2.13 Atmospheric Sensors

Atmospheric sensors are designed to provide measures of air temperature, vapor, atmospheric constituents, aerosols the etc. as well as wind and earth radiation budget. Figure 2.13.1 shows the important atmospheric constituents for green house effect gases, ozone layer and acid rain.

As remote sensing techniques cannot meet the direct measurement of these physical magnitude, it is necessary to estimate them from spectral measurement of atmospheric scattering, absorptance or emission.

The spectral wave length range is very wide from the near ultraviolet to the millimeter radio wave depending on the objects to be measured (See 1.4).

There are two types of atmospheric sensor, that is, active and passive . Because the active sensor is explained in section 2.15 "Laser Radar" or Lidar,only the passive type sensors will be introduced here.

Two directions of atmospheric observation are usually adopted; one is nadir observation and the other is limb observation as shown in Figure 2.13.1. The nadir observation is superior in the horizontal resolution compared to vertical resolution. It is mainly useful in the troposphere but not in the stratosphere where the atmospheric density is very low.

The limb observation method is to measure the limb of the earth with an oblique angle. In this case, not only atmospheric emission but also atmospheric absorption of the light of the sun, the moon and the stars are measured, as shown in Figure 2.13.1. Compared with the nadir observation, the limb observation has higher vertical resolution and higher measurability in the stratosphere. The absorption type of limb observation has rather high S/N but observation direction or area is limited except for the stars.

There are two types of atmospheric sensors, that is, sensors with a fixed observation direction, called sounders and scanners.

The main element of optical sensor is a spectrometer with a very high spectral resolution such as the Michelson spectrometer, Fabry-Perot spectrometer and other spectrometers with grating and prism.

Figure 2.13.2 shows the structure of Michelson spectrometer called IMG which will be borne on ADEOS (Advanced Earth Observing Satellite to be launched in 1995 by Japan).


Copyright © 1996 Japan Association of Remote Sensing All rights reserved