Atmospheric Monitoring with Passive Techniques

A special problem in optical atmospheric monitoring is the ever-present turbulence of the air that inalces the recorded light intensity vary substan- 

The background distributed radiation source (the sky, walls, ground or an artificial heat blanket) must then have a different temperature than the gas to be imaged (see also Fig. 7.29 and [6.180]). 

An enhanced correlation set-up with no moving parts, can be achieved by modulating the absorption spectrum of the gas in a reference cell. Then the optical path does not need to be divided into two paths, but the external gas can be observed through the modulated ceil as indicated in Fig. 6.78, where the special case of light transmission over a considerable distance by means of fibre optics is indicated [6.181]. This can be performed using pressure, Stark 

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We will discuss two active remote-sensing techniques for the atmosphere — the long-path absorption technique and the lidar technique. However, we will first consider a passive technique, in which lasers play an important part in the detection scheme. This optical heterodyne technique is even more frequently used for signal recovery in connection with the active optical remote-sensing methods. The field of laser monitoring of the atmosphere is covered in several monographs and articles [10.70-10.76]. 

Spectroscopy in the microwave region yields valuable information on the rotational structure of molecules [7.50-52]. Apart from this type of fundamental experiment, important applications of microwave radiometry include atmospheric and ocean monitoring [7.53-56]. Passive, as well as active techniques are used for different types of remote sensing. Active micro-wave techniques are pursued with radar systems. 

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