During the polar mesosphere summer echo (PMSE) campaign in 1988 the first multiple-frequency mesospheric measurements were carried out using the European incoherent scatter (EISCAT) 224-MHz radar. A case study of nearly simultaneous measurements of coherent backscatter, collected on two closely spaced frequencies on July 3, 1988, is presented in this paper. The data are used to investigate the frequency coherence of the radar echoes and to perform frequency domain interferometry (FDI) analysis. The FDI technique provides precise information about the thickness and relative position of isolated scattering layers. The results indicate that scattering layers with thicknesses in the range 85–120 m are sometimes present in the polar summer mesosphere. Such a layer is shown to exist for a period of approximately 10 min, and its position is tracked as it descends over more than 1 km in altitude and transits from one range gate to the next. In addition, the FDI technique is used to study a case where a sudden frequency jump is observed in the Doppler spectrum. Spectral jumps appear to be a characteristic of the 224-MHz PMSE echoes observed at EISCAT. The FDI analysis and Doppler sorting are employed to study the coherence spectrum of the two-frequency data before, during, and after the jump. The frequency jump as well as the signatures that are observed in the coherence spectrum can be explained if the radar is observing a thin scattering layer which has been rippled by a steepened gravity wave.
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