Choice of wavelengths
Gaseous absorption
Atmospheric scattering
Reflection and emission from the earth's surface
Reflection from a plane surface Choice of wavelengths
Gaseous absorption
Atmospheric scattering
Reflection and emission from the earth's surface
Reflection from a plane surfaceIt is obviously important for the remote sensing and the earth from a satellite, or other platform, to choose wavelengths where the atmosphere is sufficiently transparent. The only (and important) exception is when selective atmospheric absorption is used to infer the properties of some atmospheric constituent.
The atmosphere is never completely transparent to electromagnetic radiation. However, at some wavelengths it has good transparency, whereas at other wavelengths it is very absorbing. It also scatters radiation through the molecules, aerosols and cloud particles, which can affect the visibility considerably.
Atmospheric scattering varies more uniformly and slowly with wavelength, generally decreasing as the wavelength increases, or the particle, or molecular, size decreases.
Chief molecular absorbers in the atmosphere are water vapour and carbon dioxide, with contributions also from ozone, methane, nitrous oxide, and others. The general atmospheric transmittance across the whole spectrum of wavelengths is also shown in Figure 4. Wavelengths used for remote sensing of the surface are chosen in windows where the atmos-pheric transmittance approaches 100%.
In general, atmospheric absorption tends to become stronger from the visible to the infrared. Water vapour causes most absorption in the near infrared from 0.7um to 6 um (but with C0 2 bands at 2.7 um and 4.3 um), there is strong C02 absorption around 15 um, then intense water vapour absorption takes over right through to about 1 mm wavelength
In the microwave region, the O2 molecule has strong absorption bands at about 2.5 and 5 mm and H2O has a band at 1.35 cm.
Atmospheric particles and molecules scatter radiation, thus causing a solar reflection
in the atmosphere, which lowers the surface contrast. The particles have sizes between
less than 0.1 um and over 1 um. When A becomes greater than particle diameter, the
scattering becomes less efficient. The scattering cross-section b is defined by:
For molecular scattering,
For aerosol scattering
but this varies with aerosol type.
Generally, molecular scattering is important from the UV to about 0.8 um, aerosol scattering up to about 2 um, cloud scattering to about 1 mm and scattering from rain to about I cm. Scattering can also be used to examine the properties of the particulates, such as in radar detection of rain, or lidar detection of volcanic aerosols.
The visible transmittance of aerosols is about 0.7 to 0.9. The transmittance of clouds is normally less than 0. 1, except for high, thin ice clouds.
In both active and passive remote sensing we are concerned with the
intensity of either reflection or emission of the earth's surface. The reflection
intensity depends on the surface refractive index, the absorption coefficient and the
angles of incidence and reflection. Of course, we are generally not looking in a direction
such that i1 = i2 in Figure 2, that is specular
reflection. Most natural surfaces are rough and the reflected radiation, due to multiple
scatterings, leaves the surface in every direction. However, the reflected intensity still
does depend to some extent on the solar angle, and water surfaces give specular
reflection.
For a passive system, the angles of incidence and reflection are determined by the
relative positions of the Earth and the Sun, as shown in Figure 6. The resultant
bi-directional reflectance is then a function of the three angles shown.
Figure 6: Bi-directional reflectance from a diffuse (rough) surface such as
vegetation.
4.5 Reflection from a plane surface
Despite difficulties with a diffuse (rough) surface, it is till instructive to consider reflection from a plane surface (e.g. water). We define the reflection coefficient as:
where I2 and I1 are reflected and incident intensities. R also depends on the polarisation of, and we define R1 and Rr as components of the reflectivity parallel and perpendicular to the plane of incidence respectively. In figure 2, I is in the refection plane and r is at right angles to the paper. In figures 7a and 7b, the dependence of R on i1 is shown for both a rather transmitting (e.g. water or glass in the visible), or absorbing surface. Surprisingly, R increases for the latter at vertical incidence. This is because the absorption is so strong that it occurs near the surface, leading to strong reflection (known as anomalous dispersion).
Polarisation components I and r have different reflection properties. In figure 7a, the reflected r component is zero at an angle of 56.3 degrees, leading to complete polarisation. At large angles of incidence, the reflectance of both components approaches one. This behaviour is also observed with rough or vegetated surfaces; but not to the same extent.
Figure 7: Reflection coefficients versus incidence from a plane surface of a
dielectric medium which is (a) transparent and (b) absorbing.
