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Angal A.,Science Systems And Applications Inc. | Xiong X.,NASA | Wu A.,Sigma Space | Chander G.,SGT Inc. | Choi T.,Sigma Space
IEEE Transactions on Geoscience and Remote Sensing | Year: 2013

In recent years, there has been a significant increase in the use of remotely sensed data to address global issues. With the open data policy, the data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Enhanced Thematic Mapper Plus (ETM+) sensors have become a critical component of numerous applications. These two sensors have been operational for more than a decade, providing a rich archive of multispectral imagery for analysis of mutitemporal remote sensing data. This paper focuses on evaluating the radiometric calibration agreement between MODIS and ETM+ using the near-simultaneous and cloud-free image pairs over an African pseudo-invariant calibration site, Libya 4. To account for the combined uncertainties in the top-of-atmosphere (TOA) reflectance due to surface and atmospheric bidirectional reflectance distribution function (BRDF), a semiempirical BRDF model was adopted to normalize the TOA reflectance to the same illumination and viewing geometry. In addition, the spectra from the Earth Observing-1 (EO-1) Hyperion were used to compute spectral corrections between the corresponding MODIS and ETM+ spectral bands. As EO-1 Hyperion scenes were not available for all MODIS and ETM+ data pairs, MODerate resolution atmospheric TRANsmission (MODTRAN) 5.0 simulations were also used to adjust for differences due to the presence or lack of absorption features in some of the bands. A MODIS split-window algorithm provides the atmospheric water vapor column abundance during the overpasses for the MODTRAN simulations. Additionally, the column atmospheric water vapor content during the overpass was retrieved using the MODIS precipitable water vapor product. After performing these adjustments, the radiometric cross-calibration of the two sensors was consistent to within 7%. Some drifts in the response of the bands are evident, with MODIS band 3 being the largest of about 6% over 10 years, a change that will be corrected in Collection 6 MODIS processing. © 1980-2012 IEEE.

Xiong X.,NASA | Angal A.,Science Systems And Applications Inc. | Choi T.,Sigma Space | Sun J.,Sigma Space | Johnson E.,Raytheon Co.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

MODIS reflective solar bands (RSB) calibration is provided by an on-board solar diffuser (SD). On-orbit changes in the SD bi-directional reflectance factor (BRF) are tracked by a solar diffuser stability monitor (SDSM). The SDSM consists of a solar integration sphere (SIS) with nine detectors covering wavelengths from 0.41 to 0.94 μm. It functions as a ratioing radiometer, making alternate observations of the sunlight through a fixed attenuation screen and the sunlight diffusely reflected from the SD during each scheduled SD/SDSM calibration event. Since launch, Terra and Aqua MODIS SD/SDSM systems have been operated regularly to support the RSB on-orbit calibration. This paper provides an overview of MODIS SDSM design functions, its operation and calibration strategies, and on-orbit performance. Changes in SDSM detector responses over time and their potential impact on tracking SD on-orbit degradation are examined. Also presented in this paper are lessons learned from MODIS SD/SDSM calibration system and improvements made to the VIIRS SD/SDSM system, including preliminary comparisons of MODIS and VIIRS SDSM on-orbit performance. © 2012 SPIE.

Mishchenko M.I.,NASA | Zakharova N.T.,Sigma Space | Videen G.,U.S. Army | Khlebtsov N.G.,Russian Academy of Sciences | Wriedt T.,IWT - Foundation Institute of Materials Engineering
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2010

The T-matrix method is among the most versatile, efficient, and widely used theoretical techniques for the numerically exact computation of electromagnetic scattering by homogeneous and composite particles, clusters of particles, discrete random media, and particles in the vicinity of an interface separating two half-spaces with different refractive indices. This paper presents an update to the comprehensive database of T-matrix publications compiled by us previously and includes the publications that appeared since 2007. It also lists several earlier publications not included in the original database.

Yao M.-S.,Sigma Space | Yao M.-S.,Trinnovim LLC | Cheng Y.,NASA
Journal of Climate | Year: 2012

The response of cloud simulations to turbulence parameterizations is studied systematically using the GISS general circulation model (GCM) E2 employed in the Intergovernmental Panel on Climate Change's (IPCC) FifthAssessment Report (AR5).Without the turbulence parameterization, the relative humidity (RH) and the low cloud cover peak unrealistically close to the surface; with the dry convection or with only the local turbulence parameterization, these two quantities improve their vertical structures, but the vertical transport of water vapor is still weak in the planetary boundary layers (PBLs); with both local and nonlocal turbulence parameterizations, the RH and low cloud cover have better vertical structures in all latitudes due to more significant vertical transport of water vapor in the PBL. The study also compares the cloud and radiation climatologies obtained from an experiment using a newer version of turbulence parameterization being developed at GISS with those obtained from the AR5 version. This newer scheme differs from the AR5 version in computing nonlocal transports, turbulent length scale, and PBL height and shows significant improvements in cloud and radiation simulations, especially over the subtropical eastern oceans and the southern oceans. The diagnosed PBL heights appear to correlate well with the low cloud distribution over oceans. This suggests that a cloudproducing scheme needs to be constructed in a framework that also takes the turbulence into consideration.

Del Genio A.D.,NASA | Chen Y.,Columbia University | Kim D.,Lamont Doherty Earth Observatory | Yao M.-S.,Sigma Space
Journal of Climate | Year: 2012

The relationship between convective penetration depth and tropospheric humidity is central to recent theories of the Madden-Julian oscillation (MJO). It has been suggested that general circulation models (GCMs) poorly simulate the MJO because they fail to gradually moisten the troposphere by shallow convection and simulate a slow transition to deep convection. CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data are analyzed to document the variability of convection depth and its relation to water vapor during the MJO transition from shallow to deep convection and to constrain GCM cumulus parameterizations. Composites of cloud occurrence for 10 MJO events show the following anticipated MJO cloud structure: shallow and congestus clouds in advance of the peak, deep clouds near the peak, and upper-level anvils after the peak. Cirrus clouds are also frequent in advance of the peak. The Advanced Microwave Scanning Radiometer for Earth Observing System (EOS) (AMSR-E) columnwater vapor (CWV) increases by~5 mmduring the shallow- deep transition phase, consistent with the idea of moisture preconditioning. Echo-top height of clouds rooted in the boundary layer increases sharply with CWV, with large variability in depth when CWV is between;46 and 68 mm. International Satellite Cloud Climatology Project cloud classifications reproduce these climatological relationships but correctly identify congestus-dominated scenes only about half the time. A version of the Goddard Institute for Space Studies Model E2 (GISS-E2) GCM with strengthened entrainment and rain evaporation that produces MJO-like variability also reproduces the shallow-deep convection transition, including the large variability of cloud-top height at intermediate CWV values. The variability is due to small grid-scale relative humidity and lapse rate anomalies for similar values of CWV.

Abshire J.B.,NASA | Riris H.,NASA | Allan G.R.,Sigma Space | Weaver C.J.,University of Maryland Baltimore County | And 5 more authors.
Tellus, Series B: Chemical and Physical Meteorology | Year: 2010

We report initial measurements of atmospheric CO2 column density using a pulsed airborne lidar operating at 1572 nm. It uses a lidar measurement technique being developed at NASA Goddard Space Flight Center as a candidate for the CO2 measurement in the Active Sensing of CO2 Emissions over Nights, Days and Seasons (ASCENDS) space mission. The pulsed multiple-wavelength lidar approach offers several new capabilities with respect to passive spectrometer and other lidar techniques for high-precision CO2 column density measurements. We developed an airborne lidar using a fibre laser transmitter and photon counting detector, and conducted initial measurements of the CO2 column absorption during flights over Oklahoma in December 2008. The results show clear CO2 line shape and absorption signals. These follow the expected changes with aircraft altitude from 1.5 to 7.1 km, and are in good agreement with column number density estimates calculated from nearly coincident airborne in-situ measurements. © 2010 The Authors Tellus B © 2010 International Meteorological Institute in Stockholm.

Del Genio A.D.,NASA | Barbara J.M.,Sigma Space
Icarus | Year: 2012

An automated cloud tracking algorithm is applied to Cassini Imaging Science Subsystem high-resolution apoapsis images of Saturn from 2005 and 2007 and moderate resolution images from 2011 and 2012 to define the near-global distribution of zonal winds and eddy momentum fluxes at the middle troposphere cloud level and in the upper troposphere haze. Improvements in the tracking algorithm combined with the greater feature contrast in the northern hemisphere during the approach to spring equinox allow for better rejection of erroneous wind vectors, a more objective assessment at any latitude of the quality of the mean zonal wind, and a population of winds comparable in size to that available for the much higher contrast atmosphere of Jupiter. Zonal winds at cloud level changed little between 2005 and 2007 at all latitudes sampled. Upper troposphere zonal winds derived from methane band images are ∼10ms -1 weaker than cloud level winds in the cores of eastward jets and ∼5ms -1 stronger on either side of the jet core, i.e., eastward jets appear to broaden with increasing altitude. In westward jet regions winds are approximately the same at both altitudes. Lateral eddy momentum fluxes are directed into eastward jet cores, including the strong equatorial jet, and away from westward jet cores and weaken with increasing altitude on the flanks of the eastward jets, consistent with the upward broadening of these jets. The conversion rate of eddy to mean zonal kinetic energy at the visible cloud level is larger in eastward jet regions (5.2×10 -5m 2s -3) and smaller in westward jet regions (1.6×10 -5m 2s -3) than the global mean value (4.1×10 -5m 2s -3). Overall the results are consistent with theories that suggest that the jets and the overturning meridional circulation at cloud level on Saturn are maintained at least in part by eddies due to instabilities of the large-scale flow near and/or below the cloud level. © 2012 .

Simmon R.,Sigma Space
34th International Symposium on Remote Sensing of Environment - The GEOSS Era: Towards Operational Environmental Monitoring | Year: 2011

Although data visualization is a powerful tool in Earth science, the resulting imagery is often complex and difficult to interpret for non-experts. Students, journalists, web site visitors, or museum attendees often have difficulty understanding false-color imagery and data-driven maps. Many visualizations are designed for data exploration or peer communication. Different techniques are necessary for communication with a broad audience. Data visualization combines ideas from cognitive science, graphic design, and cartography, and applies them to the challenge of presenting data clearly. Visualizers on NASA's Earth Observatory web site team use these techniques to craft images based on remote sensing data. Imagery of the eruption of Iceland's Eyjafjallajökull Volcano serves as a case study, showing specific applications of general design techniques. By using color carefully, precisely aligning disparate data sets, and highlighting important features, we crafted an image that clearly conveys the complex distribution of airborne ash.

News Article | May 22, 2012

Sigma Space Corporation produces pioneering instrumentation in lidar, laser ranging, altitude determination, spectroscopy, and radiometry for remote sensing and defense applications. We also proudly supply advanced science and engineering services to our government customers. Comprehensive capabilities: Sigma offers its customers integrated solutions - from requirements definition and design concept, to product development, integration and testing. Responsiveness: Our size and customer accessibility to all levels of the company allows us to provide a quick and flexible response to our customers' needs. Cost effectiveness: We have an efficient management structure with low overhead and streamlined project management. Sigma can provide technical services as needed at a customer's location or at our corporate facilities. Technical expertise: We have assembled a staff of talented managers, engineers, scientists, and technicians with a variety of backgrounds including: Program and risk management - Engineering (Optical, Electrical, Mechanical, Thermal, Systems) - Science research (multiple Earth and Space science disciplines) - Software engineering and development - Computer systems and network administration - Computer-aided manufacturing - Calibration and validation studies Investment in infrastructure: We are a company committed to growth through corporate investment. As a result, the corporate facilities that we have developed enable us to provide our customers with the flexible, "on-demand" design and manufacturing services necessary for their changing needs. Our facilities and equipment include state-of-the-art CAD tools, optical and electronic laboratories, computer-controlled machine shop, class 10K clean room, and fiber splicing and assembly areas.

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