Charles K. Gatebe

Use of In Situ and Airborne Multiangle Data to Assess MODIS- and Landsat-Based Estimates of Directional Reflectance and Albedo

The quantification of uncertainty in satellite-derived global surface albedo products is a critical aspect in producing complete, physically consistent, and decadal land property data records for studying ecosystem change. A challenge in validating albedo measurements acquired from space is the ability to overcome the spatial scaling errors that can produce disagreements between satellite and field-measured values. Here, we present the results from an accuracy assessment of MODIS and Landsat-TM albedo retrievals, based on collocated comparisons with tower and airborne Cloud Absorption Radiometer (CAR) measurements collected during the 2007 Cloud and Land Surface Interaction Campaign (CLASIC). The initial focus was on evaluating inter-sensor consistency through comparisons of intrinsic bidirectional reflectance estimates. Local and regional assessments were then performed to obtain estimates of the resulting scaling uncertainties, and to establish the accuracy of albedo reconstructions during extended periods of precipitation. In general, the satellite-derived estimates met the accuracy requirements established for the high-quality MODIS operational albedos at 500 m (the greater of 0.02 units or ±10% of surface measured values). However, results reveal a high degree of variability in the root-mean-square error (RMSE) and bias of MODIS visible (0.3-0.7 μm) and Landsat-TM shortwave (0.3-5.0 μm) albedos; where, in some cases, retrieval uncertainties were found to be in excess of 15 %. Results suggest that an overall improvement in MODIS shortwave albedo retrieval accuracy of 7.8%, based on comparisons between MODIS and CAR albedos, resulted from the removal of sub-grid scale mismatch errors when directly scaling-up the tower measurements to the MODIS satellite footprint.

Projections of rapidly rising surface temperatures over Africa under low mitigation

An analysis of observed trends in African annual-average near-surface temperatures over the last five decades reveals drastic increases, particularly over parts of the subtropics and central tropical Africa. Over these regions, temperatures have been rising at more than twice the global rate of temperature increase. An ensemble of high-resolution downscalings, obtained using a single regional climate model forced with the sea-surface temperatures and sea-ice fields of an ensemble of global circulation model (GCM) simulations, is shown to realistically represent the relatively strong temperature increases observed in subtropical southern and northern Africa. The amplitudes of warming are generally underestimated, however. Further warming is projected to occur during the 21st century, with plausible increases of 4?6 ?C over the subtropics and 3?5 ?C over the tropics by the end of the century relative to present-day climate under the A2 (a low mitigation) scenario of the Special Report on Emission Scenarios. High impact climate events such as heat-wave days and high fire-danger days are consistently projected to increase drastically in their frequency of occurrence. General decreases in soil-moisture availability are projected, even for regions where increases in rainfall are plausible, due to enhanced levels of evaporation. The regional dowscalings presented here, and recent GCM projections obtained for Africa, indicate that African annual-averaged temperatures may plausibly rise at about 1.5 times the global rate of temperature increase in the subtropics, and at a somewhat lower rate in the tropics. These projected increases although drastic, may be conservative given the model underestimations of observed temperature trends. The relatively strong rate of warming over Africa, in combination with the associated increases in extreme temperature events, may be key factors to consider when interpreting the suitability of global mitigation targets in terms of African climate change and climate change adaptation in Africa.

Characterization of errors in the use of integrating-sphere systems in the calibration of scanning radiometers

Laboratory measurements were performed to characterize the geometrical effects in the calibration of the NASAs cloud absorption radiometer (CAR). The measurements involved three integrating sphere sources (ISSs) operated at different light levels and experimental setups to determine radiance variability. The radiance gradients across the three ISS apertures were 0.2%-2.6% for different visible, near-infrared, and shortwave infrared illumination levels but <15% in the UV. Change in radiance with distance was determined to be 2%-20%, being highest in the UV. Radiance variability due to the edge effects was found to be significant; as much as 70% due to the sphere aperture and <10% due to the CAR telescopes secondary mirror.

Biomass Burning, Land-Cover Change, and the Hydrological Cycle in Northern Sub-Saharan Africa

The Northern Sub-Saharan African (NSSA) region, which accounts for 20%-25%of the global carbon emissions from biomass burning, also suffers from frequent drought episodes and other disruptions to the hydrological cycle whose adverse societal impacts have been widely reported during the last several decades. This paper presents a conceptual framework of the NSSA regional climate system components that may be linked to biomass burning, as well as detailed analyses of a variety of satellite data for 2001-2014 in conjunction with relevant model-assimilated variables. Satellite fire detections in NSSA show that the vast majority (greater than 75%) occurs in the savanna and woody savanna land-cover types. Starting in the 2006-2007 burning season through the end of the analyzed data in 2014, peak burning activity showed a net decrease of 2-7% /yr in different parts of NSSA, especially in the savanna regions. However, fire distribution shows appreciable coincidence with land-cover change. Although there is variable mutual exchange of different land cover types, during 2003-2013, cropland increased at an estimated rate of 0.28% /yr of the total NSSA land area, with most of it (0.18% /yr) coming from savanna.During the last decade, conversion to croplands increased in some areas classified as forests and wetlands, posing a threat to these vital and vulnerable ecosystems. Seasonal peak burning is anti-correlated with annual water-cycle indicators such as precipitation, soil moisture, vegetation greenness, and evapotranspiration, except in humid West Africa (5 deg-10 deg latitude),where this anti-correlation occurs exclusively in the dry season and burning virtually stops when monthly mean precipitation reaches 4 mm/d. These results provide observational evidence of changes in land-cover and hydrological variables that are consistent with feedbacks from biomass burning in NSSA, and encourage more synergistic modeling and observational studies that can elaborate this feedback mechanism.

Observing System Simulations for Small Satellite Formations Estimating Bidirectional Reflectance

Abstract The bidirectional reflectance distribution function (BRDF) gives the reflectance of a target as a function of illumination geometry and viewing geometry, hence carries information about the anisotropy of the surface. BRDF is needed in remote sensing for the correction of view and illumination angle effects (for example in image standardization and mosaicing), for deriving albedo, for land cover classification, for cloud detection, for atmospheric correction, and other applications. However, current spaceborne instruments provide sparse angular sampling of BRDF and airborne instruments are limited in the spatial and temporal coverage. To fill the gaps in angular coverage within spatial, spectral and temporal requirements, we propose a new measurement technique: use of small satellites in formation flight, each satellite with a VNIR (visible and near infrared) imaging spectrometer, to make multi-spectral, near-simultaneous measurements of every ground spot in the swath at multiple angles. This paper describes an observing system simulation experiment (OSSE) to evaluate the proposed concept and select the optimal formation architecture that minimizes BRDF uncertainties. The variables of the OSSE are identified; number of satellites, measurement spread in the view zenith and relative azimuth with respect to solar plane, solar zenith angle, BRDF models and wavelength of reflection. Analyzing the sensitivity of BRDF estimation errors to the variables allow simplification of the OSSE, to enable its use to rapidly evaluate formation architectures. A 6-satellite formation is shown to produce lower BRDF estimation errors, purely in terms of angular sampling as evaluated by the OSSE, than a single spacecraft with 9 forward-aft sensors. We demonstrate the ability to use OSSEs to design small satellite formations as complements to flagship mission data. The formations can fill angular sampling gaps and enable better BRDF products than currently possible.

BRDF Analysis of Savanna Vegetation and Salt-Pan Samples

In this paper, laboratory-based bidirectional reflectance distribution function (BRDF) analysis of vegetation leaves, soil, and leaf-litter samples is presented. The leaf litter and soil samples, numbered 1 and 2, were obtained from a site located in the savanna biome of South Africa (Skukuza: 25.0deg S, 31.5deg E). A third soil sample, number 3, was obtained from Etosha Pan, Namibia (19.20deg S, 15.93deg E, altitude of 1100 m). In addition, BRDF of local fresh and dry leaves from tulip polar tree (Liriodendron tulipifera) and black locust tree (Robinia pseudoacacia) were studied. It is shown how the BRDF depends on the incident and scatter angles, sample size (i.e., crushed versus whole leaf), soil samples fraction size, sample status (i.e., fresh versus dry leaves), vegetation species (i.e., poplar versus locust), and the vegetations biochemical composition. As a demonstration of the application of the results of this paper, airborne BRDF measurements acquired with NASAs Cloud Absorption Radiometer over the same general site where the soil and leaf-litter samples were obtained are compared to the laboratory results. Good agreement between laboratory and airborne-measured BRDF is reported.

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

Relative trajectories for multi-angular earth observation using science performance optimization

Distributed Space Missions (DSMs) are gaining momentum in their application to earth science missions owing to their unique ability to increase observation sampling in spatial, spectral and temporal dimensions simultaneously. This paper identifies a gap in the angular sampling abilities of traditional monolithic spacecraft and proposes to address it using small satellite clusters in formation flight. The science performance metric for the angular dimension is explored using the Bidirectional Reflectance-distribution Function (BRDF), which describes the directional variation of reflectance of a surface element. Previous studies have proposed the use of clusters of nanosatellites in formation flight, each with a VNIR imaging spectrometer, to make multi-spectral reflectance measurements of a ground target, at different zenith and azimuthal angles simultaneously. In this paper, a tradespace of formation flight geometries will be explored in order to optimize or maximize angular spread and minimize BRDF estimation errors. The simulated formation flight solutions are applied to the following case studies: Snow albedo estimation in the Arctic and vegetation in the African savannas. Results will be compared to real data from previous airborne missions (NASAs ARCTAS Campaign in 2008 and SAFARI Campaign in 2000).

Cloud optical parameters from airborne observation of diffuse solar radiation accomplished in USA and USSR in different geographical regions

Cloud optical parameters (optical thickness, single scattering albedo, and ground albedo) are obtained from airborne experiments with NASA’s Cloud Absorption Radiometer and analysed taking into account observational and processing uncertainties. The analytical approach of the inverse asymptotic formulas of the transfer theory, which uses observed values of solar diffuse radiance, is applied. The method is free from a priori restrictions and links put to desired parameters. The algorithms and first results of processing have been presented earlier. The first results, being the solution of the inverse problem, showed strong fluctuations in values, which required the regularization of the solution. The dependence of uncertainties of the result on viewing direction was revealed. Hence, here attention is focused on uncertainties of observation, angle function calculation, and processing approach, which is taken into account for result averaging, and the regularization procedure is described. Calculating the uncertainties of the processing approach is accomplished analytically using formulas for the retrieval of the optical parameters. The values of the desired parameters obtained in eight observational spectral channels – above, below, and within the cloud – at 16 levels are presented. The final results are compared to the optical parameters of extended cloud layers obtained earlier using a similar method of inverse asymptotic formulas from spectral data of Russian aircraft solar irradiance measurements in different regions, made in the 1970s/1980s at Leningrad (now Saint Petersburg) University in the USSR.

Evaluation of Hyperspectral Snapshot Imagers onboard Nanosatellite Clusters for Multi-Angular Remote Sensing

Hyperspectral snapshot imagers are capable of producing 2D spatial images with a single exposure at selected and numerous wavelength bands instead of 1D spatial at all spectral band images like in push-broom instruments. Snapshot imagers are critical technologies for multi-angle remote sensing using distributed space missions. They help to relax the attitude control requirements of clusters of small satellites whose narrow field-of-view payloads point at the same ground spot or to increase the footprint area of small satellite constellations with wide field-of-view payloads. This paper reviews the existing spectral imagers for multi-angle remote sensing, performs a feasibility study to incorporate existing state-of-the-art snapshot imagers and proposes baseline imagers to serve as payload for the distributed nanosatellites. The overall approach includes an extensive trade study to identify the optics, spectral elements, their parameters and compare the identified choices both qualitatively and quantitatively. The proposed baseline design has an telescope aperture diameter of 7 cm, focal plane pixel size of 20 μm, 1000 pixels per side of the focal plane array sampling the scene and acousto-optic tunable filters or waveguide spatial heterodyne imagers that simulate a swath up to 90 km, image up to 86 wavebands with an SNR above 100. The tradeoff between spectral and spatial ranges sampled by the two baseline imager options has been highlighted.