The perfect sequences are so ideal that all out-of-phase autocorrelation coefficients are zero, and if the sequences are used as the coding modulating signal for the phase-modulated radar, there will be no interference of side lobes theoretically. However, it has been proved that there are no binary perfect sequences of period 4 〈 n ≤ 12100. Hence, the almost perfect sequences with all out-of-phase autocorrelation coefficients as zero except one are of great practice in engineering. Currently, the cyclic difference set is one of most effective tools to analyze the binary sequences with perfect periodic autocorrelation function. In this article, two characteristic formulas corresponding to the autocorrelation and symmetric structure of almost perfect sequences are calculated respectively. All almost perfect sequences with period n, which is a multiple of 4, can be figured out by the two formulas. Several almost perfect sequences with different periods have been hunted by the program based on the two formulas and then applied to the simulation program and practical application for ionospheric sounding.
On the basis of the analysis of the system sounding principle, this paper introduced a new type of ionospheric oblique backscattering sound system, which is based on the pseudo- ran- dom noise phase modulated pulse compression. According to the high requirements of real-time and a large amount of computation and echo characteristics, a high-speed real-time signal processing system was established and the design of system hardware and software was focused on. The sounding results indicate that the system is equipped to handle the data fast and has a high degree of software features. It is of great significance for the realization of fast, real-time ionospheric sounding means.
