Phase difference method at UHF frequency band for hydrometeor detection

The advancements in microelectronics, high-frequency electronics, and FPGA enabled the development and further availability of software-defined receivers (SDR). This article deals with the possibilities of using the sources of electromagnetic energy for the detection of the hydrometeors in the lowest layer of the atmosphere in real time. Our effort was targeted toward the space and time localization of precipitation and its intensity by using frequencies around one GHz.

Electromagnetic signal attenuation by water vapor and by precipitation either in liquid or solid form in these bands is immeasurable and it is basically lost in the noise. The use of signal attenuation by the liquid precipitation, typically by using the remote directional connection, is effective in microwave bands over 10 GHz.

Refractivity is the physical value that impacts electromagnetic signal penetration through the atmosphere. It depends on the air pressure, temperature, and saturated water vapor, and from the point of view of electromagnetic signal propagation it represents the properties of the material environment which influences its speed and dispersion. Determination of the refractivity value is a quite complicated process. It requires sophisticated instrumentation based on highly accurate time normal. This work is bringing a method that is using atmospheric refractivity changes (its gradient) between different trajectories along which electromagnetic energy is spread from the broadcaster to a simple antenna system. A coherent SDR receiver is used for the evaluation of time delays between the signals coming to different antennas. The time shifts are converted to the frequency changes towards the frequency at which the observation was done. An advantage of this method comes from the fact that it does not depend on the signal strength until it is available. It is a fully passive method using external sources of the electromagnetic signal and the signal does not need to be decoded. Phase difference between the signals is related to the frequency of the source signal and depends on the properties of the atmosphere.

Performed experimental measurements recognized that the evaluation of the phase changes by a coherent receiver is correlated with different atmospheric phenomena and showed the possibility to detect them. The phase change reacted to different atmospheric phenomena mostly, but not exclusively, related to hydrometeors. Detection of the local as well as frontal showers and thunderstorms with high-intensity precipitation was clearly shown as well as the relaxation of the atmospheric humidity after the time-limited precipitation. Further to that the method reacts to atmospheric turbulence and to the thermal inversion process.

The results come from a certain number of time-limited experiments. Continual monitoring in selected localities is envisaged in close future.