This paper is the first paper of the sequence devoted to modern methods of protection of coherent pulse radars against combined interferences (additive internal noise mixture masking the active jamming and clutter (passive jamming)). It compares the ultimate capabilities of the known and relatively new varieties of the interference protection (anti-jam and anti-clutter) systems under the hypothetical conditions of exact knowledge of statistical characteristics of signals and interferences. The ultimate capabilities of systems are understood in the sense that their efficiency is calculated for the hypothetical conditions of exact knowledge of statistical characteristics of input actions. The obtained estimates determine the upper bounds of efficiency in the real conditions of a priori uncertainty of parameters of signals and interferences. The losses of efficiency related to the transition to simplified systems of space-time signal processing (STSP) are also analyzed. The second paper deals with peculiarities (high-speed) of the considered anti-jam and anti-clutter systems in real conditions of parametric a priori uncertainty that is overcome by using different kinds of estimates of a priori unknown parameters of interferences. The third paper is devoted to the substantiation of general-purpose STSP system based on adaptive lattice filters.