Track-via-missile

Track-via-missile or TVM refers to a missile guidance technique which combines features of semi-active radar homing (SARH) and radio command guidance.

Explanation
TVM guidance requires a radar ground station and a missile with a radar receiver. As with semi-active homing missiles, the ground-based radar illuminates the target with radar energy which is then reflected off the target and detected by the missile. However, unlike a SARH missile, the missile itself does not compute interception with this information. Instead, data from the radar returns is relayed back to the ground station via a data link.

Advantages

 * Unlike an active radar homing missile, the missile does not alert the target to the fact that it is homing in on it by illuminating it with radio waves. Typically, the target will be aware that it is being illuminated by the SAM radar, but it will not know for certain if it has been engaged. Modern phased-array radars, by virtue of their thin beams and low side lobes make detection by the aircraft even more difficult.
 * Unlike semi-active radar homing missiles, the electronics needed to calculate and follow an interception path do not need to be built into each missile, reducing their complexity, weight and cost. It is also possible to make the missiles more accurate by using more sophisticated algorithms for calculating interception than would be possible in the limited processor in a missile. In addition, it is possible for operators to adjust the missile's flight path throughout the engagement, even during the terminal homing phase.
 * Unlike radio command guided missiles, because the missile's radar receiver is much closer to the target than the ground station, more accurate tracking information can be generated for the system's computer. It is also more difficult to jam or spoof the tracking signal.

It is also possible for the ground station to receive direct radar reflections from the target (rather than the data downloaded by the missile) and combine the two sources of information to generate the interception course. This adds an extra element of 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 Russian S-300PS/S-300F/S-300-PMU (NATO designation SA-10 "Grumble") anti-aircraft/anti-Cruise missile (with Anti-ballistic missile capability.
 * The U.S. MIM-104 Patriot anti-aircraft and anti-cruise missile system (with some limited anti-ballistic missile capability)