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NASA NTRS2008

The On-Orbit Performance of the CALIOP Lidar on CALIPSO

CALIPSO is a joint NASA - CNES satellite currently in its third year of operation in low earth orbit. The satellite is making optical measurements of the Earth s atmosphere to help quantify the impact of aerosols and clouds on the Earth s radiation budget. To do this, it carries three instruments: CALIOP, a two-wavelength polarization-sensitive elastic backscatter lidar; the IIR a three band thermal imaging radiometer; and the WFC a visible single-band imager. CALIOP utilizes a Nd:YAG laser which incorporates a harmonic crystal to provide laser light at both 1064 nm and 532 nm. This beam is expanded and transmitted into the atmosphere at near nadir. The laser light scattered from clouds and aerosols back to the satellite, along with any solar background light, is collected by a one meter diameter beryllium telescope. The captured light is separated into its two wavelengths and optically filtered. The 1064 nm component is detected with an avalanche photodiode, while the 532 nm component is further resolved into two linear polarization components which are then detected by matching photomultiplier tubes. This presentation will describe the lidar and give examples of its on-orbit performance.

Winker, David↗
NASA NTRS1980

Atmospheric effects on radiometric imaging from satellites under low optical thickness conditions

The interference that the atmosphere poses to analyzing the imagery taken by satellite-borne instruments is discussed, assuming a cloud-free, planar, and horizontally uniform atmosphere. An approximate explicit formula is derived for the earth-atmosphere system nadir-beam reflectivity in terms of the atmospheric parameters, object pixel reflectivity and surrounding area reflectivity, for the limiting case of an optically thin atmosphere. The concepts of the forward-scattering and the backward-scattering optical thickness are introduced, and it is shown that the atmospheric effects in a spectral band depend in a specific fashion on these atmospheric parameters and on the surface spectral reflectivity. In addition, contrast transmittance through the atmosphere, which affects the possibilities of photointerpretation, is discussed.

Otterman, J.↗
NASA NTRS1985

On stereo viewing of SAR images

Stereo viewing is indispensable for the measurement of homologue points in two overlapping images and the subsequent reconstruction of the three-dimensional topographic relief. Aspects of stereo viewing geometry are discussed along with problems of radar stereoscopy, taking into account stereo evaluation with an exaggeration factor, the viewability of real images, and radar stereoscopic computations. Attention is given to the definition of radar stereoscopic exaggeration, questions of image simulation, and the accuracy obtained with parallax measurements in radar images. An evaluation with a large set of about 40 radar stereo models demonstrates that same-side arrangements provide good stereo viewability. This result was confirmed for aircraft radar with look angles off-nadir of 60-80 deg and intersection angles between 0.2 deg and 23 deg, and for satellite radar (Seasat) with look angles of 20 deg and interaction angles of 1.2 deg - 4.8 deg.

Leberl, F. W.↗
NASA NTRS1991

Optical design of the Moderate Resolution Imaging Spectrometer - Tilt (MODIS-T) for the Earth Observing System (Eos)

The Moderate Resolution Imaging Spectrometer (MODIS) is an Earth viewing sensor that is planned as a facility instrument for the Earth Observing System (Eos) scheduled to begin functioning in the late 1990's. The MODIS is composed of two mutually supporting sensors one of which is MODIS-T, where 'T' signifies a tiltable along-track field of view. MODIS-T is a 32 channel imaging spectrometer with a required 10 nm to 15 nm spectral resolution (FWHM) in the 400 nm to 880 nm spectral range with less than 2.3 percent instrument induced linear polarization. The instrument provides at nadir a 33 km by 1500 km swath with a 1.1 km spatial resolution and an along-track pointing capability of +/- 50 deg about nadir. The heart of the optical design consists of a f/3 grating-type reflecting Schmidt camera.

Maymon, Peter W.↗
NASA NTRS2008

An Overview of the Hurricane Imaging Radiometer (HIRAD)

Accurate observations of ocean surface vector winds (OSVW) with high spatial and temporal resolution are critically important to improve both our understanding and predictability of tropical cyclones. As the successful NASA QuikSCAT satellite continues to age beyond its planned life span, many members of the tropical cyclone research and operational community recognize the need to develop new observational technologies and strategies to meet the essential need for OSVW information. This concern has been expressed in both the "Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond" developed by the National Research Council Committee on Earth Science and Applications from Space and the "Interagency Strategic Research Plan for Tropical Cyclone The Way Ahead" developed by the Joint Action Group for Tropical Cyclone Research (JAG-TCR) sponsored by the Office of the Federal Coordinator for Meteorology. One innovative technology development which offers the potential for new, unique remotely sensed observations of tropical cyclone OSVW and precipitation is the Hurricane Imaging Radiometer (HIRAD). This new instrument is passive microwave synthetic thinned aperture radiometer under development at the NASA Marshall Space Flight Center that will operate at the C-Band frequencies of 4, 5, 6, and 7 GHz. These frequencies have been successfully demonstrated by the NOAA nadir-staring Stepped Frequency Microwave Radiometer (SFMR) as useful for monitoring tropical cyclone ocean surface wind speeds and rain rates from low altitude reconnaissance aircraft. The HIRAD design incorporates a unique antenna design as well as several technologies that have been successfully demonstrated by the University of Michigan Lightweight Rain Radiometer sponsored by NASA Earth Science Technology Office Instrument Incubator Program. HIRAD will be a compact, lightweight, low-power instrument with no moving parts that will produce imagery of ocean wind surface wind parameters and rain rate during the strong wind and heavy rain hurricane conditions that hamper the observational capabilities of higher frequency passive microwave radiometers or scatterometers. It will also produce imagery of sea surface temperature under cloudy and lightly precipitating skies eliminating the need for additional thermal infrared imagers. The strategic plan for HIRAD includes a roadmap for ocean surface wind speed and OSVW technology development using flight demonstrations on piloted aircraft, uninhabited aerial vehicle systems, and satellite platforms. The roadmap will include exit opportunities for technology transfer from research to operations based on satisfactory demonstrations. The first aircraft version of HIRAD will be singular polarization sensor designed to observe ocean surface wind speed and rainfall. The second aircraft version of HIRAD will be dual polarization sensor designed to observe OSVW as a prototype for a future satellite sensor.

Source record↗
NASA NTRS2006

Simulating Visible/Infrared Imager Radiometer Suite Normalized Difference Vegetation Index Data Using Hyperion and MODIS

The success of MODIS (the Moderate Resolution Imaging Spectrometer) in creating unprecedented, timely, high-quality data for vegetation and other studies has created great anticipation for data from VIIRS (the Visible/Infrared Imager Radiometer Suite). VIIRS will be carried onboard the joint NASA/Department of Defense/National Oceanic and Atmospheric Administration NPP (NPOESS (National Polar-orbiting Operational Environmental Satellite System) Preparatory Project). Because the VIIRS instruments will have lower spatial resolution than the current MODIS instruments 400 m versus 250 m at nadir for the channels used to generate Normalized Difference Vegetation Index data, scientists need the answer to this question: how will the change in resolution affect vegetation studies? By using simulated VIIRS measurements, this question may be answered before the VIIRS instruments are deployed in space. Using simulated VIIRS products, the U.S. Department of Agriculture and other operational agencies can then modify their decision support systems appropriately in preparation for receipt of actual VIIRS data. VIIRS simulations and validations will be based on the ART (Application Research Toolbox), an integrated set of algorithms and models developed in MATLAB(Registerd TradeMark) that enables users to perform a suite of simulations and statistical trade studies on remote sensing systems. Specifically, the ART provides the capability to generate simulated multispectral image products, at various scales, from high spatial hyperspectral and/or multispectral image products. The ART uses acquired ( real ) or synthetic datasets, along with sensor specifications, to create simulated datasets. For existing multispectral sensor systems, the simulated data products are used for comparison, verification, and validation of the simulated system s actual products. VIIRS simulations will be performed using Hyperion and MODIS datasets. The hyperspectral and hyperspatial properties of Hyperion data will be used to produce simulated MODIS and VIIRS products. Hyperion-derived MODIS data will be compared with near-coincident MODIS collects to validate both spectral and spatial synthesis, which will ascertain the accuracy of converting from MODIS to VIIRS. MODIS-derived VIIRS data is needed for global coverage and for the generation of time series for regional and global investigations. These types of simulations will have errors associated with aliasing for some scene types. This study will help quantify these errors and will identify cases where high-quality, MODIS-derived VIIRS data will be available.

Ross, Kenton W.↗
NASA NTRS2003

MRO's High Resolution Imaging Science Experiment (HiRISE): Science Expectations

The Mars Reconnaissance Orbiter (MRO) is expected to launch in August 2005, arrive at Mars in March 2006, and begin the primary science phase in November 2006. MRO will carry a suite of remote-sensing instruments and is designed to routinely point off-nadir to precisely target locations on Mars for high-resolution observations. The mission will have a much higher data return than any previous planetary mission, with 34 Tb of returned data expected in the first Mars year in the mapping orbit. The mapping orbit is nearly polar, 255 x 320 km above the surface, 12 orbits per day. The HiRISE camera, features a 0.5 m telescope, 12 m focal length, and 14 CCDs. Basic capabilities are summarized.

A McEwen↗
NASA NTRS1997

Utility of BRDF Models for Estimating Optimal View Angles in Classification of Remotely Sensed Images

Statistical classification of remotely sensed images attempts to discriminate between surface cover types on the basis of the spectral response recorded by a sensor. It is well known that surfaces reflect incident radiation as a function of wavelength producing a spectral signature specific to the material under investigation. Multispectral and hyperspectral sensors sample the spectral response over tens and even hundreds of wavelength bands to capture the variation of spectral response with wavelength. Classification algorithms then exploit these differences in spectral response to distinguish between materials of interest. Sensors of this type, however, collect detailed spectral information from one direction (usually nadir); consequently, do not consider the directional nature of reflectance potentially detectable at different sensor view angles. Improvements in sensor technology have resulted in remote sensing platforms capable of detecting reflected energy across wavelengths (spectral signatures) and from multiple view angles (angular signatures) in the fore and aft directions. Sensors of this type include: the moderate resolution imaging spectroradiometer (MODIS), the multiangle imaging spectroradiometer (MISR), and the airborne solid-state array spectroradiometer (ASAS). A goal of this paper, then, is to explore the utility of Bidirectional Reflectance Distribution Function (BRDF) models in the selection of optimal view angles for the classification of remotely sensed images by employing a strategy of searching for the maximum difference between surface BRDFs. After a brief discussion of directional reflect ante in Section 2, attention is directed to the Beard-Maxwell BRDF model and its use in predicting the bidirectional reflectance of a surface. The selection of optimal viewing angles is addressed in Section 3, followed by conclusions and future work in Section 4.

Valdez, P. F.↗
NASA NTRS2009

Assessment of the Short-Term Radiometric Stability between Terra MODIS and Landsat 7 ETM+ Sensors

The Landsat 7 (L7) Enhanced Thematic Mapper (ETM+) sensor was launched on April 15th, 1999 and has been in operation for over nine years. It has six reflective solar spectral bands located in the visible and shortwave infrared part of the electromagnetic spectrum (0.5 - 2.5 micron) at a spatial resolution of 30 m. The on-board calibrators are used to monitor the on-orbit sensor system changes. The ETM+ performs solar calibrations using on-board Full Aperture Solar Calibrator (FASC) and the Partial Aperture Solar Calibrator (PASC). The Internal Calibrator Lamp (IC) lamps, a blackbody and shutter optics constitute the on-orbit calibration mechanism for ETM+. On 31 May 2003, a malfunction of the scan-line corrector (SLC) mirror assembly resulted in the loss of approximately 22% of the normal scene area. The missing data affects most of the image with scan gaps varying in width from one pixel or less near the centre of the image to 14 pixels along the east and west edges of the image, creating a wedge-shaped pattern. However, the SLC failure has no impacts on the radiometric performance of the valid pixels. On December 18, 1999, the Moderate Resolution Imaging Spectroradiometer (MODIS) Proto-Flight Model (PFM) was launched on-board the NASA's EOS Terra spacecraft. Terra MODIS has 36 spectral bands with wavelengths ranging from 0.41 to 14.5 micron and collects data over a wide field of view angle (+/-55 deg) at three nadir spatial resolutions of 250 m, 500 in 1 km for bands 1 to 2, 3 to 7, and 8 to 36, respectively. It has 20 reflective solar bands (RSB) with spectral wavelengths from 0.41 to 2.1 micron. The RSB radiometric calibration is performed by using on-board solar diffuser (SD), solar diffuser stability monitor (SDSM), space-view (SV), and spectro-radiometric calibration assembly (SRCA). Through the SV port, periodic lunar observations are used to track radiometric response changes at different angles of incidence (AOI) of the scan mirror. As a part of the AM Constellation satellites, Terra MODIS flies approximately 30 minutes behind L7 ETM+ in the same orbit. The orbit of L7 is repetitive, circular, sunsynchronous, and near polar at a nominal altitude of 705 km (438 miles) at the Equator. The spacecraft crosses the Equator from north to south on a descending node between 10:00 AM and 10:15 AM. Circling the Earth at 7.5 km/sec, each orbit takes nearly 99 minutes. The spacecraft completes just over 14 orbits per day, covering the entire Earth between 81 degrees north and south latitude every 16 days. The longest continuous imaging swath that L7 sensor can collect is for a 14-minute subinterval contact period which is equivalent to 35 full WRS-2 scenes. On the other hand, Terra can provide the entire corresponding orbit with wider swath at any given ETM+ collection without contact time limitation. There are six spectral matching band pairs between MODIS (bands 3, 4, 1, 2, 6, 7) and ETM+ (bands 1, 2, 3, 4, 5, 7) sensor. MODIS has narrower spectral responses than ETM+ in all the bands. A short-term radiometric stability was evaluated using continuous ETM+ scenes within the contact period and the corresponding half orbit MODIS scenes. The near simultaneous earth observations (SNO) were limited by the smaller swath size of ETM+ (187 km) as compared to MODIS (2330 km). Two sets of continuous granules for MODIS and ETM+ were selected and mosaiced based on pixel geolocation information for non cloudy pixels over the North American continent. The Top-of- Atmosphere (TOA) reflectances were computed for the spectrally matching bands between ETM+ and MODIS over the regions of interest (ROI). The matching pixel pairs were aggregated from a finer to a coarser pixel resolution and the TOA reflectance values covering a wide dynamic range of the sensors were compared and analyzed. Considering the uncertainties of the absolute calibration of the both sensors, radiometric stability was verified for the band pairs. The Railroad Valley Playa, Nada (RVPN) was included in the path of this continuous orbit, which served as a verification point between the shortterm and the long-term trending results from previous studies. This work focuses on monitoring the short-term on-orbit stability of MODIS and the ETM+ RSB. It also provides an assessment of the absolute calibration differences between the two sensors over their wide dynamic ranges.

Choi, Taeyoung↗
NASA NTRS1973

Evaluation of ERTS data for certain oceanographic uses

The author has identified the following significant results. A significant result was reported 13 July on anomalous dark patches observed in certain ERTS-1 images. This sunglint related feature is possible whenever the solar elevation is greater than 55 deg and the surface waters are calm. It is observed in all MSS bands. Although the bright specular point is nearly 300 nm from nadir no bright specular returns are possible with a 0930 local time descending path-only diffuse glitter, that may reach reflectance levels of 1-2% under wind speeds of 5-10 m/sec and solar elevations greater than 60 deg.

Strong, A. E.↗
NASA NTRS1991

Retrieval and molecule sensitivity studies for the global ozone monitoring experiment and the scanning imaging absorption spectrometer for atmospheric chartography

The Global Ozone Monitoring Experiment (GOME) and the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) are diode based spectrometers that will make atmospheric constituent and aerosol measurements from European satellite platforms beginning in the mid 1990's. GOME measures the atmosphere in the UV and visible in nadir scanning, while SCIAMACHY performs a combination of nadir, limb, and occultation measurements in the UV, visible, and infrared. A summary is presented of the sensitivity studies that were performed for SCIAMACHY measurements. As the GOME measurement capability is a subset of the SCIAMACHY measurement capability, the nadir, UV, and visible portion of the studies is shown to apply to GOME as well.

Chance, Kelly V.↗
NASA NTRS2010

Solar Array Disturbances to Spacecraft Pointing During the Lunar Reconnaissance Orbiter (LRO) Mission

The Lunar Reconnaissance Orbiter (LRO), the first spacecraft to support NASA s return to the Moon, launched on June 18, 2009 from the Cape Canaveral Air Force Station aboard an Atlas V launch vehicle. It was initially inserted into a direct trans-lunar trajectory to the Moon. After a five day transit to the Moon, LRO was inserted into the Lunar orbit and successfully lowered to a low altitude elliptical polar orbit for spacecraft commissioning. Successful commissioning was completed in October 2009 when LRO was placed in its near circular mission orbit with an approximate altitude of 50km. LRO will spend at least one year orbiting the Moon, collecting lunar environment science and mapping data, utilizing a suite of seven instruments to enable future human exploration. The objective is to provide key science data necessary to facilitate human return to the Moon as well as identification of opportunities for future science missions. LRO's instrument suite will provide the high resolution imaging data with sub-meter accuracy, highly accurate lunar cartographic maps, mineralogy mapping, amongst other science data of interest. LRO employs a 3-axis stabilized attitude control system (ACS) whose primary control mode, the "Observing Mode", provides Lunar nadir, off-nadir, and inertial fine pointing for the science data collection and instrument calibration. This controller combines the capability of fine pointing with on-demand large angle full-sky attitude reorientation. It provides simplicity of spacecraft operation as well as additional flexibility for science data collection. A conventional suite of ACS components is employed in the Observing Mode to meet the pointing and control objectives. Actuation is provided by a set of four reaction wheels developed in-house at NASA Goddard Space Flight Center (GSFC). Attitude feedback is provided by a six state Kalman filter which utilizes two SELEX Galileo Star Trackers for attitude updates, and a single Honeywell Miniature Inertial Measurement Unit (MIMU) to provide body rates for attitude propagation. Rate is computed by differentiating accumulated angle provided by the MIMU. The Observing Mode controller is required to maintain fine pointing while a large fully-articulated solar array (SA) maintains its panel normal to the solar incidence. This paper describes the disturbances to the attitude control resulting from the SA articulation. Observing Mode performance in the presence of this disturbance was assessed while the spacecraft was in an initial elliptical low altitude orbit during the commissioning phase, which started about two weeks after launch and lasted for 90 days. LRO demonstrated excellent pointing performance during Observing Mode nadir and inertial attitude target operations during this phase. Transient LRO attitude errors observed during commissioning resulted primarily from three sources, Diviner instrument calibrations, RW zero crossings, and SA articulation. Even during times of considerable disturbance from SA articulation, the attitude errors were maintained below the statistical attitude error requirement level of 15 arc-sec (3 sigma).

Calhoun, Philip↗
NASA NTRS1996

SIR-C/X-SAR Observations of Rain Storms

Teh SIR-C/X-sar radar observations of rain storms are the first multi-polarization and multi-frequency observations of percipitation from space. In addition to numerous, often dramatic images of severe weather systems obtained by forming a synthetic aperture in the usual side-looking attitude, several data takes were performed while the radar antennas were parallel to the ground and the radar beams were pointing at nadir. These oppoprtunities coincided with the passage of the Shuttle over Tropical Cyclone Odille in the southern Indian Ocean during the first flight, and over Typhoon Seth in the Western Pacific during the second flight. The resulting observations, or more apropriately, the resulting measurements, demonstrate for the first time the capability of a spaceborne multi-frequency multi-polarization microwave radar system to quantify precipitation rates, to detect hydrometer phase, and to classify rain type.

SIR-C↗
NASA NTRS2011

Evaluation of Detector-to-Detector and Mirror Side Differences for Terra MODIS Reflective Solar Bands Using Simultaneous MISR Observations

The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the five Earth-observing instruments on-board the National Aeronautics and Space Administration (NASA) Earth-Observing System(EOS) Terra spacecraft, launched in December 1999. It has 36 spectral bands with wavelengths ranging from 0.41 to 14.4 mm and collects data at three nadir spatial resolutions: 0.25 km for 2 bands with 40 detectors each, 0.5 km for 5 bands with 20 detectors each and 1 km for the remaining 29 bands with 10 detectors each. MODIS bands are located on four separate focal plane assemblies (FPAs) according to their spectral wavelengths and aligned in the cross-track direction. Detectors of each spectral band are aligned in the along-track direction. MODIS makes observations using a two-sided paddle-wheel scan mirror. Its on-board calibrators (OBCs) for the reflective solar bands (RSBs) include a solar diffuser (SD), a solar diffuser stability monitor (SDSM) and a spectral-radiometric calibration assembly (SRCA). Calibration is performed for each band, detector, sub-sample (for sub-kilometer resolution bands) and mirror side. In this study, a ratio approach is applied to MODIS observed Earth scene reflectances to track the detector-to-detector and mirror side differences. Simultaneous observed reflectances from the Multi-angle Imaging Spectroradiometer (MISR), also onboard the Terra spacecraft, are used with MODIS observed reflectances in this ratio approach for four closely matched spectral bands. Results show that the detector-to-detector difference between two adjacent detectors within each spectral band is typically less than 0.2% and, depending on the wavelengths, the maximum difference among all detectors varies from 0.5% to 0.8%. The mirror side differences are found to be very small for all bands except for band 3 at 0.44 mm. This is the band with the shortest wavelength among the selected matching bands, showing a time-dependent increase for the mirror side difference. This study is part of the effort by the MODIS Characterization Support Team (MCST) in order to track the RSB on-orbit performance for MODIS collection 5 data products. To support MCST efforts for future data re-processing, this analysis will be extended to include more spectral bands and temporal coverage.

Wu, Aisheng↗
NASA NTRS1989

Moderate Resolution Imaging Spectrometer - A progress report (April 1989)

The Moderate Resolution Imaging Spectrometer (MODIS) is a facility instrument to be flown on the first Earth Observing System (scheduled launch in the late 1990s). The MODIS has two components. One component is a 110-deg-scan-angle instrument called MODIS-N (nadir). This instrument has 40 selected bands supporting observations of the land surface, the oceans, and atmosphere in the visible, NIR, short-wave IR (1.0-3.0 microns), and thermal IR (3.0-15.0 microns). The other component is a 90-deg-scan-angle scanning instrument that can tilt fore and aft along the satellite track, called MODIS-T (tilt). Both MODIS-N and MODIS-T are nearing the end of detailed design studies. The driving scientific requirements include absolute calibration accuracy 2 percent, instrument-induced polarization less than 2 percent, SNR reaching 800:1 for observing ocean color at large solar zenith angles, and dynamic range allowing observations of cloud characteristics and snow-covered areas.

Salomonson, V. V.↗
NASA NTRS2011

Aquarius Instrument and Salinity Retrieval

Aquarius has been designed to map the surface salinity field of the global ocean from space a parameter important for understanding ocean circulation and its relationship to climate and the global water cycle. Salinity is measured remotely from space by measuring the thermal emission from the ocean surface. This is done at the low frequency end of the microwave spectrum (e.g. 1.4 GHz) where the emission is sufficiently sensitive to changes in salinity to be detected with sophisticated radiometers. The goal is to monitor the seasonal and interannual variation of the large scale features of the surface salinity field in the open ocean by providing maps on a monthly basis with a spatial resolution of 150 km and an accuracy of 0.2 psu. These are challenging requirements that have led to some unique features of the instrument. These include: a) The addition of a co-located scatterometer to help provide a correction for roughness; b) The addition of a polarimetric channel (third Stokes parameter) to the radiometer to help correct for Faraday rotation; c) Asun-synchronous orbit with a 6 pm ascending equatorial crossing to minimize Faraday rotation and with the antennas looking away from the sun toward the nighttime side to minimize contamination by radiation from the sun; and d) An antenna designed to limit side lobes in the direction of rays from the sun. In addition, achieving the accuracy goal of 0.2 psu requires averaging over one month and to do this requires a highly stable radiometer. Aquarius has three separate radiometers that image in pushbroom fashion with the three antenna beams looking across track. The antenna is a 2.5-m diameter, offset parabolic reflector with three feed horns and the three beams are arranged to image with the boresight aligned to look across track, roughly perpendicular to the spacecraft heading and pointing away from the Sun. The three beams point at angles of theta = 25.8 deg., 33.8 deg. and 40.3 deg. with respect to the spacecraft nadir which correspond to local incidence angles at the surface of 28.7 deg., 37.8 deg. and 45.6 deg., respectively. The resolution of the three radiometer beams (axes of the 3dB ellipse) is: 76 x 94 km for the inner beam, 84 x 120 km for the middle beam to 96 x 156 km for the outer beam. Together they cover a swath of about 390 km. Aquarius will map the global ice-free ocean every 7-days from which monthly average composites will be derived. This will provide a snapshot of the mean field, as well as resolving the seasonal to interannual variations over the three-year baseline of the mission.

Le Vine, D. M.↗
NASA NTRS1982

Contrast reduction by the atmosphere and retrieval of nonuniform surface reflectance

A radiative transfer model is developed which gives the upward radiance at nadir for any 1-D Lambertian surface reflectance. This model is used to depict the atmospheric effect on the transmittance of contrast for any 1-D surface reflectance. Here by contrast we mean a general variation of the radiation field across the image. With the aid of this model an inversion algorithm is developed for retrieval of true surface reflectance from high resolution satellite data (e.g., Landsat). This inversion technique can be a useful tool for extraction of surface reflectance from satellite data in the case of a surface reflectance variable in one dimension only (e.g., seashore or near borders of big fields). A sensitivity study of the inversion procedure on the knowledge of atmospheric parameters and sensor calibration was performed. It is shown that this inversion technique is stable even in the presence of errors in the sensor calibration and the atmospheric parameters. The method was applied to Landsat data in two wavelengths. The results show reasonable dependence of the derived surface reflectance on the distance from the seashore.

Mekler, Y.↗
NASA NTRS2001

Cloud and Radiation Studies during SAFARI 2000

Though the emphasis of the Southern Africa Regional Science Initiative 2000 (SAFARI-2000) dry season campaign was largely on emission sources and transport, the assemblage of aircraft (including the high altitude NASA ER-2 remote sensing platform and the University of Washington CV-580, UK MRF C130, and South African Weather Bureau JRA in situ aircrafts) provided a unique opportunity for cloud studies. Therefore, as part of the SAFARI initiative, investigations were undertaken to assess regional aerosol-cloud interactions and cloud remote sensing algorithms. In particular, the latter part of the experiment concentrated on marine boundary layer stratocumulus clouds off the southwest coast of Africa. Associated with cold water upwelling along the Benguela current, the Namibian stratocumulus regime has received limited attention but appears to be unique for several reasons. During the dry season, outflow of continental fires and industrial pollution over this area can be extreme. From below, upwelling provides a rich nutrient source for phytoplankton (a source of atmospheric sulphur through DMS production as well as from decay processes). The impact of these natural and anthropogenic sources on the microphysical and optical properties of the stratocumulus is unknown. Continental and Indian Ocean cloud systems of opportunity were also studied during the campaign. Aircraft flights were coordinated with NASA Terra Satellite overpasses for synergy with the Moderate Resolution Imaging Spectroradiometer (MODIS) and other Terra instruments. An operational MODIS algorithm for the retrieval of cloud optical and physical properties (including optical thickness, effective particle radius, and water path) has been developed. Pixel-level MODIS retrievals (11 km spatial resolution at nadir) and gridded statistics of clouds in th SAFARI region will be presented. In addition, the MODIS Airborne Simulator flown on the ER-2 provided high spatial resolution retrievals (50 m at nadir). These retrievals will be discussed and compared with in situ observations.

Platnick, Steven↗