaussian-beam wave model, pointing errors become an important consideration in a reliable laser communication link. Off-axis scintillations increase even more rapidly for large diameter beams and can in some cases lead to a scintillation saturation for pointing errors greater than 1 (mu) rad off the optical beam axis.

Atmospheric turbulence causes intensity fluctuations in the received beam of a ground to space laser communication link. Theoretical analysis of the beam propagation through atmospheric turbulence to a satellite shows that very accurate pointing of the transmitter beam will reduce the fading of the signal. A new technique for tracking through turbulence is described that uses enhanced backscatter resulting from reciprocal scattering paths. Laboratory experiments have been conducted and data compared to computer simulation. A plan has been developed for demonstrating the technique on a Low Earth Orbiting satellite.

Wave-front distortion introduced as light passes through the atmosphere results in short exposure images which exhibit random warping amongst other effects. Our aim is to remove the warping to restore images to their true geometry, but this is not easy as the true geometry is generally not known. To do so, we need to understand the effect of atmospheric turbulence on short exposure images. The individual images are corrected and summed to produce a final image, which therefore has local motion blur removed and can approach the theoretical resolution limit of our optical/imaging system. An important by-product of the process is a sequence of detailed shift maps which provide, in effect, a visualization of the instantaneous turbulence field.

In the clear atmosphere, refractive index turbulence severely limits a variety of optical remote sensing techniques, such as imagers of laser remote sensors. To calculate the limiting effect of turbulence, the structure function constant C_n² of the refractive index fluctuations must be known. Certain cases also require knowledge of the inner scale l₀ of refractive index turbulence. The largest values of C_n² occur in the atmospheric boundary layer. For the boundary layer over land, a number of models has been developed and experimentally tested so far. However, only few data are available for the boundary layer over sea. This paper describes a simple model for the marine boundary layer. The model uses Monin-Obukhov similarity relationships between C_n² and l₀ and the turbulent fluxes of heat and momentum. The turbulent fluxes are calculated from a bulk parameterization scheme. Main input parameters are air temperature, sea surface temperature and wind speed. Model predictions are compared with experimental data from the SWAP 1993 campaign in the North Sea. Here C_n² and l₀ were measured with two scintillometers over path lengths of 100 m and 190 m. The comparison demonstrates the capability of this modelling approach.

The two techniques for sensing full aperture tilt with a laser guide star (LGS) are described. The first technique exploits a full aperture beam transmitting through the main optical train. The two auxiliary telescopes, which are separated from the transmitter in transverse directions, are used to measure a laser beacon image motion. The contribution of the down propagation path to the tilt that is measured with an auxiliary telescope is eliminated by averaging LGS image over a laser beacon angular extent. Such averaging requires FOV of the receiver which greatly exceeds the tilt angular correlation scale. A second method exploits a small aperture beam transmitted from behind a portion of a primary mirror of the main telescope. A laser beacon image motion is measured simultaneously with the main and auxiliary telescopes. A full aperture tilt is determined by subtracting the tilt measured with the main telescope from that measured with an auxiliary one. This method does not require transmitting laser irradiance through the main optical train, and it might be used for the mesospheric sodium layer. A scheme for measuring uncontrolled motion of the main telescope is also considered.

Experimental measurements and computer simulations show that the range of applicability of deconvolution from wavefront sensing can be extended by augmentation with Richardson-Lucy blind deconvolution.

Computer simulations are used to demonstrate the feasibility of operating a low-order adaptive optics system for observation of bright low-earth orbiting satellites during daylight hours. The system considered includes speckle post- processing of partially compensated focal plane images.

A high-resolution space-born Earth observation adaptive optic system contains a sensor of wave aberration in its exit pupil. The output signal of this sensor is used to minimize the aberration in such or other feedback loop. The complexity of known wave aberration measurement techniques stimulates the search of alternative methods, one of them being considered in this report. The proposed method of wave aberration measurement is comprised with the following steps: Taking the measurement of phase characteristic of the optical system based on the current Earth surface image analysis. Calculation of the unknown wave aberration function using the measured phase characteristic as an input data in Gerchberg and Saxton algorithm. The problems of the further investigation of that measurement technique are discussed.

Statistical method of maximum likelihood is applied to the problem of measurement of a wavefront turbulent distortions and restoration of distorted images. It is shown, that synthesized optimal algorithm may be realized by an adaptive system with a controlled phase element. If a coherence function is a result of input field processing, the optimal procedure may be realized as a recurrent computing algorithm.