Which missle is that? The Sunburn travels at Mach 3, and the newer Yahont travels at Mach 2 - 2.5 but carries a sophisticated guidance system supposedly impervious to ECM, ECCM, and utilizes Artificial Intelligence.
ECM is Electronic Countermeasures. ECCM is Electronic Counter-Countermeasures. The former attempts to prevent enemies from utilizing the electromagnetic spectrum, the latter attempts to defeat enemy ECM.
There is no such thing as a guidance system immune to ECM. Please do not accept the manufacturers brochure as being a fair and impartial analysis of a weapons system...