An Improved High-Resolution Processing Method for a Frequency Domain Interferometric Imaging (FII) Technique

Abstract

Frequency hopping [also recently called range imaging (RIM) or frequency domain interferometric imaging (FII)] is a pulse compression technique used to improve the range resolution Δr of Doppler radars limited by their minimum transmitted pulse length. This technique can be seen as an extension of the dual-frequency domain interferometry (FDI) technique, since it consists of transmitting more than two adjacent frequencies. Similarly to antenna array processing used for angular scanning, RIM/FII enables range scanning along the vertical line of sight to obtain a range profile (classically called “brightness” in the literature of the field of antenna array processing). The performances of RIM/FII can be improved by using high-resolution methods such as the maximum likelihood method (or the Capon method), the singular value decomposition method with the multiple signal classification (MUSIC) algorithm, and the newly introduced improved maximum likelihood method (the Lagunas–Gasull method). The applications of such methods would permit us to investigate in detail the small-scale dynamics within the stratified atmosphere where very thin structures, such as temperature sheets, coexist with thin turbulent layers. First, simulations are presented in order to compare the performances of the Lagunas–Gasull method with respect to the other methods already discussed by Palmer et al. and Luce et al. The second part of this paper is devoted to presenting some preliminary results with a 45-MHz miniradar profiler located at Toulon, France (43.7°N, 5.58°E), and to show other applications of the Lagunas–Gasull method on data collected with the VHF middle- and upper-atmosphere (MU) radar located at Shigaraki, Japan (34.85°N, 136.10°E). These results demonstrate the applicability of the Lagunas–Gasull method on two different VHF Doppler radars.