While ground spectrometers and sources can operate at a continuum of wavelengths

Question

While ground spectrometers and sources can operate at a continuum of wavelengths (i.e hyperspectral), from space hyperspectral sensing is often quite hard. Most sensors are multispectral with a comparatively small number of bands.   As an example, consider LANDSAT Enhanced Thematic Mapper (ETM) which is the workhorse satellite for the USGS and helps map out the land properties over the entire globe.  

For this sattelite,

a) Why is the sensor good for observing land but not useful for oceans or atmosphere?

b) Based on the LANDSAT channels available, why is it not reasonable for a single channel to discriminate a surface ?

c) Using multiple channels, which LANDSAT channels do you think (do not look it up but reason it out) would be the best to observe vegetation and separate it from both soils and man-made surfaces ?

d) Besides the surface reflection, we have to worry about the atmosphere getting in the way. In particular, the atmosphere can attenuate the solar reflected light from the surface. Keeping this in mind, which Landsat channels should you avoid. Hint. atmospheric transmission window. If a window does not transmit well, explain what in the atmosphere is causing the problem.

e) solar illumination can be different so that even for the same material, the reflectance at different illumination angles may modify the reflectance. How can our vegetation indicator considered help eliminate this concern?

f) Based on this understanding, provide a brief write up based on Google research of 3 separate examples of how vegetation indices are used in environmental studies. This should include high resolution sensors such as Landsat (30 meters) as well as lower resolution sensors such as MODIS

g) We mentioned black body radiation. Find a formula for this radiation and plot the black body spectra for both the earth (300K) and the sun (5800K). Why is the sun yellow?

Explanation / Answer

The landsat satellite are based on the principal of passive remote sensing. The images are generated on the basis of reflectance of the sunlight by the object. since reflectance is not very prominent rather it is negligible from oceans and atmosphere hence study of atmosphere and ocean is not possible. Various objects reflects light in different wavelength range, and each channel is designed for measuring the reflectance in particular wavelength range, hence a single channel is not enough to differentiate between the various object. We all are aware that the satellite imaging depends upon the surface reflectance. The vegetation index is ratio of the surface reflectance in different wavelength ranges. In this regard it is imperative to note that the plants absorbs most of the light in visible range and reflect light is Near infrared band. Within the visible band the absorption in red band is maximum, whereas as from soil the reflectance in red band is maximum, in contrast for NIR water has high absorption whereas vegetation has high reflectance, hence to highlight the vegetation and differentiate between the various features, we can use the ratio of the channel for max absorption to the channel for maximum reflectance. Thus the suitable bands are band 4 (Red) and Band 5 (NIR). We should avoid using band 11. It has been observed that the spectra has been variedly influenced and these includes: Modification of spectral as well as spatial characteristics of incident light Attenuation of transmitted light Scattering

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