Following on the success of the NASA-CNES CALIPSO/CALIOP space-based cloud-aerosol lidar, which is now approaching its 14th year of continuous operation, NASA Langley Research Center has been advancing space-based lidar mission concepts and related technologies to address future cloud, aerosol and ocean science objectives identified in the 2017 Decadal Survey for Earth Science and Applications from Space. Recently, scientists at NASA Langley have been able to use data from CALIOP to retrieve global ocean biomass, which is a measurement that was never originally envisioned for this instrument, and represents a new scientific frontier for space-based lidar. CALIOP’s measurements complement ocean color records, extending ocean retrievals into nighttime and to high latitudes, and creating a more complete picture of global ocean biomass. Due to detector bandwidth limitations, however, CALIOP, as with ocean color, only obtains column-averaged measurements that are biased towards the surface. To address this limitation, Politecnico di Milano is developing a lidar detector based on a single photon avalanche diode (SPAD) array. This detector is expected to achieve a depth resolution of <1 m in the ocean, and have a detection efficiency and dynamic range that far exceeds that of the analog-readout, PMT-based receiver on CALIOP. Naturally, these same characteristics will also benefit cloud and aerosol retrievals, making SPAD arrays an excellent detector candidate for future space-based atmosphere and ocean lidar missions. Here, we describe potential advantages of fast (~THz) photon counting SPAD arrays, and how they are being developed for this application.
High Spectral Resolution Lidar (HSRL) is typically realized using an absorption filter to separate molecular returns from particulate returns. NASA Langley Research Center (LaRC) has designed and built a Pressure-Tuned Wide-Angle Michelson Interferometer (PTWAMI) as an alternate means to separate the two types of atmospheric returns. While absorption filters only work at certain wavelengths and suffer from low photon efficiency due to light absorption, an interferometric spectral filter can be designed for any wavelength and transmits nearly all incident photons. The interferometers developed at LaRC employ an air spacer in one arm, and a solid glass spacer in the other. Field widening is achieved by specific design and selection of the lengths and refractive indices of these two arms. The principal challenge in using such an interferometer as a spectral filter for HSRL aboard aircraft is that variations in glass temperature and air pressure cause changes in the interferometer’s optical path difference. Therefore, a tuning mechanism is needed to actively accommodate for these changes. The pressure-tuning mechanism employed here relies on changing the pressure in an enclosed, air-filled arm of the interferometer to change the arm’s optical path length. However, tuning using pressure will not adjust for tilt, mirror warpage, or thermally induced wavefront error, so the structural, thermal, and optical behavior of the device must be well understood and optimized in the design and manufacturing process. The PTWAMI has been characterized for particulate transmission ratio, wavefront error, and tilt, and shows acceptable performance for use in an HSRL instrument.
An integrated Structural-Thermal-Optical-Performance (STOP) model was developed for a field-widened Michelson interferometer which is being built and tested for the High Spectral Resolution Lidar (HSRL) project at NASA Langley Research Center (LaRC). The performance of the interferometer is highly sensitive to thermal expansion, changes in refractive index with temperature, temperature gradients, and deformation due to mounting stresses. Hand calculations can only predict system performance for uniform temperature changes, under the assumption that coefficient of thermal expansion (CTE) mismatch effects are negligible. An integrated STOP model was developed to investigate the effects of design modifications on the performance of the interferometer in detail, including CTE mismatch, and other threedimensional effects. The model will be used to improve the design for a future spaceflight version of the interferometer. The STOP model was developed using the Comet SimApp™ Authoring Workspace which performs automated integration between Pro-Engineer®, Thermal Desktop®, MSC Nastran™, SigFit™, Code V™, and MATLAB®. This is the first flight project for which LaRC has utilized Comet, and it allows a larger trade space to be studied in a shorter time than would be possible in a traditional STOP analysis. This paper describes the development of the STOP model, presents a comparison of STOP results for simple cases with hand calculations, and presents results of the correlation effort to bench-top testing of the interferometer. A trade study conducted with the STOP model which demonstrates a few simple design changes that can improve the performance seen in the lab is also presented.
The NASA Earth Observing System Simulators Suite (NEOS3) is a modular framework of forward simulations tools for remote sensing of Earth’s Atmosphere from space. It was initiated as the Instrument Simulator Suite for Atmospheric
Remote Sensing (ISSARS) under the NASA Advanced Information Systems Technology (AIST) program of the Earth
Science Technology Office (ESTO) to enable science users to perform simulations based on advanced atmospheric and
simple land surface models, and to rapidly integrate in a broad framework any experimental or innovative tools that they
may have developed in this context. The name was changed to NEOS3 when the project was expanded to include more advanced modeling tools for the surface contributions, accounting for scattering and emission properties of layered
surface (e.g., soil moisture, vegetation, snow and ice, subsurface layers). NEOS3 relies on a web-based graphic user
interface, and a three-stage processing strategy to generate simulated measurements. The user has full control over a
wide range of customizations both in terms of a priori assumptions and in terms of specific solvers or models used to
calculate the measured signals.This presentation will demonstrate the general architecture, the configuration procedures
and illustrate some sample products and the fundamental interface requirements for modules candidate for integration.
We use combined multi-year measurements from the surface and space for assessing the spatial and temporal distribution of aerosol properties within a large (~400x400 km) region centered on Cape Cod, Massachusetts, along the East Coast of the United States. The ground-based Aerosol Robotic Network (AERONET) measurements at Martha’s Vineyard Coastal Observatory (MVCO) site and Moderate Resolution Imaging Spectrometer (MODIS) sensors on board the Terra and Aqua satellites provide horizontal and temporal variations of aerosol optical depth, while the Cloud- Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) offers the altitudes of aerosol-layers. The combined ground-based and satellite measurements indicated several interesting features among which were the large differences in the aerosol properties observed in July and February. We applied the climatology of aerosol properties for designing the Two-Column Aerosol Project (TCAP), which is supported by the U.S. Department of Energy’s (DOE’s) Atmospheric Radiation Measurement (ARM) Program. The TCAP field campaign involves 12-month deployment (started July 1, 2012) of the ground-based ARM Mobile Facility (AMF) and Mobile Aerosol Observing System (MAOS) on Cape Cod and complimentary aerosol observations from two research aircraft: the DOE Gulfstream-1 (G-1) and the National Aeronautics and Space Administration (NASA) B200 King Air. Using results from the coordinated G-1 and B200 flights during the recent (July, 2012) Intensive Observation Period, we demonstrated that the G-1 in situ measurements and B200 active remote sensing can provide complementary information on the temporal and spatial changes of the aerosol properties off the coast of North America.
High spectral resolution lidars (HSRLs) designed for aerosol and cloud remote sensing are increasingly being deployed
on aircraft and called for on future space-based missions. The HSRL technique relies on spectral discrimination of the
atmospheric backscatter signals to enable independent, unambiguous retrieval of aerosol extinction and backscatter.
NASA Langley Research Center is developing a tilted pressure-tuned field-widened Michelson interferometer (FWMI)
to achieve the spectral discrimination for an HSRL system. The FWMI consists of a cubic beam splitter, a solid glass
arm, and a sealed air arm. The spacer that connects the air arm mirror to the main part of the interferometer is designed
to minimize thermal sensitivity. The pressure of the sealed air-arm air can be accurately controlled such that the
frequency of maximum interference can be tuned with great precision to the transmitted laser wavelength. In this paper,
the principle of the tilted pressure-tuned FWMI for HSRL is presented. The pressure tuning rate, the tilted angle
requirement and challenges in building the real instrument are discussed.
High spectral resolution lidars (HSRLs) are increasingly being deployed on aircraft and called for on future space-based
missions. The HSRL technique relies on spectral discrimination of the atmospheric backscatter signals to enable
independent, unambiguous retrieval of aerosol extinction and backscatter. A compact, monolithic field-widened
Michelson interferometer is being developed as the spectral discrimination filter for an HSRL system at NASA Langley
Research Center. The interferometer consists of a cubic beam splitter, a solid glass arm, and an air arm. The spacer that
connects the air arm mirror to the main part of the interferometer is designed to optimize thermal compensation such that
the maximum interference can be tuned with great precision to the transmitted laser wavelength. In this paper, a
comprehensive radiometric model for the field-widened Michelson interferometeric spectral filter is presented. The
model incorporates the angular distribution and finite cross sectional area of the light source, reflectance of all surfaces,
loss of absorption, and lack of parallelism between the air-arm and solid arm, etc. The model can be used to assess the
performance of the interferometer and thus it is a useful tool to evaluate performance budgets and to set optical
specifications for new designs of the same basic interferometer type.
One month of MFRSR data collected at two sites in the central California (USA) region during the CARES campaign
are processed and the MFRSR-derived AODs at 500 nm wavelength are compared with available AODs provided by
AERONET measurements. We find that the MFRSR and AERONET AODs are small (~0.05) and comparable. A
reasonable quantitative agreement between column aerosol size distributions (up to 2 μm) from the MFRSR and
AERONET retrievals is illustrated as well. Analysis of the retrieved (MFRSR and AERONET) and in situ measured
aerosol size distributions suggests that the contribution of the coarse mode to aerosol optical properties is substantial for
several days. The results of a radiative closure experiment performed for the two sites and one-month period show a
favorable agreement between the calculated and measured broadband downwelling irradiances (bias does not exceed
about 3 Wm-2), and thus imply that the MFRSR-derived aerosol optical properties are reasonable.
High spectral resolution lidars (HSRLs) designed for aerosol and cloud remote sensing are increasingly being deployed
on aircraft and called for on future space-based missions. The HSRL technique relies on spectral discrimination of the
atmospheric backscatter signals to enable independent, unambiguous retrieval of aerosol extinction and backscatter. A
compact, monolithic field-widened Michelson interferometer is being developed as the spectral discrimination filter for
an HSRL system at NASA Langley Research Center. The Michelson interferometer consists of a cubic beam splitter, a
solid glass arm, and an air arm. The spacer that connects the air arm mirror to the main part of the interferometer is
designed to optimize thermal compensation such that the frequency of maximum interference can be tuned with great
precision to the transmitted laser wavelength. In this paper, a comprehensive radiometric model for the field-widened
Michelson interferometeric spectral filter is presented. The model incorporates the angular distribution and finite cross
sectional area of the light source, reflectance of all surfaces, loss of absorption, and lack of parallelism between the airarm
and solid arm, etc. The model can be used to assess the performance of the interferometer and thus it is a useful tool
to evaluate performance budgets and to set optical specifications for new designs of the same basic interferometer type.
High spectral resolution lidars (HSRLs) have recently shown great value in aerosol measurements form
aircraft and are being called for in future space-based aerosol remote sensing applications. A quasi-monolithic
field-widened, off-axis Michelson interferometer had been developed as the spectral discrimination filter for
an HSRL currently under development at NASA Langley Research Center (LaRC). The Michelson filter
consists of a cubic beam splitter, a solid arm and an air arm. The input light is injected at 1.5° off-axis to
provide two output channels: standard Michelson output and the reflected complementary signal. Piezo packs
connect the air arm mirror to the main part of the filter that allows it to be tuned within a small range. In this
paper, analyses of the throughput wavephase, locking error, AR coating, and tilt angle of the interferometer are
described. The transmission ratio for monochromatic light at the transmitted wavelength is used as a figure of merit for
assessing each of these parameters.
We outline a new method, called the ratio method, developed to retrieve aerosol optical depth (AOD) under broken
cloud conditions and present validation results from sensitivity and case studies. Results of the sensitivity study
demonstrate that the ratio method, which exploits ratios of reflectances in the visible spectral range, has the potential for
accurate AOD retrievals under different observational conditions and random errors in input data. Also, we examine the
performance of the ratio method using aircraft data collected during the Cloud and Land Surface Interaction Campaign
(CLASIC) and the Cumulus Humilis Aerosol Processing Study (CHAPS). Results of the case study suggest that the ratio
method has the ability to retrieve AOD from multi-spectral aircraft observations of the reflected solar radiation.
Long-term measurements of the global distributions of clouds, trace gases, and surface reflectance are needed for
the study and monitoring of global change and air quality. The Geostationary Imaging Fabry-Perot Spectrometer
(GIFS) instrument is an example of a next-generation satellite remote sensing concept. GIFS is designed
to be deployed on a geostationary satellite, where it can make continuous hemispheric imaging observations of
cloud properties (including cloud top pressure, optical depth, and fraction), trace gas concentrations, such as tropospheric
and boundary layer CO, and surface reflectance and pressure. These measurements can be made with
spatial resolution, accuracy, and revisit time suitable for monitoring applications. It uses an innovative tunable
imaging triple-etalon Fabry-Perot interferometer to obtain very high-resolution line-resolved spectral images of
backscattered solar radiation, which contains cloud and trace gas information. An airborne GIFS prototype and
the measurement technique have been successfully demonstrated in a recent field campaign onboard the NASA
P3B based at Wallops Island, Virginia. In this paper, we present the preliminary GIFS instrument design and
use GIFS prototype measurements to demonstrate the instrument functionality and measurement capabilities.
The NASA Langley Research Center and the NASA Goddard Space Flight Center, have collaborated to design, build and fly a combination backscatter and Differential Absorption Lidar (DIAL) instrument for the measurement of aerosols, temperature and ozone from the NASA DC-8. The AROTAL (Airborne Raman Ozone Temperature and Aerosol Lidar) instrument was flown on two separate Arctic missions to look at ozone loss processes during the late winter-early spring, and to validate measurements made by the SAGE III satellite instrument. Results from this instrument have demonstrated that the SAGE III instrument is in agreement with the lidar retrievals to better than ten per cent.
Recent assessments of global climate change conclude that the radiative effect of aerosols is one of the largest uncertainties in our ability to predict future climate change. A myriad of new sensors and satellite missions are being designed to address this major question confronting credible prediction of climate change. The NASA Langley Airborne A-Band Spectrometer (LAABS) is a recently developed aircraft instrument that provides high spectral resolution (~0.03 nm) radiance measurements of reflected sunlight over the oxygen A-band spectral region centered near 765 nm. High resolution O2 A-band spectrometry of reflected sunlight is a promising new approach for remote sensing of aerosol and cloud optical properties. While the LAABS instrument provides valuable data on a stand-alone basis, greater scientific return may be realized by combining the A-band spectra with coincident lidar measurements that supply additional information on the vertical distribution of the aerosol. In particular, an instrument suite that combines LAABS with the new airborne High Spectral Resolution Lidar (HSRL) has the potential to provide a comprehensive suite of aerosol and cloud optical property measurements never before achieved. In this paper, we investigate the combined use of LAABS and HSRL measurements to infer aerosol single scatter albedo. We explore the information content of the O2 A-band reflectance spectra and, in particular, the advantages offered by high resolution A-band spectrometers such as LAABS. The approach for combined LAABS/HSRL retrievals is described and results from simulation studies are presented to illustrate their potential for retrieval of single scatter albedo.
The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite will be launched in April of 2005, and will make continuous measurements of the Earth's atmosphere for the following three years. Retrieving the spatial and optical properties of clouds and aerosols from the CALIPSO lidar backscatter data will be confronted by a number of difficulties that are not faced in the analysis of ground-based data. Among these are the very large distance from the target, the high speed at which the satellite traverses the ground track, and the ensuing low signal-to-noise ratios that result from the mass and power restrictions imposed on space-based platforms. In this work we describe an integrated analysis scheme that employs a nested, multi-grid averaging technique designed to optimize tradeoffs between spatial resolution and signal-to-noise ratio. We present an overview of the three fundamental retrieval algorithms (boundary location, feature classification, and optical properties analysis), and illustrate their interconnections using data product examples that include feature top and base altitudes, feature type (i.e., cloud or aerosol), and layer optical depths.
The Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) is the primary instrument on the CALIPSO satellite, which is scheduled to launch in 2005. CALIOP will provide profiles of total backscatter at two wavelengths, from which aerosol and cloud profiles will be derived. The instrument also measures the linear depolarization of the backscattered return, allowing discrimination of cloud phase and the identification of the presence of non-spherical aerosols. CALIOP is complete and has been tested in a ground-based configuration. This paper provides information on basic characteristics and performance of CALIOP.
We have designed and built two versions of a space-qualifiable, single-frequency Nd:YAG laser. Our approach to frequency stabilization of the seeded oscillator is a variation of the “ramp and fire” technique. In this design, the length of the pulsed laser cavity is periodically varied until a resonance with the seed laser is optically detected. At that point the pulsed laser is fired, ensuring that it is in resonance with the seed laser. For one of the lasers the resulting single frequency pulses are amplified and frequency tripled. This system operates at 50 Hz and provides over 50 mJ/pulse of single-frequency 355 nm output. It has been integrated into the GLOW (Goddard Lidar Observatory for Winds) mobile Doppler lidar system for field testing. The second laser is a 20o Hz oscillator only system that is frequency doubled for use in the High Spectral Resolution Lidar (HSRL) system being built at NASA Langley Research Center. It provides 4 mJ of single-frequency 532 nm output that has a spectral purity of >10,000. In this paper we describe the design details, environmental testing, and integration of these lasers into their respective lidar systems.
12 Atmospheric remote sensing with the O2 A-band has a relatively long history, but most of these studies were attempting to estimate surface pressure or cloud-top pressure. Recent conceptual studies have demonstrated the potential of spaceborne high spectral resolution O2 A- band spectrometers for retrieval of aerosol and cloud optical properties. The physical rationale of this new approach is that information on the scattering properties of the atmosphere is embedded in the detailed line structure of the O2 A-band reflected radiance spectrum. The key to extracting this information is to measure the radiance spectrum at very high spectral resolution. Instrument performance requirement studies indicate that, in addition to high spectral resolution, the successful retrieval of aerosol and cloud properties from A-band radiance spectra will also require high radiometric accuracy, instrument stability, and high signal-to-noise measurements. To experimentally assess the capabilities of this promising new remote sensing application, the NASA Langley Research Center is developing an airborne high spectral resolution A-band spectrometer. The spectrometer uses a plane holographic grating with a folded Littrow geometry to achieve high spectral resolution (0.5 cm-1 and low stray light in a compact package. This instrument will be flown in a series of field campaigns beginning in 2001 to evaluate the overall feasibility of this new technique. Results from these campaigns should be particularly valuable for future spaceborne applications of A-band spectrometers for aerosol and cloud retrievals.
Recently NASA Langley Research Center's (LaRC) Aerosol Research Branch conducted an aircraft exhaust particle experiment involving tow ground based lidar systems and NASA's B737-100, T39 and OV10 aircraft. The experiment took place at LaRC in February and March of 1996. During flight, exhaust particles exiting the two wing-mounted engines of the B737 become quickly entrained into the aircraft's wingtip vortices. The LaRC lidar systems were used to measure the distribution and optical properties of these exhaust particles as the B737 overflew the lidar facility. Two lidar systems, located in a common facility, were utilized for this experiment. One system was a fixed zenith- viewing lidar with a 48-inch receiver and a 2J transmitter, and the second was a scanning lidar with a 14-inch receiver and a 600 mJ transmitter. Two measurement geometries were employed for the experiment. In the first geometry, the B737 flew upwind of the lidar facility and perpendicular to the ambient wind. The second design had the aircraft fly directly over the facility, and parallel to the ambient wind.Under either scenario data were acquired at 20 and 30 Hertz, by the fixed zenith and scanning system respectively, as the ambient wind carried the vortex pair across the field of view of the lidars. The two supporting aircraft were used to collect in-situ particle data and to measure atmospheric turbulence, respectively. In this paper all aspects of the experiment will be discussed including the lidar systems, the geometry of the experiment, and the aircraft used. Also, selected data obtained during the experiment will be presented.
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