Knowledge of the wind field at various scales is widely recognized at the present as fundamental to advancing our understanding and prediction of the weather development. Wind profiles are measured only at a few locations by radiosondes mainly in the northern hemisphere and there on the continents.
Coherent lidar airborne shear sensor (CLASS) is a development and flight-validation program to demonstrate the feasibility of airborne CO2 coherent lidar systems for predictive detection of hazardous windshear conditions.
Three potential techniques for making wind measurements from space are discussed, and coherent detection Doppler lidar chosen for further consideration. The impact of different laser transmitter wavelengths on the design of a Doppler lidar are considered. Specifically covered in the paper are coherent detection systems based on the 2.1μm Tm:Ho:YAG laser and the 9.1μm 12C1802 laser. Both systems engineering aspects and subsystem impacts are discussed.
Technology requirements are identified for 5 year operation of a LAWS type instrument between on-orbit maintenance visits. This paper statuses identified technology maturity levels. While LAWS is certainly an advanced technology instrument, the technologies required for most LAWS subsystems have either been previously demonstrated as flight hardware or will be demonstrated in the near future. The transmitter laser is the principle subsystem requiring technology enhancement. The basic LAWS subsystems are delineated as follows: transmitter laser; receiver-processor; optical; mechanical and support; command, control, communications; and electrical power. The following paragraphs present each of these subsystems and current as well as projected technology status for assemblies of each.
Lack of global wind data is a major limitation to progress in both meteorology and atmospheric science. Current observing networks are incapable of meeting this requirement so users must look to space-borne instrumentation; whence the interest in Doppler wind lidars which could provide winds in cloud free air globally. This paper seeks to relate the provision of such instruments to user requirements for global winds, outlining some of the relevant activities of the European Space Agency.
A long-life Nd:YAG laser transmitter module for spaceborne elastic back-scattering LIDAR is under construction supported by European Space Agency contract. This paper presents preliminary tests and results of a space qualifiable laser with 5 watt average power at 532 nm and expected lifetime of 109 shots.
As part of the atmospheric research program, European Space Agency has required the development of an high power (0.5-1 J) tunable narrow bandwidth Alexandrite laser. We have developed a Q-switched laser based on a master-slave ring oscillator to obtain almost single longitudinal and transverse mode emission.
We summarize the results of an investigation into the relative merits of diffractive and radially-variable unstable resonator couplers for use in heterodyne-detection lidar systems, emphasizing overall lidar system efficiency.
An experimental evaluation of different resonator designs has been performed in order to verify the feasibility of a pulsed CO2 laser source for spaceborne Doppler LIDAR with a self-sustained TEA discharge channel. A laser with 10 J of energy/pulse, 1.2 is FWHM pulselength, less than 200 kHz frequency chirp has been demonstrated using a 5x5x100 cm3 active medium volume and a modified Self Filtering Unstable Resonator (SFUR) optical design.
Chirp measurements have been conducted on the pulsed output of an unstable resonator oscillating on the P20 line of the 10.6 micron band of an e-beam sustained CO2 gain medium. The mode volume was approximately 5 liters at a pressure between 1/3 and 1/2 atm. The measurements were conducted at pulse lengths of the order of 10 microseconds and are useful in characterizing the device for use in radar applications. A heterodyne technique was used, mixing the oscillator signal with a cw laser operating on the P20 line, but offset approximately 40 MHz in order to resolve uncertainties in the sense of the frequency deviation. Chirp values of less than 400 kHz are observed during the first 10 μsec.
Design criteria for a compact, sealed, high power CO2 waveguide laser are discussed. The laser is capable of being operated continuous wave (cw) and with amplitude or frequency modulation (AM or FM) according to the type of intracavity crystal used. It is anticipated that the device will find application in a variety of coherent detection based systems.
FM chirp/pulse compression has long been used in conventional radar systems . The main advantages of such a technique are: 1. Efficient use of the average power available at the transmitter. 2. Increased system accuracy, both in range and velocity measurements. 3. Reduction of jamming vulnerability. We have explored the use of this technique for laser radar systems and in this paper describe an electro-optically FM modulated CO2 waveguide with post detection pulse compression by a surface acoustic wave (SAW) com-pression filter. The CO2 laser has been FM chirp modulated by a CdTe intracavity modulator. A frequency deviation of 95 MHz in 2.1 psec was attained in this fashion. Following heterodyne detection, the chirped pulse was compressed to 15 nsec using a SAW compression filter. This corresponded to a compression factor of 130. The suppression of unwanted sidelobes with a weighting filter was also demonstrated.
The performance of coherent receivers operating over the range 9.18 µm to 10.74 µm, using Hg1-xCdxTe photodetectors maintained in the temperature range 77K-120K, and featuring state-of-the-art GaAs FET preamplifiers tested at 100K - 300K temperatures, will be reported.
Heterodyne Signal-to-Noise ratios are computed by integrating transmitter-LO irradiance products. Expansion of beams in orthogonal functions simplifies calculations and yields simple, interesting results.
We have used the techniques of Siegman I and others2'3'4 to calculate the receiving efficiency of monostatic coherent lidars vs. range for various telescope focal settings. The receiving efficiency q(z) is defined as
In optical heterodyne detection the alignment of the local oscillator beam and the signal beam is of vital importance. A method for measuring signal-to-noise ratio in a heterodyne system using a blackbody radiation source is presented. Also, a method for measuring phase front differences between signal beam and local oscillator is described.
A review of three-frequency nonlinear heterodyne detections is given. A corresponding system using a single-mode and a two-mode CO2 lasers is configured, and its performance was examined. Several experimental results and cases are presented.
A processing strategy for lidar signals based on established concepts of Kalman filtering is described. Features of relevance to lidar include use of stochastic and non-stationary system models, the need for non-linear and multi-dimensional models in many applications, and the ability to 'self-tune' the filter in response to data input in the absence of a vriori information.
We describe and illustrate the use of nonstationary Wiener filtering for smoothing and differentiating path-integrated concentration estimates provided by the DIAL technique. The primary advantage of the method lies in its ability to provide filtered estimates that are smoothed relative to the local uncertainty in the input data. The approach is derived and illustrated on both synthetic and actual lidar data.
A 10 MHz high-speed data acquisition and processing system was explored, for the demand of signal processing on CO2 lidar. Laser autodyne signal was collected by the system and its frequency spectrum and power spectrum were analysed. The results of the experiment coincides with the theo-retical prediction.
A data acquisition system by single-chip microcomputer was designed. It is suitable to the future devlopment of the miniature tidar signal processing epuipment . The characteristics of frequecy response, SNR, D* and NEP of FM-CW CO2 coherent tidar were discussed.
The wide range of potential lidar applications is considered and serves to emphasize the necessity of matching technical solutions to the information required. The strong heritage of CO2 lidar technology arises from twenty years of detailed system and component investigation.
This paper discusses the use of an integrating sphere in a comparison technique to calibrate hard targets. Real targets do not completely depolarize the incident laser radiation. The directional-hemispherical, conical-hemispherical, and bihemispherical reflectance of a real target is a (4x4) matrix. This matrix can be only be measured if the walls of the integrating sphere do not completely depolarize the incident radiation on the first strike. The best method to measure the monostatic reflectance of a target is via a direct measurement.
An in-flight laser anemometer developed by CROUZET for true air speed measurements and aircraft certification has been used to provide new informations about the aerosols backscatter coefficient at 10 pm in the lower atmosphere. The CO2 laser anemometer has been flown on a Mirage III up to an altitude of 13 km. In 1987, 10 flights over the central part of France were taken from May through October during various meteorological conditions.
Satellite occultation data from the SAGE II satellite experiment have previously been used to develop a global climatology of the free tropospheric aerosol. This paper reports the results of analyses of data obtained close to locations where 10.6 μm backscatter measurements have been made. Five such locations have been identified and the SAGE II data used to study the local seasonal variations of the 1µm aerosol extinction. Intercomparison with 10.6 μm backscatter data has been used to obtain a value for the interwavelength scattering/extinction conversion factor, which is compared with the results of a previous experimental study.
The NOAA Doppler lidar trailer was transported from Boulder, Colorado, to the 3.231km level of Hawaii's Mauna Loa volcano (lat. 19.55°N, long. 155.56°W) in No-vember 1988 to participate in the NASA-sponsored Mauna Loa Backscatter Intercomparison Experiment (MABIE) for 1988. Our purpose was multifold. Among the aerosol studies our goals were to gather a statistically meaningful set of vertical backscatter pro-files at two wavelengths in the clean Pacific environment, to compare data from several microphysical sensors located at the GMCC observatory 3 km away, to assess the representativeness of the ground-based GMCC samplers with respect to the air mass over-head, and to understand the depth of the upslope and downslope flows that have historically affected the GMCC samplers. We were highly successful on all counts, having gathered 243 vertical profiles at 10.59 gm, 49 profiles at 9.25 vim, 278 GMCC intercom-parisons, and 404 wind profiles and cross sections. Our data-gathering period extended over 24 days through December 11. We calibrated the system on seven different days, usually at both wavelengths, to insure accuracy in our results. We also acquired data close in time to nearby SAGE 11. sampling, and twice took data simultaneously with GMCC's ruby lidar.
For global measurements of the wind field in the troposphere and lower stratosphere satellite-borne Doppler lidar sensors have been proposed (ALAI)IN: Atmospheric LAser Doppler INstrument, LAWS: Laser Atmospheric Wind Sounder). The design of the system components requires information about the coherent backscatter values at the wavelength of CO2 lasers. Experimental results of coherent backscatter measured by existing ground-based and airborne Doppler lidars are collected in the Backscatter Experiment (GLOBE). It is desirable to cover large altitude ranges in different geographical regions and during different meteorological conditions. For contribution to GLOBE, aerosol backscatter profiles have been measured From a mountain position using the CO, Laser Doppler Anemometer and the Nd:YAG Microlidar.
An active/passive CO2 laser radar target acquisition sensor has been developed through cooperation of the United States Army, Navy and Air Force. The Tri-Service Laser Radar (TSLR) produces pixel-registered range, velocity, active signal intensity and passive thermal emission data. Fusion of the active and passive information allows for improved characterization and discrimination between manmade objects and natural background clutter. Simple feature synergy of the pixel-registered active and passive infor-mation also provides robust cues for wire detection algorithms.
This paper describes an experimental integrated optronic system for detection and tracking of moving objects. The system is based on a CO2 waveguide laser Doppler ra-dar with homodyne receiver and galvanometer mirror beam scanner. A "hot spot" seeker consisting of a thermal imager with image processor transmits the coordinates of IR-emitting, i.e. potentially powered, objects to the laser radar scanner. The scanner addresses these "hot" locations operating in a large field-of-view (FOV) random ac-cess mode. Hot spots exhibiting a Doppler shifted laser signal are indicated in the thermal image by velocity-to-colour encoded markers. After switching to a small FOV scanning mode, the laser Doppler radar is used to track fast moving objects. Labora-tory and field experiments with moving objects including rotating discs, automobiles and missiles are described.
This paper describes a method how to classify targets - especially helicopters - by evaluating their vibration signatures. The mechanical vibrations of the targets are mainly caused by movements of engines, turbines, gears, etc.. Different types of targets have different vibration spectra which were measured in fieldtrials by an experimental heterodyne laser radar. The signals show the type characteristic vibration signatures which were evaluated by mathematical algorithms. Therefore classification of non-cooperative targets is feasible.
This paper discusses the influence of fast scanning motions on the performance of laser Doppler radars and presents some experimental results obtained with a CO2 homo-dyne system. The experiments show in which way the signal returns from a moving reference target (a rotating disc) and its stationary surroundings depend on the angular frequency of a fast rotating mirror slewing the laser beam around.
In scanning laser radar systems, the question of trade off between coverage rate and other system parameters like detection probability often arises. This is especially true for pulsed systems , as the risk of missing the target is greater than for CW-systems. Beam shaping e.g. in the form of a fan beam 1 is a convenient way of reducing the scan rate while still maintaining a high frame rate and a wide field of view. In this case a linear detector array matches the beam footprint. For extended targets the coverage rate and the angular resolution often dictates the beam size. For point targets the coverage rate and the detection probability and false alarm rates are driving the beam choice. This paper will discuss how the choice of beam size and shape as well as scan rate will affect the performance of a scanning coherent laser radar looking for point targets. Different target signal statistics and atmospheric effects are taken into account.