In the past few years the technique of Frequency Domain Interferometry(FDI) has been developed on VHF radar. By using this technique, the characteristics of a very thin atmospheric layer structure, which is embedded in the radar volume and can not be resolved by the conventional VHF radar with only one operational frequency, can be determined through the calculation of the coherence and the phase from two echo signals with slightly different operational frequencies. In this article the results of a frequency domain interferometry experiment carried out on Chung-Li radar are presented. The use of four discrete frequencies and three different frequency separations distinguishes this study from previous investigations. It is shown experimentally that the signal-to-noise power ratio indeed affects significantly the magnitude of the observed coherence. For the first time the dependence of coherence on frequency separation is also investigated. The results show that coherence decreases with frequency spacing with the roll-off rate ranging from -0.2/MHz to -1.2/MHz, depending on the atmospheric structure. After best fitting the theoretical model to the observed coherence taken from a height range with no atmospheric layer existing, the effective range resolution can be found. Employing this information, the thicknesses of two prominent and lasting layers locating at height 7.2 km and 7.8 km are estimated through the least square fitting process, and found to be 114 and 186 meters, respectively.
