Stuart Pilorz

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.

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.

Estimating chlorophyll content and bathymetry of Lake Tahoe using AVIRIS data

Abstract An AVIRIS image was obtained at Lake Tahoe on 9 August 1990, along with in situ data. Profiles of percent transmission of monochromatic light, stimulated chlorophyll fluorescence, photosynthetically available radiation, and spectral upwelling and downwelling irradiance, and upwelling radiance were measured. Chlorophyll- a + phaeopigments, total particulate absorption, detritus absorption, and absorption due to colored dissolved organic matter were measured on discrete samples. Spectral reflectance at the surface was measured with a handheld spectroradiometer. Image pre-processing included increasing the instrument signal-to-noise ratio by filtering to reduce patterned noise and spatial resampling, and application of LOWTRAN-7 as an atmospheric correction. Several analyses were then performed illustrating the utility of the AVIRIS over a dark water scene. The water-leaving radiance measured by the AVIRIS compares very well with the upwelling radiance measured in-water, everywhere but in the very short wavelength channels. After recalibrating one AVIRIS channel, the chlorophyll concentration derived from the image compares extremely well with that measured with bottle samples. Application of the pigment algorithm to the rest of the lake was confounded by covarying absorption at 440 nm by colored dissolved organic matter (CDOM), underscoring the importance of accurate calibration of the instrument in the short-wavelength channels. Surface spectroradiometer measurements made along a transect of varying depth were used to condition a multiple linear regression bathymetry model. By applying the model coefficients to a portion of the image, a bathymetry map of the shallow parts of the lake was constructed which compares favorably with published lake soundings, indicating the potential for a bottom-reflectance correction to coastal ocean color imagery.

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.

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.

Voyager photopolarimeter observations of Uranian ring occultations

Abstract The Voyager 2 photopolarimeter (PPS) observed the stars σ Sagitarii and§b Persei as they were occulted by the Uranian ring system on January 26, 1986 (A. L. Lane et al. 1986, Science 233, 65–70). Both occultations passed through the ring system so that orbital radii were sampled at two different longitudes in the ring plane for each occultation. The σ Sagitarii occultation was a grazing occultation and only ϵ, Δ, and 1986U1R occulted the star. β Persei was occulted by each of the nine classical rings twice, and each of these rings was identified on ingress and egress, although the 6 ring in β Per ingress is a marginal detection, and the η ring in β Per ingress was not detected. The orbits of the nine classical rings have been successfully modeled by numerous Earth-based stellar occulations (R. G. French et al. 1988, Icarus 73, 349–378), and Voyager occultation profiles of the rings from the radio science occultation (RSS, D. L. Gresh et al. 1989, Icarus 78, 131–168) and the ultraviolet spectrometer occultations (UVS, J. B. Holberg et al. 1987, Astron. J. 94, 178–188) have been published. The PPS occultations provide high resolution profiles of the rings (∼ 10 m/point for σ Sgr, ∼100 m/point for β Per) which contain information on waves (L. J. Horn et al. 1988, Icarus 76, 485–492), edge sharpness, vertical thickness, and azimuthal variations in radial structure within the rings. The PPS profiles of the e ring show structure similar to that seen in the radio, UVS, and Earth-based occultations, although the magnitude of peaks and valleys in optical depth changes with azimuth. 1986U1R, γ, η, α, and 6 all show varying degrees of azimuthal inhomogeneity. The η ring is not present in the β Per ingress data although it shows up clearly in egress, and the γ ring is ony 0.6 km wide at β Per ingress and is opaque to the starlight while on the egress cut it is 2.6 km wide. We also present the results of a rigorous statistical search of the data for previously undetected rings. Several features were identified in the β Per data that were more statistically significant than some of the known ring profiles (6 and η in β Per ingress), but the low signal-to-noise ratio for this occultation prevents the identification of any new rings.

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 (

Sua pan surface bidirectional reflectance: a case study to evaluate the effect of atmospheric correction on the surface products of the multi-angle imaging SpectroRadiometer (MISR) during SAFARI 2000

This paper presents a validation case study of Multi-angle Imaging SpectroRadiometer (MISR) surface products where its bidirectional reflectance (BRF) measurements during the Southern Africa Regional Science Initiative (SAFARI 2000) campaign are compared with those coincidently evaluated on the ground and from the air, using the Portable Apparatus for Rapid Acquisition of Bidirectional Observations of Land and Atmosphere (PARABOLA) and Cloud Absorption Radiometer observations, respectively. The presence of haze and smoke during the campaign provided a case study to evaluate the effect of atmospheric correction on MISR surface products. Two surface types were considered in the analyses: the bright desert-like surface of the Pan and the dark grassland that surrounds it. The results show that for the dark surface the BRF values retrieved from MISR are in good agreement, within 5%, with those obtained from field data. For the bright desert-like pan surface, better agreement, within ~10%, was found in all channels on the clear day but only in the forward scattering on the hazy day. A comparison of MISR aerosol retrievals to those obtained from three independent ground measurements suggests that, in the presence of a highly reflective surface, small uncertainties in the MISR aerosol retrievals become magnified at larger optical depths, causing errors in the surface BRF retrievals

Ground-based validation of the EOS Multi-angle Imaging SpectroRadiometer (MISR) aerosol retrieval algorithms and science data products

A plan for the ground-based validation of MISR aerosol retrieval is outlined. Activities occur in two phases: (1) pre-launch, work is focused on technique development and MISR algorithm validation using conventional ground-based methods and a MISR simulator (AirMISR) operating from the ER-2 aircraft to simulate MISR on-orbit observations. (2) Post-launch, the validation program relies on ground campaigns, underflights with the MISR simulator and the use of local measurements of aerosol loading and properties and irradiance measurements derived from the AERONET and ISIS networks.