Temperature Profiler RASS
Figure 1: RASS from the company Metek with a 1 290 MHz continuous wave radar
Temperature Profiler RASS
A Radio acoustic sounding system (RASS) or Temperature Profiler is a combination of an acoustic system (sonar) and a radar. It is often called Sodar. Mostly existing wind profilers are completed with an additional loudspeaker system to a RASS or vice versa: a sonar combined with an additional radar.
Operating principle
With one or more loudspeaker towers, strong sound pressure (in the region 135 dBA) is generated in a pulse-like manner, which propagates in the atmosphere as a longitudinal wave. This produces density modulations according to the wavelength of the sound in the air, which then forms the cause of a Bragg reflection for the continuous wave radar.
The air pressure should decrease continuously with the height. Thus the speed of sound should also increase continuously. With different sound frequencies, the maximum of reflected energy is found for the radar, which then allows conclusions to be drawn about the current wavelength of the longitudinal wave. The time interval between emitted sound pulse and registered Bragg reflection is the measure for the height of the air layer with exactly this Bragg wavelength. The speed of sound in the air depends on the air density and this is also dependent on the temperature. Deviations from the expected continuous decrease of the air density is a sure sign for a temperature jump. Thus a virtual air temperature can be calculated from the changed wavelength corresponding to the radiated sound frequency.