Vegetation water content estimation for corn and soybeans using spectral indices derived from MODIS near- and short-wave infrared bands

Several recent studies have found that the brightness of clear sky systematically increases near clouds. Understanding this increase is important both for a correct interpretation of observations and for improving our knowledge of aerosol-cloud interactions. However, while the studies suggested several processes to explain the increase, the significance of each process is yet to be determined. This study examines one of the suggested processes three-dimensional (3-D) radiative interactions between clouds and their surroundings by analyzing a large dataset of MODIS (Moderate Resolution Imaging Spectroradiometer) observations over the Northeast Atlantic Ocean. The results indicate that 3-D effects are responsible for a large portion of the observed increase, which extends to about 15 km away from clouds and is stronger (i) at shorter wavelengths (ii) near optically thicker clouds and (iii) near illuminated cloud sides. This implies that it is important to account for 3-D radiative effects in the interpretation of solar reflectance measurements over clear regions in the vicinity of clouds.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008GL037089

MODIS Observations of Enhanced Clear Sky Reflectance Near Clouds

The point-spread function (PSF) of a camera can seriously affect the accuracy of radiometric calibration and measurement We found that the PSF can produce a 37% difference between the apparent measured radiance of two plaques of different sizes with the same illumination This difference can be removed by deconvolution with the measured PSF To determine the PSF, many images of a collimated beam from a He-Ne laser are averaged Since our optical system is focused at infinity, it should focus this source to a single pixel Although the measured PSF is very sharp, dropping 4 and 6 orders of magnitude and 8 and 100 pixels away from the point source, respectively, we show that the effect of the PSF as far as 100 pixels away cannot be ignored without introducing an appreciable error to the calibration We believe that the PSF should be taken into account in all optical systems to obtain accurate radiometric measurements

Effects of point-spread function on calibration and radiometric accuracy of CCD camera

Several recent studies have found that clouds are surrounded by a transition zone of rapidly changing aerosol optical properties and particle size. Characterizing this transition zone is important for better understanding aerosol-cloud interactions and aerosol radiative effects, and also for improving satellite retrievals of aerosol properties. This letter presents a statistical analysis of a monthlong global data set of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar observations over oceans. The results show that the transition zone is ubiquitous over all oceans and extends up to 15 km away from clouds. They also show that near-cloud enhancements in backscatter and particle size are strongest at low altitudes, slightly below the top of the nearest clouds. Also, the enhancements are similar near illuminated and shadowy cloud sides, which confirms that the asymmetry of Moderate Resolution Imaging Spectroradiometer reflectances found in an earlier study comes from 3-D radiative processes and not from differences in aerosol properties. Finally, the effects of CALIPSO aerosol detection and cloud identification uncertainties are discussed. The findings underline the importance of accounting for the transition zone to avoid potential biases in studies of satellite aerosol products, aerosol-cloud interactions, and aerosol direct radiative effects.

/pdf/global-calipso-observations-of-aerosol-changes-near-clouds-1spgb35h11.pdf

Global CALIPSO Observations of Aerosol Changes Near Clouds

Observations of the Earth are extremely challenging; its large angular extent floods scientific instruments with high flux
within and adjacent to the desired field of view. This bright light diffracts from instrument structures, rattles around and
invariably contaminates measurements. Astrophysical observations also are impacted by stray light that obscures very
dim objects and degrades signal to noise in spectroscopic measurements. Stray light is controlled by utilizing low
reflectance structural surface treatments and by using baffles and stops to limit this background noise. In 2007 GSFC
researchers discovered that Multiwalled Carbon Nanotubes (MWCNTs) are exceptionally good absorbers, with potential
to provide order-of-magnitude improvement over current surface treatments and a resulting factor of 10,000 reduction in
stray light when applied to an entire optical train. Development of this technology will provide numerous benefits
including: a.) simplification of instrument stray light controls to achieve equivalent performance, b.) increasing
observational efficiencies by recovering currently unusable scenes in high contrast regions, and c.) enabling low-noise
observations that are beyond current capabilities. Our objective was to develop and apply MWCNTs to instrument
components to realize these benefits. We have addressed the technical challenges to advance the technology by tuning
the MWCNT geometry using a variety of methods to provide a factor of 10 improvement over current surface treatments
used in space flight hardware. Techniques are being developed to apply the optimized geometry to typical instrument
components such as spiders, baffles and tubes. Application of the nanostructures to alternate materials (or by contact
transfer) is also being investigated. In addition, candidate geometries have been tested and optimized for robustness to
survive integration, testing, launch and operations associated with space flight hardware. The benefits of this technology
extend to space science where observations of extremely dim objects require suppression of stray light.

/pdf/multiwalled-carbon-nanotubes-for-stray-light-suppression-in-4jvuv2jwdn.pdf

Multiwalled carbon nanotubes for stray light suppression in space flight instruments

A general modelling formalism is used to extend near-field point spread function (PSF) measurements over a wide-field off-axis angular range, covering a dynamic range of 5 to 6 orders of magnitude, using bidirectional reflectance distribution function (BRDF) measurements of the scan mirror and other key optical elements. Line spread function (LSF) model results were compared with measured near-field LSF measurements for the Moderate Resolution Imaging Spectroradiometer (MODIS) of the Earth Observing System (EOS), demonstrating excellent agreement between model and measurements. When realistic effects of sensor scattered light are taken into account, significant radiometric bias errors are produced near high-contrast structured scenes (e.g. bright clouds over dark oceans and land; broken snow and ice scenes). Image restoration using an asymptotically exact PSF is shown to produce results significantly different from those produced by traditional near-field 5 × 5, 7 × 7, ..., PSF kernel matrix inversion techniques. The results reported have implications for future remote-sensor specifications and testing, in-flight and surface-based calibration comparisons, and the assessment of radiometric bias errors in the presence of moderate- to high-contrast Earth scenes.

https://iopscience.iop.org/article/10.1088/0026-1394/37/5/14/pdf

B Guenther

Papers

Satellite-Earth remote sensor scatter effects on Earth scene radiometric accuracy