Sensor Zenith Angle Research Articles

Comparison of Bayesian land surface temperature algorithm performance with Terra MODIS observations

An approach to land surface temperature (LST) estimation that relies upon Bayesian inference has been tested against multiband infrared radiometric imagery from the Terra MODIS (Moderate Resolution Imaging Spectroradiometer) instrument. The algorithm employed requires minimal knowledge of surface emissivity, starting from a parsimoniously chosen (hence, uninformative) range of prior band emissivity knowledge. Two estimation methods have been tested. The first is the iterative contraction mapping of joint expectation values for LST and surface emissivity described in a previous paper. In the second method, the Bayesian algorithm is reformulated as a maximum a posteriori (MAP) search for the maximum joint a posteriori probability for LST, given observed sensor aperture radiances and a priori probabilities for LST and emissivity. Two MODIS data granules each for daytime and night-time were used for the comparison. The granules were chosen to be largely cloud-free, with limited vertical relief in those portions of the granules for which the sensor zenith angle . Level 1b radiances were used to obtain 500 LST estimates per granule for comparison with the Level 2 MODIS LST product. The Bayesian LST estimators accurately reproduce standard MODIS product LST values. In particular, the mean discrepancy for the MAP retrievals is , and its standard deviation does not exceed 1K. The ±68% confidence intervals for individual LST estimates associated with assumed uncertainty in surface emissivity are of order 0.8K. The appendix presents a proof of convergence of the iterative contraction mapping algorithm. The expectation values of surface temperature in multiple bands, and jointly in all bands, converge to a fixed point, within a stipulated convergence criterion. Provided the support for the calculation of the expectation value brackets the maximum in the joint posterior probability for LST, the fixed point converges to the MAP LST estimate in the limit .

Viewing Geometry Dependencies in MODIS Cloud Products

Abstract Characterizing the earth’s global cloud field is important for the proper assessment of the global radiation budget and hydrologic cycle. This characterization can only be achieved with satellite measurements. For complete daily coverage across the globe, polar-orbiting satellites must take observations over a wide range of sensor zenith angles. This paper uses Moderate Resolution Imaging Spectroradiometer (MODIS) Level-3 data to determine the effect that sensor zenith angle has on global cloud properties including the cloud fraction, cloud-top pressure, effective radii, and optical thickness. For example, the MODIS cloud amount increases from 57% to 71% between nadir and edge-of-scan (∼67°) observations, for clouds observed between 35°N and 35°S latitude. These increases are due to a combination of factors, including larger pixel size and longer observation pathlength at more oblique sensor zenith angles. The differences caused by sensor zenith angle bias in cloud properties are not readily apparent in monthly mean regional or global maps because the averaging of multiple satellite overpasses together “washes out” the zenith angle artifact. Furthermore, these differences are not constant globally and are dependent on the cloud type being observed.

The role of the critical angle in brightness reversals on sunglint images of the sea surface

A wide variety of oceanic and atmospheric phenomena are often observed in and around the sunglint region on optical images of the sea surface. The appearance of these phenomena depends strongly on the viewing geometry with areas on the sea surface that are rougher (or smoother) than the background appearing as either brighter or darker than the background depending on their position relative to the specular point. To understand these sea surface signature variations, this paper introduces the concept of a critical sensor viewing angle, defined as the sensor zenith angle at which different sea surface roughness variances produce identical sunglint radiance. It is when the imaging geometry transitions through the critical angle that a surface feature goes through a brightness reversal. Knowledge of where this transition takes place is important for properly interpreting the characteristics of the sea surface signature of these phenomena. The theory behind the concept of the critical angle is presented and then applied to sunglint imagery acquired over the ocean from space by the Moderate Resolution Imaging Spectroradiometer onboard NASA's Aqua and Terra satellites.

Comparison of multitemporal compositing methods for burnt area detection in Southeast Asian conditions

This study investigated the usability of different multitemporal compositing methods for burnt area detection purposes in the humid tropical conditions of insular Southeast Asia, characterised by persisting cloud cover, varying fire‐induced spectral changes and large amount of small burn scars. Six monthly composites of the Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance images (MOD09) were built using six different algorithms. The performance and usability of the compositing algorithms were evaluated using three criteria: separability between burnt and unburnt areas, homogeneity and average sensor zenith angle. Maximum surface temperature method (band 31, 11.0 µm) produced the most homogeneous composites with preference to close‐to‐nadir observations. However, these composites showed unexpectedly low separability between burnt and unburnt areas, mainly due to low spatial resolution of band 31 (1000 m). Overall, taking into account the performance of the compositing methods and the large amount of small burnt areas in insular Southeast Asia, a minimum NIR (band 2, 0.86 µm) method, combined with pre‐compositing cloud and cloud shadow removal, was seen as the most suitable compositing method for burnt area detection in this region. Its strengths were 250 m spatial resolution in pixel selection and high burnt/unburnt separability combined with reasonably good performance on homogeneity and average sensor zenith angle.

Assessment of multitemporal compositing techniques of MODIS and AVHRR images for burned land mapping

The performance of several criteria to generate multitemporal composites of daily Moderate Resolution Imaging Spectrometer (MODIS) and Advanced Very High Resolution Radiometer (AVHRR) images for burned land mapping was tested using data acquired over the Iberian Peninsula in 2001, 2003 and 2004. The experiment was based on four tests that assessed the discriminability between burned and unburned areas, the presence of artifacts (clouds and clouds shadows), the verticality of the sensor viewing angle, and the spatial coherency of the composite images. Seven different compositing techniques were tested, based on maximizing normalized difference vegetation index (NDVI) and brightness/surface temperature, and minimizing reflectance and sensor zenith angles. The composite criterions that provide the most accurate images for burned land mapping were based on maximizing brightness/surface temperatures, either as the only criterion or in conjunction with minimizing sensor zenith angle or near infrared (NIR) reflectance. These composites present high discrimination capacity between burned and unburned areas, remove most clouds and cloud shadows, offer high spatial coherency and present middle-to-low sensor zenith angles. Traditional compositing criterion based on maximizing NDVI values provided poor results in most tests. Finally, standard NASA MODIS composite provides close to nadir observation angles, and good spatial coherency, but it offered lower discrimination between burned and unburned areas that those composites based on thermal data.

Light scattering from the spherical‐shell atmosphere: Earth curvature effects measured by SeaWiFS

It is commonly known that the Earth's atmosphere is a spherical‐shell atmosphere (SSA) rather than a plane‐parallel atmosphere (PPA). Thus, the light scattered by the Earth's atmosphere is governed by physics of the radiative transfer equation (RTE) with proper boundary conditions in the SSA system. In satellite and aircraft remote sensing, however, the PPA model is usually assumed to compute the lookup tables and convert the sensor‐measured signals to the desired physical and optical quantities.The PPA assumption is a simple yet very good approximation for solar and sensor zenith angles < ∼80°. Note that the solar and sensor zenith angles here are defined as a measure at the local surface. In the PPA assumption, however, the solar zenith angles ≥90° is not defined. When the solar zenith angles ≥90° in the PPA system; i.e., when the Sun is below the horizon, there will be no light scattered out to the top of the atmosphere (TOA) or at the bottom of the surface. We would experience complete darkness in this situation.

The Rayleigh lookup tables for the SeaWiFS data processing: Accounting for the effects of ocean surface roughness

In the atmospheric correction of ocean colour remote sensing, it is important to account for the effects of ocean surface roughness (wind speed) in the computation of Rayleigh radiance lookup tables, in particular, for the large solar and/or sensor zenith angles (>∼60°). In the paper, both simulated and the SeaWiFS-retrieved results that demonstrate the effects of the ocean surface wind speed on the Rayleigh radiance computations for the various solar and sensor-viewing geometries as well as on the performance of atmospheric corrections are presented and discussed. An improved set of Rayleigh lookup tables, in which the Rayleigh scattering radiance is also a function of the sea surface wind speed, were generated and implemented into the SeaWiFS data processing system in May 2000.

Evaluation of the NDVI and sensor zenith angle values associated with the Global Land AVHRR 1-km data set

This study was initiated to assess the quality of the data available from the Global Land AVHRR 1-km data set. The normalized difference vegetation index (NDVI) and sensor zenith angle (SZ) values associated with the Global Land AVHRR 1-km data set were compared to those values included in the NOAA/NASA Pathfinder AVHRR Land (global, 8-km) data set at coincident spatial and temporal scales. The NDVI and SZ data were averaged within 10 by 10 grid cells and compared for each of the 10-day composite intervals during the 1 April 1992 through 31 March 1993 time period. The results indicate that the sensor zenith angles of the two data sets varied considerably, however, the NDVI values were quite similar.

Oceanic lidar: radiative transfer in the atmosphere at operating altitudes from 100 m to 100 km

The feasibility of measuring water-column parameters of the sea with a fluorescence lidar under daylight conditions and at flight altitudes between 100 m and 100 km is studied by modeling the atmospheric radiative transfer. Parameters to be measured are fluorescence of gelbstoff and chlorophyll and Raman scattering of water molecules. A cloudless and stratified atmosphere with various conditions of near-surface visibility and ozone concentration is taken into consideration. Solar zenith angles are varied between 0° and 60°. Lidar specifications are set to 1 J output energy, 10 ns pulse duration, 0.1 mrad beam divergence, 0.1 mrad detection angle, and 400 cm(-1) detection bandwidth. Signal recovery is carried out over the effective pulse length of the returned signal, which is roughly 20-30 ns. Sensor zenith angles are set between 0° and 60°. As a result of the study the recommended range of excitation wavelengths for high altitudes should be chosen between 350 and 400 nm. Under these circumstances, and with the iven laser and sensor specifications, oceanic lidar measurements should also be possible at flight altitudes of up to 100 km under clear visibility conditions, even at noon.

Satellite-derived reflectance of snow-covered surfaces in Northern Minnesota

Its high albedo and low thermal conductivity make snow a key component of the global energy balance. Snow depth and reflectance are crucial inputs to General Circulation Models (GCMs). Satellite data from the Landsat Thematic Mapper (TM) have been used to estimate the reflectance of snow and snow-covered surfaces integrated over part (0.45–0.90 μm) of the reflective range of the electromagnetic spectrum. The integrated reflectance, R 1, has been computed for snow-covered agricultural and forested areas, and a frozen lake in northern Minnesota using Landsat TM scenes acquired in November 1984 and January 1985. TM-derived reflectances, corrected for atmospheric effects, are mapped for subscenes within the Minnesota TM scenes and reflectances have been color coded. The average R 1 within the November 1984 subscene was 0.429 ± 0.176 whereas the average R 1 within the January 1985 subscene was 0.669 ± 0.236. Snow reflectance is known to increase with solar and sensor zenith angle. The solar zenith angle was 70° at the scene center of the 17 November 1984 scene, and 75° at the scene center of the 04 January 1985 scene. Both areas were predominately snow covered. Lowest reflectances (near 0%) were found in the non-snow-covered agricultural fields. Reflectances of lake ice in the January scene averaged 0.957 ± 0.0008. In the future, the Moderate Resolution Imaging Spectrometer (MODIS), as part of the Earth Observing System, will enable calculation of R 1 over most of the reflective part of the electromagnetic spectrum. R 1 may then be used, along with bidirectional reflectance data, to calculate snow albedo, an important input to GCMs and regional energy balance models.

Optical sky polarimetry and photometry at Mauna Loa Observatory affected by stratospheric dust from El Chichon

The Dave vector model is shown to be a good representation of the atmospheric effects on the photometry and polarimetry of the sky above Mauna Loa Observatory (MLO) on 6 Nov. 1982 produced by the El Chichon veil. Observations and modeling were accomplished at 0.36, 0.400, and 0.500 μm based on fundamental aerosol data. The effects of nonsphericity of the aerosols are included as a previously validated approximation in the input data. Extensive use was made of ground and air truth data available at MLO and the National Weather Service. From the modeling, the vertical number density of the El Chichon aerosol is deduced. The sky is found to be unusually bright from the aerosol scattering (radiance of 0.4 MW/m2/sr at a wavelength of 0.500 μm in the southerly and westerly directions at a sensor zenith angle of 60°) and the corresponding percent polarization low (~3%). Associated meteorological observations indicate stratospheric heating.

Simulated directional radiances of vegetation from satellite platforms

A computer modeling and simulation investigated the effects of off-nadir viewing, canopy geometry and density, solar zenith angle, and atmospheric conditions on the radiance of several vegetated targets. Effects on the normalized difference (ND) index were also investigated, and two wavelengths of 0.68 and 0.80 micron were simulated at nine sensor zenith angles, five sensor azimuths, and nine solar zenith angles. Seven canopies of low, medium, and high densities were observed, and bidirectional reflectance functions of other canopies were discussed. A modified Turner's (1974) atmospheric radiative transfer model was used for 2300 simulation runs. Off-nadir viewing effects were more pronounced in the red than the infrared, and the variability was found to be a function of canopy geometry. The ND index eliminated much of the variability seen in the individual bands, while the overall variability decreased with increasing biomass, and increased with increasing solar zenith angles.