Atmospheric Window

One important practical consequence of the interaction of electromagnetic radiation with matter and of the detailed composition of our atmosphere is that only light in certain wavelength regions can penetrate the atmosphere well. These regions are called atmospheric windows.

The following figure shows the amount of absorption at different wavelengths in the atmosphere. Figure 1 shows the half-absorption altitude. Half Absorption Altitude is defined to be the altitude in the atmosphere (measured from the Earth's surface) where 1/2 of the radiation of a given wavelength incident on the upper atmosphere has been absorbed. Windows correspond to those regions where the half-absorption altitude is very small.

The dominant windows in the atmosphere are seen to be in the visible and radio frequency regions, while X-Rays and UV are seen to be very strongly absorbed and Gamma Rays and IR are somewhat less strongly absorbed. We see clearly the argument for getting above the atmosphere with detectors on space-borne platforms in order to observe at wavelengths other than the visible and RF regions.

Within the electromagnetic spectrum, which spread from radio waves to the gama rays, the earth's atmosphere has two windows regions. They are

1. Near visible window

2. Radio window

Radio Window

The radio window is the range of frequencies of electromagnetic radiation that the earth's atmosphere lets through. The wavelengths in the radio window run from about one centimetre to about eleven-metre waves.

Radio Window region includes microwave and radio wave frequencies.

Ultra high frequency UHF 300–3000 MHz 1 m – 100 mm

television broadcasts, microwave ovens, mobile phones, wireless LAN, Bluetooth, GPS and Two-Way Radios such as Land Mobile, FRS and GMRS Radios

Super high frequency SHF 3–30 GHz 100 mm – 10 mm

microwave devices, wireless LAN, most modern Radars

Extremely high frequency(EHF) 30–300 GHz 10 mm – 1 mm

Radio astronomy, high-frequency microwave radio relay

Fine spectal Classification

P band

L band 1 to 2 GHz

S band 2 to 4 GHz (2300 to 2500 and 2700 to 3700)

C band 4 to 8 GHz

X band 8 to 12 GHz

Kuband 12 to 18 GHz

K band 18 to 26.5 GHz

Kaband 26.5 to 40 GHz

Q band 30 to 50 GHz

U band 40 to 60 GHz

V band 50 to 75 GHz

E band 60 to 90 GHz

W band 75 to 110 GHz

F band 90 to 140 GHz

D band 110 to 170 GHz

References:

1. http://csep10.phys.utk.edu/astr162/lect/light/windows.html

2. http://en.wikipedia.org/wiki/Radio_window