ECM resistance to the system. Disadvantages TVM also has some disadvantages. For example, the data link could potentially be jammed, which is not possible with an active homing or “fire and forget” missile. Additionally, this technique requires the ground-based radar to be active throughout the engagement, potentially aiding aircraft equipped with anti-radiation missiles as they attempt to detect and engage the SAM radar. Another potential disadvantage compared to active radar homing is that the missile must rely on the ground-based radar for guidance, so if the target is able to put an obstacle between itself and the fixed radar system (e.g. a hill), or if it manages to get outside of the radar's tracking envelope (e.g., fly outside of the tracking “fan” of a PATRIOT radar, or fly outside the effective range of another system) then the missile will not be able to detect reflected radiation from the target and thus will be unable to continue the engagement. Examples Most very modern long-range SAM systems use the track-via-missile technique. This includes: The Chinese HQ-9 medium- to long-range, semi-active radar homing missile. The U.S. MIM-104 Patriot anti-aircraft and anti-cruise missile system (with some limited anti-ballistic missile capability) References Missile guidance