“Track-via-missile” refers to a missile

Missile

Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...

 guidance technique which combines features of semi-active radar homing

Semi-active radar homing

Semi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external ...

 (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

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 illuminates the target with radar energy which is then reflected off the target and detected by the missile. However, unlike an 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

Data link

In telecommunication a data link is the means of connecting one location to another for the purpose of transmitting and receiving information. It can also refer to a set of electronics assemblies, consisting of a transmitter and a receiver and the interconnecting data telecommunication circuit...

.

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
  Surface-to-air missile
  A surface-to-air missile or ground-to-air missile is a missile designed to be launched from the ground to destroy aircraft or other missiles...
  
   radar, but it will not know for certain if it has been engaged. Modern phased-array
  Phased array
  In wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna 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
  Radar jamming and deception
  Radar jamming and deception is the intentional emission of radio frequency signals to interfere with the operation of a radar by saturating its receiver with noise or false information...
  
   or spoof
  Spoofing attack
  In the context of network security, a spoofing attack is a situation in which one person or program successfully masquerades as another by falsifying data and thereby gaining an illegitimate advantage.- Spoofing and TCP/IP :...
  
   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

Electronic countermeasures

An electronic countermeasure is an electrical or electronic device designed to trick or deceive radar, sonar or other detection systems, like infrared or lasers. It may be used both offensively and defensively to deny targeting information to an enemy...

 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

Surface-to-air missile

A surface-to-air missile or ground-to-air missile is a missile designed to be launched from the ground to destroy aircraft or other missiles...

 systems use the track-via-missile technique. This includes:

• The Chinese HQ-9
  HQ-9
  The HQ-9 is China’s new generation medium- to long-range, active radar homing air defence missile.Initially an indigenous design with limited capabilities, the HQ-9 missile has undergone a redesign to incorporate Russian rocket technology after the acquisition of S-300 5V55-series missiles from...
  
   medium- to long-range, semi-active radar homing missile. There are allegations that China illegally acquired Patriot missile technology to develop the HQ-9.
• The Russia
  Russia
  Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
  
  n S-300PS/S-300F/S-300-PMU (NATO designation SA-10 "Grumble") anti-aircraft/anti-Cruise missile
  Cruise missile
  A cruise missile is a guided missile that carries an explosive payload and is propelled, usually by a jet engine, towards a land-based or sea-based target. Cruise missiles are designed to deliver a large warhead over long distances with high accuracy...
  
   (with Anti-ballistic missile
  Anti-ballistic missile
  An anti-ballistic missile is a missile designed to counter ballistic missiles .A ballistic missile is used to deliver nuclear, chemical, biological or conventional warheads in a ballistic flight trajectory. The term "anti-ballistic missile" describes any antimissile system designed to counter...
  
   capability.
• The Russia
  Russia
  Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
  
  n S-300V (NATO designation SA-12 “Gladiator”/“Giant”) anti-aircraft/anti-cruise missile (with Anti-ballistic missile capability.
• The U.S. MIM-104 Patriot
  MIM-104 Patriot
  The MIM-104 Patriot is a surface-to-air missile system, the primary of its kind used by the United States Army and several allied nations. It is manufactured by the Raytheon Company of the United States. The Patriot System replaced the Nike Hercules system as the U.S. Army's primary High to Medium...
  
  anti-aircraft and anti-cruise missile system (with some limited anti-ballistic missile capability)