Over the past 20 years, active sensor techniques have emerged as crucial airborne and ground-based measurement techniques, providing the Earth sciences community with unique measurements for scientific disciplines in atmospheric chemistry, dynamics, studies of the Earth radiation budget, climate, meteorology, altimetry and crystal dynamics.' Two techniques, Lidar (Light Detection and Ranging) and DIAL (Differential Absorption and Lidar) have demonstrated the ability to conduct range resolved measurements of aerosols, cloud dynamics, height of the planetary boundary layer, humidity, trace species, and atmospheric winds. Experiments are routinely conducted from scientific aircraft in national and international campaigns. Additionally, airborne measurements have been conducted to demonstrate centimeter accuracy in Lidar ranging experiments. New applications are also emerging in aviation meteorology and in chemical agent, stand-off detection.2 To date, an active sensor experiment at optical frequencies (i.e. lasers) has not been conducted from space for Earth sciences experiments. However, under sponsorship from NASA's Office of Aeronautics and Space Technology, OAST, an experiment is planned for 1992 to conduct a technological Lidar experiment from the Space Shuttle with a Nd:YAG laser, at the fundamental, first and second harmonic wavelengths (1064 nm, 532 nm, 355 nm). Experimental plans and details of the instrumentation have been previously published.3 In this conference, a status of the Laser Transmitter Module [LTM] for the Space Shuttle experiment has been presented.4 A second laser experiment to measure the vertical profiles of water vapor in the lower atmosphere is also under development for a scientific experiment from the NASA U-2.5 The laser transmitters being developed for this experiment are using alexandrite technology. Experimental designs and concepts for this experiment have also been presented in this conference.6 From a technological perspective, this experiment is important since the DIAL experiment must be configured to be totally autonomous, and will develop systems technology in associated E/O systems technology including a laser wavemeter/spectrometer, detectors and information systems.

The airborne lidar system of the Aerosol Research Branch, NASA Langley Research Center, has been used since 1978 to study the behavior of the stratospheric aerosol on a global scale. The system, in its current configuration, can operate at a wavelength of either 0.6943 pm (ruby laser) or 0.530 μm (frequency doubled Nd:YAG laser). It has a 35-cm diameter receiving telescope and is capable of making simultaneous dual-polarization measurements on the back-scattered signal. The lidar has been flown onboard the NASA Electra, P-3 Orion, and CV 990 aircraft. The advantages of an airborne lidar system over its ground-based counterpart are the ability for it to be directed to any region of interest and to survey atmospheric aerosols over a wide range of latitudes and longitudes. The 'present system has been used for four areas of study. These are (1) satellite correlative measurements, (2) volcanic stratospheric injection studies, (3) polar vortex behavior, and (4) polar stratospheric cloud observations.

High resolution moisture profiles have been shown to be good indicators of dynamic structures in the atmosphere such as fronts. Since the fundamental work of the Norwegian school of meteorology in the early 1900's, it has been recognized that knowledge of the motion of cyclones and their associated cold and warm fronts can have significant impact on the forecast of midlatitude weather. Fronts, by their very nature, are characterized by discontinuities in a number of atmosphere parameters. Generally, there are abrupt changes in atmospheric temperature, moisture, and wind, both magnitude and direction, on either side of a front. The study of fronts is difficult because they don't exist just at the Earth's surface but have a complicated sloping structure aloft. Frequently, frontal surfaces can be observed to extend from the Earth's surface through much of the troposphere.

The airborne differential absorption lidar (DIAL) system developed at the NASA Langley Research Center has been used to investigate ozone (03) and aerosol distributions in numerous international field experiments over the past 3 years. This paper discusses the measurements of 03 and aerosols during the 1985 and 1987 NASA Global Tropospheric Experiments to study the troposphere above the Amazon rain forest of Brazil in the dry and wet seasons and during the 1987 Airborne Antarctic Ozone Experiment to investigate the development of the 03 hole in the stratosphere over Antarctica.

An injection-locked, pulsed Doppler lidar for atmospheric wind monitoring is described. The lidar produces up to 50 pulses per second with energies of about 1 J per pulse. Transverse mode suppression is effected with an output coupler whose radial reflectivity is parabolically-tapered. The lidar has been used to measure winds in several unique field experiments.

The NASA Space Station Program includes development of a polar orbiting platform as part of the Earth observing system (Eos) Program. A lidar facility is in-turn being developed for the Eos polar platforms to make atmospheric science measurements utilizing techniques of laser atmospheric backscatter (optical radar), Differential Absorption Lidar (DIAL) and altimetry. The science for this facility has been developed by the Lidar Atmospheric Sounder and Altimetry (LASA) panel for the Earth observing system and will be addressed in an Announcement of Opportunity which will be released early in 1988 to solicit science proposals and experiment proposals for the Eos missions.

Graphite/epoxy (Gr/E) composites offer significant advantages over other candidate materials for optical instrument structures where stiffness, strength and stability are required at minimal weight and cost. This paper presents Composite Optics' experience in design and fabrication using graphite/epoxy for telescope assemblies, camera assemblies, and other precision optical instrument support structures. Approaches and solutions developed by Composite Optics, Incorporated, for specific design challenges will be discussed and illustrated by actual fabricated optical instrument hardware.

Army combat forces involved in global military operations require knowledge of the terrain and accurate positioning and navigation capability to effectively perform their missions. Combat critical data from satellite-based systems to augment ground and airborne data collection, processing, and dissemination systems are crucial for the delivery and use of the needed information and intelligence in near-real time. The Army is developing ground-based testbed systems to utilize terrain and weather data collected from space-based platforms to enhance Army commanders' battlefield capabilities, and is researching new applications for the NAVSAT Global Positioning System and the Defense Advanced Research Projects Agency-sponsored (DARPA) LIGHTSAT program that are unique to the Army. In addition, the Army is designing experiments to be conducted on the Space Shuttle.

The U.S. Army Chemical Research, Development and Engineering Center (CRDEC) is currently engaged in an extensive multi-year exploratory development program to exploit laser radar for Chemical/Biological (CB) Stand-Off Detection. At present, the only near term capability for the detection of chemical agents at a distance is the use of passive infrared sensors. These sensors can detect only chemical vapors. Active (laser) infrared (IR) systems employing Differential Scattering and Absorption Lidar (DISC/DIAL) are being developed for the detection of chemical agents in all physical forms: vapor, aerosols, and rains, as well as liquid surface contamination. In addition, an ultraviolet (UV) system employing laser induced fluorescence is being developed for the detection of biological agent clouds consisting of pathogens and toxins. The principles of operation of these systems and the history of their development will be briefly discussed. The IR and UV breadboard systems have recently participated in an extensive field test employing battlefield concentration of simulants and interferents with excellent results. These data will be discussed along with the necessary development efforts required to adapt the DISC/DIAL technology to Army and other service needs.

A generalized windshear hazard index is defined, which is derived from considerations of wind conditions and an aircraft's present and potential altitude. Based on a systems approach to the windshear threat, lidar appears to be a viable methodology for windshear detection and avoidance, even in conditions of moderately heavy precipitation. The proposed airborne CO₂ and Ho:YAG lidar windshear detection systems analyzed in this paper can each give the pilot information about the line-of-sight component of windshear threat from his present position to a region extending 1 to 3 km in front of the aircraft. This constitutes a warning time of 15 to 45 s. The technology necessary to design, build and test such a brassboard 10.6-μm CO₂ lidar is at hand.

The feasibility of using a pulsed coherent lidar for an on-board airline wind shear monitoring system was studied using detailed computer simulations of the lidar wind measuring process. Lidar performance was studied using NASA-LaRC-provided data fields of an actual microburst event that occurred on August 2, 1985 at the Dallas-Ft. Worth International Airport. Both CO₂ and Ho:YAG lidar systems performed well in the dry micro-burst test case. Both lidar systems are able to measure wind shear in the severe weather of the wet microburst to ranges greater than 1.4 km giving minimum warning times of approximately 15 sec.

The present paper describes the conceptual design and potential scientific and engineering applications of the Geodynamics Laser Ranging System (GLRS). The latter instrument is a spaceborne laser ranging and altimeter system. It is currently scheduled to be launched in the 1997 time frame as a facility on the first European polar orbiting platform which is being developed as part of the multiple satellite Earth Observing System (Eos). In the geodynamics or retroranging mode, the system will use the green and ultraviolet output from a subnanosecond pulse, frequency-tripled, Nd:YAG laser to measure the geometric range from the spacecraft to arrays of strategically placed groundbased retro-reflectors. The use of two colors, in conjunction with a two picosecond resolution streak tube receiver to measure the interpulse propagation delay introduced by atmospheric dispersion, will permit measurement of the geometric range with an absolute accuracy of 0.5 cms or better. Large radar cross- sections, on the order of a million square meters from the passive ground retroreflectors permit the use of small (0.18 m) receiver telescope diameters and optical trackers with the high slew rates and fast settling times necessary to interrogate large numbers of targets in a typical ten minute pass over the array. This data can be processed to provide decimeter accuracy satellite orbital ephemeris to collocated instruments, perform centimeter accuracy global geodetic measurements, provide data on the motion of tectonic plates or ice floes, and produce high spatial resolution information on regional crustal deformation caused by seismic, volcanic, or other forces such as oil or water withdrawal. Other potential scientific uses include the measurement of atmospheric surface pressure, ocean wave height and surface roughness, cloudtop heights, subnanosecond time transfer between widely separated sites, and longterm general relativity experiments. In the engineering arena, GLRS could provide preliminary surveys in support of land management applications or large scale engineering projects such as interstate highways as well as important information on long term ground stability relevant to the location of nuclear power plants, pipelines, aquaducts, dams, etc. The unconverted fundamental Nd:YAG radiation at 1.06 micrometers is directed to a nadir or near-nadir viewing altimetric channel and viewed by a somewhat larger (0.5 m diameter) telescope. A topographic channel provides both time-of-flight to the underlying terrain and nanosecond resolution waveforms to study geologic structures, transport properties associated with sand dunes and lava flows, and the thickness, growth, and decay of ice sheets. A second lower resolution digitizer provides data on cloud distributions and cloudtop heights.

Traveling-wave type optical modulators using LiTa0₃ crystals have been designed and fabricated for high resolution distance measurement. The modulation property of the modulator is characterized at GHz frequencies. The distance measuring-system is constructed using two of these optical modulators and is preliminarily tested. The modulators were driven by slightly different frequencies in the GHz region in order to measure the phase of the modulated signal using the heterodyne method. The displacement of a target was measured with a resolution of 15pm and with a linearity of 0.2%.

Substantial developments in solid-state lasers are required to meet the demands of remote sensing instruments designed to operate in spacecraft or aircraft. Progress in fiber spectroscopy, in eyesafe solid-state lasers both for Doppler work and to serve as a pump laser to generate mid-infrared wavelengths, and in nonlinear optics to generate tunable mid-infrared radiation to meet these demands will be presented.

A compact instrument for single shot spectrum analysis and real time wavelength measurement of pulsed, tunable lasers has been built. Its accuracy and limitations are discussed, along with its potential applications. Results of tests with a narrow-line Nd:YAG-pumped dye laser are also presented.

The present paper summarizes the key characteristics and development status of an automated dual Alexandrite laser transmitter to be used in a series of airborne water vapor DIAL experiments on the NASA ER-2 high altitude research aircraft. The Lidar Atmospheric Sensing Experiment, or LASE, is a joint effort of the Langley Research Center (LaRC) and the Goddard Space Flight Center (GSFC) with the latter center being responsible for the transmitter. The transmitter consists of five separate subsystems - a pressurized Dual Laser Head (DLH) containing two Alexandrite lasers, a Laser Control Unit (LCU), a Laser Thermal Unit (LTU), and two Lamp Driver Units (LDU's) - and weighs approximately 330 lbs. Each laser utilizes three computer- controlled tuning elements - a five plate birefringent tuner, a thin solid etalon, and a thick solid etalon - to perform inflight tuning or wavelength scans and to hold the radiated wavelength within 0.5 pm of the desired value with the aid of an external high precision wavemeter. The transmitter is designed to operate and maintain its alignment over a wide range of environmental conditions in the aircraft Q-bay compartment. Major reductions in system size and weight, relative to commercial alexandrite lasers, were necessary to meet LASE Project requirements. Furthermore, since the Q-bay experiences ambient pressure variations between 3.5 and 15 psi and ambient temperature variations between 15° and 40°C, the transmitter will be tested in an environmental chamber prior to delivery to the Project. At the present time, subsystem performance tests have been successfully completed on all flight units with the exception of the laser head itself which has undergone only breadboard level tests. In addition, the LTU and LDU's have successfully passed 5 g-rms random vibration tests.

An automatic in-flight method for tuning lasers to selected absorption lines of gases in the atmosphere has been investigated. The method has been developed as part of a system for the determination of water vapor in the atmosphere from an airborne platform. It is demonstrated by numeric simulation that a tuneable alexandrite laser can be adjusted to within 0.5 picometers of a desired wavelength within a data gathering and computational period of less than ten minutes. Numeric simulations were used to investigate potential sources of error. The method was analyzed for a NASA water vapor differential absorption instrument now being constructed, however, the method may be useful for other atmospheric gases.

Performance analyses are presented which establish that over most of the range of signals expected for a down looking differential absorption lidar (DIAL) operated at 16 km the silicon avalanche photodiode (APD) is the preferred detector for DIAL measurements of atmospheric water vapor in the 730 nm spectral region. The higher quantum efficiency of the APD's, (0.8 - 0.9) compared to a photomultiplier's (0.04 - 0.18) more than offsets the higher noise of an APD receiver. In addition to offering lower noise and hence lower random error the APD's excellent linearity and impulse recovery minimize DIAL systematic errors attributable to the detector. Estimates of the effect of detector system parameters on overall random and systematic DIAL errors are presented, and performance predictions are supported by laboratory characterization data for an APD receiver system.

Recent advances in laser diode pumped solid state laser sources permit the design and testing of laser sources with linewidths that approach the Schawlow-Townes limit of 1 Hz/mW of output power. Laser diode pumped solid state ring oscillators have been operated with cw output power levels of 25 mW at electrical efficiencies that exceed 6%. These oscillators are expected to operate for lifetimes that approach those of the laser diode sources which is now approaching 20,000 hours. The efficiency and lifetime of these narrow linewidth laser sources will enable basic physics space measurements of gravity waves, remote sensing applications including local range rate and LIDAR measurements, and laser sources for frequency and time standards. A free-flight experiment called SUNLITE (Stanford University NASA Laser In-space Technology Experiment) is being designed to measure the linewidth of this all solid state laser system.

NASA's lidar in Space Technology Experiment. (LITE) is a multimission Space Transportation System (STS) program to evaluate the capability of a lidar experiment to measure aerosols and other atmospheric parameters from a space platform. The measurements will be made at three wavelengths (1.06, 0.532 and 0.355 microns). This paper presents the results to date of the program to produce the space-qualified laser transmitter module (LTM) for this mission.

Semiconductor injection lasers are required for implementing virtually all spaceborne remote sensing systems. Their main advantages are high reliability and efficiency, and their main roles are invisioned in pumping and injection locking of solid state lasers. In some shorter range applications they may even be utilized directly as the sources.

A coherent lidar is a powerful technique for remote measurements of velocity fields, and at middle infrared wavelengths it also has advantages compared with direct-detection lidar in extended range measurement of atmospheric species. These qualities are obtained at the cost of additional complexity and more stringent requirements on the coherence properties of the lasers which are used. An attempt is made to summarize the technological requirements of certain coherent lidar applications and discuss relevant technology development, emphasizing the CO₂ and solid-state laser components.

It is possible to make wind profiles from space using current technology. These wind profiles are essential for investigating many of the interdisciplinary scientific questions of the Earth Observing System (Eos). A spaceborne wind sounder, the Laser Atmospheric Wind Sounder (LAWS) has been chosen as a facility instrument on the Eos. This paper discusses the efforts currently underway to prepare for the deployment of the LAWS instrument on an Eos polar orbiter and on the Manned Space Station. The measurement technique heritage, which includes ground-based and airborne measurements in the atmosphere, is discussed, along with the scientific and technical issues that are being addressed. Whether obtained globally from the Eos polar platform or in the w tropics from an Space Station,wind profiles from space will provide essential information for advancing the skill of numerical weather prediction, furthering our knowledge of the large-scale atmospheric circulation and climate dynamics, and improving our understanding of the global biogeochemical and hydrologic cycles. These scientific issues are also briefly discussed.

The optical setup of an airborne differential absorption lidar based on optical heterody-ne detection of topographic backscattering of CW tunable CO₂ lasers will be shown. A simplified arrangement that takes advantage of the internal ampliiicattion of the backscattered and Doppler shifted light by the operating lasers makes excellent separation of the wavelength channels possible. As a result the optomechanical system is highly compact and minimum number of elements are used; e.g. all the necessary optical signals measured by a single detector are electronically separated. The lidar is used for detection of molecular pollutants in the atmosphere from an airborne platform. False information rejection is carried out by simultaneous processing of differential absorption signal and fluctuation characteristics of the signal originating from scattering from the random rough topographic surface.