Mark C. Helmlinger

MISR aerosol optical depth retrievals over southern Africa during the SAFARI‐2000 Dry Season Campaign

This paper presents, for the first time, retrievals of aerosol optical depths from Multi-angle Imaging Spectro-Radiometer (MISR) observations over land. Application of the MISR operational algorithm to data taken over southern Africa during the SAFARI-2000 dry season campaign yields results that compare favorably with coincident surface-based measurements taken by the AERONET radiometer network.

Coordinated Airborne, Spaceborne, and Ground-Based Measurements of Massive, Thick Aerosol Layers During the Dry Season in Southern Africa

During the dry season airborne campaign of the Southern African Regional Science Initiative (SAFARI 2000), coordinated observations were made of massive thick aerosol layers. These layers were often dominated by aerosols from biomass burning. We report on airborne Sun photometer measurements of aerosol optical depth (λ = 0.354-1.557 μm), columnar water vapor, and vertical profiles of aerosol extinction and water vapor density that were obtained aboard the University of Washingtons Convair-580 research aircraft. We compare these with ground-based AERONET Sun/sky radiometer results, with ground based lidar data (MPL-Net), and with measurements from a downward pointing lidar aboard the high-flying NASA ER-2 aircraft. Finally, we show comparisons between aerosol optical depths from the Sun photometer and those retrieved over land and over water using four spaceborne sensors (TOMS, MODIS, MISR, and ATSR-2).

Early validation of the Multi-angle Imaging SpectroRadiometer (MISR) radiometric scale

The Multi-angle Imaging SpectroRadiometer (MISR) instrument consists of nine cameras, four spectral bands each, and an on-board calibrator (OBC). Experiments using the latter allow camera radiometric coefficients to be updated bimonthly. Data products are thus calibrated to a stable radiometric scale, even in the presence of instrument response changes. The camera, band, and pixel-relative calibrations are accurately determined using the OBC. Conversely, as the OBC itself is subject to response degradation, MISR also conducts annual field vicarious calibration campaigns. The first of these, conducted in June 2000 at a desert site in Nevada, has been used to establish the present absolute radiometric scale. Validation of this radiometric scale, using AirMISR, shows consistency to within 4%. Following these studies, however, it was determined that MISR radiometry is subject to scene-dependent effects due to ghosting that, for the Nevada test sites, reduces the apparent radiance by 3%. Correction for this effect is required in order to avoid radiometric errors over sites that do not exhibit the same background contrast. Additional studies are in progress, with plans to correct for scene-contrast effects in future Level 1B1 processing.

Vicarious calibration experiment in support of the Multi-angle Imaging SpectroRadiometer

On June 11, 2000, the first vicarious calibration experiment in support of the Multi-angle Imaging SpectroRadiometer (MISR) was conducted. The purpose of this experiment was to acquire in situ measurements of surface and atmospheric conditions over a bright, uniform area. These data were then used to compute top-of-atmosphere (TOA) radiances, which were correlated with the camera digital number output, to determine the in-flight radiometric response of the on-orbit sensor. The Lunar Lake Playa, Nevada, was the primary target instrumented by the Jet Propulsion Laboratory for this experiment. The airborne MISR simulator (AirMISR) on board a NASA ER-2 acquired simultaneous observations over Lunar Lake. The in situ estimations of top-of-atmosphere radiances and AirMISR measurements at a 20-km altitude were in good agreement with each other and differed by 9% from MISR measurements. The difference has been corrected by adjusting the gain coefficients used in MISR standard product generation. Data acquired simultaneously by other sensors, such as Landsat, the Terra Moderate-Resolution Imaging SpectroRadiometer (MODIS), and the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS), were used to validate this correction. Because of this experiment, MISR radiances are 9% higher than the values based on the on-board calibration. Semiannual field campaigns are planned for the future in order to detect any systematic trends in sensor calibration.

Vicarious Calibration of the GOSAT Sensors Using the Railroad Valley Desert Playa

Japans Greenhouse Gases Observing Satellite (GOSAT) was successfully launched into a sun-synchronous orbit on January 23, 2009 to monitor global distributions of carbon dioxide ( CO2) and methane (CH4). GOSAT carries two instruments. The Thermal And Near-infrared Sensor for carbon Observation Fourier-Transform Spectrometer (TANSO-FTS) measures reflected radiances in the 0.76 μm oxygen band and in the weak and strong CO2 bands at 1.6 and 2.0 μm. The TANSO Cloud and Aerosol Imager (TANSO-CAI) uses four spectral bands at 0.380, 0.674, 0.870, and 1.60 μm to identify clear soundings and to provide cloud and aerosol optical properties. Vicarious calibration was performed at Railroad Valley, Nevada, in the summer of 2009. The site was chosen for its flat surface and high spectral reflectance. In situ measurements of geophysical parameters, such as surface reflectance, aerosol optical thickness, and profiles of temperature, pressure, and humidity, were acquired at the overpass times. Because the instantaneous field of view of TANSO-FTS is large (10.5 km at nadir), the spatially limited reflectance measurements at the field sites were extrapolated to the entire footprint using independent satellite data. During the campaign, six days of measurements were acquired from two different orbit paths. Spectral radiances at the top of the atmosphere were calculated using vector radiative transfer models coupled with ground in situ data. The agreement of the modeled radiance spectra with those measured by the TANSO-FTS is within 7%. Significant degradations in responsivity since launch have been detected in the short-wavelength bands of both TANSO-FTS and TANSO-CAI.

PARABOLA III: A sphere‐scanning radiometer for field determination of surface anisotropic reflectance functions

The Portable Apparatus for Rapid Acquisition of Bidirectional Observation of the Land and Atmosphere III (PARABOLA III) is a sphere‐scanning radiometer. The original PARABOLA was built to study the relationship between surface morphology and reflected radiation properties. Follow‐on work led to the design of an improved radiometer, the PARABOLA III. This in‐situ sensor will be used to validate surface reflectances at angles measured by the Multi‐angle Imaging SpectroRadiometer (MISR), a global imager flown on the Earth Observing System (EOS)‐Terra orbital spacecraft. Derived PARABOLA III data products include the surface bidirectional reflectance factor, and sky and surface radiances for the upward and downward viewing hemispheres. This paper describes the design, calibration, and operation of the JPL PARABOLA III.

Long-Term Vicarious Calibration of GOSAT Short-Wave Sensors: Techniques for Error Reduction and New Estimates of Radiometric Degradation Factors

This work describes the radiometric calibration of the short-wave infrared (SWIR) bands of two instruments aboard the Greenhouse gases Observing SATellite (GOSAT), the Thermal And Near infrared Sensor for carbon Observations Fourier Transform Spectrometer (TANSO-FTS) and the Cloud and Aerosol Imager (TANSO-CAI). Four vicarious calibration campaigns (VCCs) have been performed annually since June 2009 at Railroad Valley, NV, USA, to estimate changes in the radiometric response of both sensors. While the 2009 campaign ( VCC2009) indicated significant initial degradation in the sensors compared to the prelaunch values, the results presented here show that the stability of the sensors has improved with time. The largest changes were seen in the 0.76 μm oxygen A-band for TANSO-FTS and in the 0.380 and 0.674 μm bands for TANSO-CAI. This paper describes techniques used to optimize the vicarious calibration of the GOSAT SWIR sensors. We discuss error reductions, relative to previous work, achieved by using higher quality and more comprehensive in situ measurements and proper selection of reference remote sensing products from the Moderate Resolution Imaging Spectroradiometer used in radiative transfer calculations to model top-of-the-atmosphere radiances. In addition, we present new estimates of TANSO-FTS radiometric degradation factors derived by combining the new vicarious calibration results with the time-dependent model provided by Yoshida (2012), which is based on analysis of on-board solar diffuser data. We conclude that this combined model provides a robust correction for TANSO-FTS Level 1B spectra. A detailed error budget for TANSO-FTS vicarious calibration is also provided.

Ground measurements of surface BRF and HDRF using PARABOLA III

The ground-based Portable Apparatus for Rapid Acquisition of Bidirectional Observations of Land and Atmosphere (PARABOLA), version 3, provides multiangle measurements of sky and ground radiances on a spherical grid of 5° in the zenith-to-nadir and azimuthal planes in eight spectral channels. The hemispherical directional reflectance factor (HDRF) can be measured directly by comparing the radiance reflected by the surface in given direction to that reflected from a reference surface simultaneously observed by the PARABOLA 3. The surface bidirectional reflectance factor (BRF) cannot be measured directly, however, because of the presence of the sky diffuse illumination. The contribution of the diffuse sky radiance to the radiance reflected by the natural target surface is computed, and removed, using an iterative technique. Two approaches are employed: the first requires knowledge of the atmospheric optical depth, and the second requires the simultaneous measurements of the radiance reflected by a standard surface panel under the same atmospheric and illumination conditions. Ground measurements of the BRF and HDRF for dry lake surfaces were obtained from the PARABOLA 3 observations with better than ± 10% accuracy. The results described in this work are used primarily for the vicarious calibration of the Multiangle Imaging Spectroradiometer (MISR) onboard the Earth Observing System (EOS) Terra platform and for validation of MISR BRF retrievals of selected Earth surface targets.

Vicarious calibration: A reflectance-based experiment with AirMISR

Abstract A vicarious reflectance-based calibration experiment for the Multiangle Imaging SpectroRadiometer (MISR) airborne simulator, AirMISR, is described as one precursor experiment of this type planned for postlaunch application to MISR itself. The experiment produces a set of multiangle near-top-of-atmosphere radiances that are compared with the multiangle AirMISR radiances, established using a laboratory calibration. The field and aircraft data were collected as part of an engineering test flight at Moffett Field, CA, on November 5, 1997. A concrete tarmac was used as the field target. Atmospheric optical depth data were collected adjacent to the target throughout the actual overflight period using a single Reagan solar radiometer. For logistical reasons, the surface hemispherical directional reflectance factor (HDRF) was determined 7 days later using the Portable Apparatus for Rapid Acquisition of Bidirectional Observation of the Land and Atmosphere III (PARABOLA III), along with the areally averaged spectral HDRF at normal incidence, obtained with an Analytical Spectral Devices (ASD) FieldSpec moderate resolution field spectrometer. AirMISR overflew the target under clear sky conditions though the aerosol turbidity was high (∼0.3 at 550 nm). Good to fair agreement has been obtained at all angles and wavelengths between the top-of-atmosphere (TOA) radiances calculated for the measured atmospheric and surface conditions and the radiances incident at AirMISR as determined from the laboratory calibration. Some systematic disagreements are present. The largest disagreements (∼15% in the blue) are found at the highest view angles and the smallest at nadir viewing (

On-Orbit Calibration of the EO-1 Hyperion and Advanced Land Imager (ALI) Sensors Using the LED Spectrometer (LSpec) Automated Facility

Vicarious calibration (VC) technology, developed in the mid-1980s, has been employed to establish the absolute radiometric calibration of Earth-viewing sensors aboard satellites and aircraft. This method has heretofore required special visits by a field team to collect surface and atmospheric measurements, at specific times necessarily coincident with a sensor overflight. With the recent creation of an autonomous calibration facility, VC data can now be made available to the sensor community without the need for each research group to deploy its own field team. Beginning in mid-November 2006, there have been ongoing efforts to expand the data processing capabilities of the Jet Propulsion Laboratory-operated LED Spectrometer (LSpec) automatic facility, which has been making continual measurements of surface reflectances and atmospheric transmittances since that time. The facility is located at Frenchman Flat, within the Nevada Test Site. Data are used to support the VC of sensors which make observations at visible and near-infrared wavelengths. An array of eight LSpecs performs the autonomous function of recording surface reflectances at 5-min intervals, thereby permitting accurate and continual real-time adjustments to high-resolution Analytical Spectral Devices spectrometer measurements, acquired every several months during on-site visits. Moreover, resident at the LSpec site is a Cimel sunphotometer, used to make atmospheric transmittance measurements. The Cimel is part of the Aerosol Robotic Network (AERONET). Measurements made by the LSpec Cimel are used by AERONET to derive values of aerosol optical depths. From continuous in situ measurement of spectral reflectance of the playa surface, along with acquired aerosol optical depths and ozone optical depths (obtained from the Ozone Mapping Instrument), an LSpec database is being produced and is available via a web-based interface. To highlight the viability of making use of LSpec-derived data, we have executed a few distinct multiple scattering radiative transfer codes in order to perform VCs of the Earth Observing-1 (EO-1) Hyperion and EO-1 Advanced Land Imager sensors.