The increasing emphasis on military missions in littoral areas has created a need for better real-time meteorological and oceanographic (METOC) data. A micro-weather station (MWS) program was initiated to develop a miniature, low- cost, covert, and autonomous platform to provide continuous, high-accuracy METOC data from restricted areas. Microsensors, and in particular micro-electromechanical systems, offer the size, cost, and performance needs of the MWS. However, there are special technical challenges involved in collecting accurate METOC data with microsensors during military missions. These challenges involve the ability of microsystems to acquire valid data at the air-sea and air-land interfaces where the METOC parameters are distorted by boundary layer effects, local structures, and contamination. Microsensors are particularly prone to fouling due to the micrometer scale of the transducers. Microsensors can be protected from the harsh environment is several ways, including placing the transducer behind a protective film, selective sampling of the environment, and using microremote sensors such as micro-lidar.
A prototype compact off-axis reflective lidar telescope has been designed and fabricated for remote sensing of atmospheric winds from space and airborne platforms. The 250 mm aperture telescope consists of two mirrors and a collimating lens to achieve a very compact size, without any central obscuration.It has no internal focal point to prevent air breakdown, and the pupil relay optics has also been eliminated. This paper presents the results of optical design and sensitivity analysis along with the predicted performance. The major design issues for lidar systems, particularly the one that utilizes coherent detection for higher sensitivity and Doppler frequency extraction, are the wavefront quality, polarization purity, and a minimum backscattering off the reflective surfaces. These design issues along with the other optical characteristics of this lidar telescope are presented. The effect on the wavefront quality of the tilt, decentration and axial spacing tolerances for the mirrors, collimating lens and quarter wave plate is discussed.
Lidar can be an effective tool for measuring the optical extinction along atmospheric paths. The Penn State University LAMP lidar recently participated in a series of atmospheric measurements at NASA's Wallops Island test facility in September 1995. The LAMP lidar was operated with the beam pointed along horizontal and vertical paths. This paper discusses the determination of atmospheric extinction coefficients with Raman lidar measurements. Lidar measurements show the presence of aerosol layers in the atmosphere as a marked departure from the expected molecular profile for a Raman lidar signal. The horizontal extinction coefficient can be determined directly from the range corrected slope of a horizontal Raman profile. The vertical extinction coefficient can be determined directly from the range corrected slope of a horizontal Raman profile. The vertical extinction coefficient can be determined by comparing the gradient of the Raman lidar profile with the gradient of the molecular atmosphere. The LAMP lidar has also been used with a bistatic receiver to measure the scattering phase function which can then be used to calculate the aerosol particle size distribution and the optical extinction coefficient. This paper will discuss the experimental method and present several representative examples from Raman lidar measurements. The extinction coefficients determined form the Raman lidar data will then be compared with the extinction coefficients determined from the bistatic receiver data.
The performance of the lidar atmospheric profile sensor (LAPS) instrument for temperature measurements in the lower troposphere has been investigated. LAPS is an automated lidar system that measures temperature from the rotational Raman return of atmospheric nitrogen and oxygen. We present the measurement technique, the data analysis and the performance of the LAPS instrument. Comparisons to radiosonde profiles are discussed.
Atmospheric turbulence adversely affects operation of commercial and military aircraft and is a design constraint. The airplane structure must be designed to survive the loads imposed by turbulence. Reducing these loads allows the airplane structure to be lighter, a substantial advantage for a commercial airplane. Gust alleviation systems based on accelerometers mounted in the airplane can reduce the maximum gust loads by a small fraction. These systems still represent an economic advantage. The ability to reduce the gust load increases tremendously if the turbulent gust can be measured before the airplane encounters it. A lidar system can make measurements of turbulent gusts ahead of the airplane, and the NASA Airborne Coherent Lidar for Advanced In-Flight Measurements (ACLAIM) program is developing such a lidar. The ACLAIM program is intended to develop a prototype lidar system for use in feasibility testing of gust load alleviation systems and other airborne lidar applications, to define applications of lidar with the potential for improving airplane performance, and to determine the feasibility and benefits of these applications. This paper gives an overview of the ACLAIM program, describes the lidar architecture for a gust alleviation system, and describes the prototype ACLAIM lidar system.
Solid-state coherent Doppler lidar sensors operating at the eyesafe 2 micrometers wavelength have experienced rapid development over the past five years. Several ground-based and airborne systems have been successfully demonstrated. CTI is currently making significant strides toward the development of an autonomous, modest-cost wind field sensor for boundary-layer profiling. High spatial resolution 3D coverage for several kilometers around the lidar location will be possible at update rates of about a minute-- depending on the scan volume and grid size. This paper summarizes the results of detailed sensor performance modeling for the boundary layer profiler and discusses preliminary scan concepts and issues.
Under NASA sponsorship, Coherent Technologies, Inc. (CTI) has designed and built the transceiver, and is developing the scanner, for an airborne scanning optical wind sensor. A scanning, single-aperture architecture was chosen for the CTI/NASA Optical Air Data System. Techniques for vector wind estimation form LOS scalar velocity measurements, the choice of scan patterns and wind models for various applications, and various other considerations that led to this decision are discussed within. Estimating wind vectors requires taking multiple scalar velocity projections along non- coplanar lines of slight. THis can be done from several apertures to the same field point, or vice versa, and may involve either fixed or scanned beams. For a scanning, interpolative systems, the choice of scan pattern and wind model are intimately related. Typically, more complicated models require more intricate scans to separate the fit parameters. Vector wind estimation error can arise from a variety of sources. Several effects can contribute to LOS velocity measurement noise, some of which stem form the scan itself. Inaccuracies in the scan deflection vector can also introduce error. Error can enter if the wind field model is not sufficiently sophisticated to account for small-scale turbulence. Finally, a surface-flux measurement technique is introduced, which promises to be less sensitive to noise and turbulence than wind vector estimation.
Broadband IR high spectral resolution observations of atmospheric emission provide key meteorological information related to atmospheric state parameters, cloud and surface spectral properties, and processes influencing radiative budgets and regional climate. Fourier transform spectroscopy (FTS), or Michelson interferometry, has proven to be an exceptionally effective approach for making these IR spectral observations with the high radiometric accuracy necessary for weather and climate applications, and are currently developing a new airborne instrument for use on an unmanned aerospace vehicle (UAV). These include the high- resolution interferometer sounder aircraft instrument developed for the NASA high altitude ER2, the atmospheric emitted radiance interferometer (AERI) and the new AERI-UAV for application in the DOE atmospheric radiation measurement program. This paper focuses on the design of the AERI-UAV which is novel in many respects. The efforts will help speed the day when this valuable instrumentation is used to improve remote sensing and radiative budget observations from space.
A motor driven two-axis mount combined with PC-based solar position and correction software provides highly accurate tracking of the sun. The system includes ta two-axis mount driven by servo motors with optical encoder position indication; servo amplifiers; a personal computer equipped with a two-axis motor controller; software for calculating solar position; and error correction software. The optical encoders have a resolution of 0.1 arcseconds per step, and the solar position software agrees to within 1.25 arcminutes with US Naval Observatory calculations of solar position. The error correction software applies linear regression via singular value decomposition to a series of manual tracking corrections. The regression creates a best-fit compensation for misalignment of the mount. Tests with simulated random correction errors show that the correction algorithm can achieve accuracy within two times the standard deviation of the manual correction errors. This accuracy was maintained following final manual correction for test periods as long as 54 hours. Various tracking options allow the user to scan repeatedly across the solar disc, track a position offset from the sun, or reflect the solar image into another instrument.
Misaligned interferometer cavity optics introduce into the retrieved spectra multiplicative noise terms. Low frequency or static tilts convey a moderate impact on system performance and are manifested as amplitude distortion of the reconstructed spectrum, through a reduction of the instrument line shape function, while dynamic tilts convey a strong impact on system performance manifested as radiometric errors through a multiplicative noise term. Dynamic alignment which is an electromechanical feedback control technique of the interferometer cavity optics eliminates some extremely exacting design and fabrication requirements and provides safety margin for thermally or vibrationally induced structural distortions. Employing both coherent and incoherent radiation, several optical or electro-optical sub aperture and full aperture measurement techniques have been employed to determine the static and dynamic alignment characteristics of a customized brassboard Bomem, Inc. instrument. The unit evaluated was configured as an adaptation of their commercial DA-2 unit and employed galvanometric alignment actuators with amplitude reduction mechanical linkages.
The accuracy of global weather forecasts is limited by the vertical resolution and accuracy of the HIRS. Higher spectral resolution sounders with 1000-1500 channels are under development but are unlikely to be operational until 2007. A concept has been developed by ITT A/CD and the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin/Madison that can provide high resolution sounding much earlier. This concept involves adding a longwave Fourier transform spectrometer to the current HIRS/3. This can be done in a low risk and low cost way that will provide advanced sounding data products without impacting the existing operational weather system capabilities.
More accurate weather forecasting requires improvements in vertical temperature profiling.T He increase in vertical temperature resolution along with wind distributions will provide important information on vertical motion fields for Mesoscale weather predictions. Such information is particularly valuable for short-term forecasts and storm tracking. New techniques beyond what can be achieved with the current filter wheel sounders are required. A Michelson interferometer is proposed for the next generation of GOES Sounders. The interferometric spectrometer will greatly increase the spectral resolution of the sounder over the filter wheel instruments, improving its ability to measure temperature and water vapor profiles. This paper describes the current baseline design for the interferometer-equipped GOES Sounder, known as the GOES High-Resolution Interferometric Sounder.
The end of the cold war has seen a shift in emphasis of Navy operations from a 'blue water' strategic role to a 'brown water' coastal role. The more changeable and complex coastal environment plus the need to operate first on scene in unfamiliar areas creates a need for environmental data which is best filled by remote sensing. This paper summarizes some of the Navy's most important and enduring environmental knowledge needs. Remote sensing from satellites, aircraft and ships will provide some of this environmental knowledge.
In addition 1:0common usage in atmospheric trace species measurements, the CO laser-based differential absorption (DIAL) and differenüal scattering (DISC) lidar sensor is also an extremely powerful tool for the remote detection, localization, identification and quantification of chemical vapors and liquids composed of the organophosphates that are used in insecticides and in deadly chemical warfare nerve agents such as Satin. The authors have designed, fabricated, and tested a C02 DIAL/DISC lidar sensor system that was optimized for the broad, but distinct spectral features, required for organophosphate detection. This paper describes the system, the field tests that were conducted, test results, and data analysis. Spectral pattern recognition techniques were used to obtain a receiver operating characteristic which relates the probability of detection and false a]ann to concentration-path-length products.
Key Words: carbon dioxide, lidar, DIAL, DISC, aerosols, chemical agents, pollution