Process of ultrashort pulse time structure reconstruction using FROG method and/or its variants is analyzed from two starting positions. The first one is in using of some general properties of pulses in a far zone of diffraction. This way permits for the increasing of stability of the inverse mathematical problem solution by reduction of the set of test functions. The possibility of reconstruction of the form of pulse emitted by laser itself from the results obtained in the far zone of diffraction is shown. The second position is in the analysis of space-and-time structure of the interference pattern formed in the nonlinear transparent matter used to produce the signal to be measured. To explain the formation of self-diffraction signal, the hypothesis of Cherenkov emission from this pattern is proposed. Special effects of spectrum transformation are discussed and some indirect confirmation of the idea is demonstrated.