System design and signal processing for the RASS method

Dr. Dominik Exel M.Sc.

Dr. Dominik Exel M.Sc.
Supervisory committee:

Univ.-Prof. Dipl.-Ing. Dr. Bernhard Zagar

A.Univ.-Prof. Dipl.-Ing. Dr. Christian Diskus

Prof. Dr.techn. habil. Stefan Johann Rupitsch

Final exam: January 07th, 2021

The radio-acoustic-sounding (RASS) principle, known from meteorology, enables contactless measurement of temperature profiles of gases. An emitted sound pulse causes a change in the refractive index in the air due to the density fluctuation. These fluctuations allow scattering of electromagnetic waves. This physical law is used to determine the propagation speed of this sound pulse with a Doppler radar.

 

Figure 1: The transmitted radar signal is scattered at the propagating sound wave fronts. To detect a measurable signal, the Bragg condition and the collocation must be met.

Based on the temperature-sound velocity relationship, the temperature of gases can be measured contactless. Adequate backscattering efficiency of the electromagnetic power is a prerequisite for reliable functionality. On the basis of the derived receive power it is shown which factors influence the system behavior. The collocation of the sources and the correct match of the wavelengths (Bragg-condition) are essential. Collocation means that the phase centers of the radar antenna and the tweeter are virtually the same. One realization of this principle is possible with a fine metal grid, which is transparent for the sound waves and acts like a mirror for the electromagnetic waves.

Figure 2: The electromagnetic waves are deflected by the metal grid. Thus the phase center of the tweeter and the radar antenna are virtually the same.

Based on the theory of signal generation, the expected low signal power and the therefore very unfavorable signal-to-noise ratio an optimized signal processing algorithm is presented and derived. It shows that the variance of the temperature estimates changes indirectly proportional to the 4th power of the Bragg cycles. The practical application of this measuring system is shown on the basis of various measurement results.

 

 

Keywords: RASS, Doppler-Radar, non-contact temperature measurement