11.4 Remote sounding techniques

These methods were originally developed for meteorology and control of industrial emissions.

1. LIDAR: compares laser back scattering or transmission. DIAL: differential absorption LIDAR, works at two frequencies (on and off the atmospheric line of interest), detects 0.01 g/$m^3$ water vapor. Disadvantage: works best from aircraft, expensive equipment.
2. SODAR: Remote sounding with sound waves. Detects turbulence, but gives little quantitative results.
3. IR window: H$_2$O line absorption in front of a strong continuum source (Sun, Moon, Jupiter). Disadvantage: Directions of observing and monitoring beam differ. The phase correction degrades as a function of the separation angle and the distance of the dominant turbulent layer.
4. Radiometric: Uses the atmospheric emission. Dedicated monitors operate mostly near the 22 GHz or 183 GHz lines (several spectral channels). The inter-line regions of the 1mm and 3mm windows are also sensitive enough, but make it difficult to remove cloud emission.

For the radiometric approach, it is useful to study the sensitivity as a function of frequency, i.e. by how much the sky emission changes for a fixed fluctuation of water vapor, which corresponds to a fixed wet path fluctuation. Fig.11.3 shows what change in $T_{sky}$ one must measure under conditions of various humidity.

There are two reasons to use the 22.2 GHz line: Clouds are easier corrected at this frequency, and receiver components are less expensive.

One notices that the 84-116 GHz window is 1-2 times as sensitive as the 22.2 GHz line, and the 210-248 GHz window 4.5-8.3 times. A dedicated receiver near the 183 GHz water line would have the highest sensitivity, but can suffer from temperature dependent saturation effects. It is better adapted for sites where the total amount of water above the instrument is typically less than 3 mm.