Track-via-missile or TVM refers to a missile guidance technique which combines features of semi-active radar homing (SARH) and radio command guidance. This avoids the problems with terminal accuracy normally seen by command guided missiles, especially at long range. It has been used on a number of long-range surface-to-air missiles (SAMs) including the MIM-104 Patriot. Explanation Command guidance has the advantage of isolating most of the equipment for the missile guidance at the launcher, where size and weight are significantly less important. In these systems, the radar that provides guidance is on the ground or ship and the missile lacks an independent guidance system. Typically two radars are used, one tracking the target and another the missile, so that they can fly independent and widely separated paths. A computer then calculates the position and velocity of the two and calculates an intercept point. The same computer then calculates the control inputs needed to fly the missile to that point and sends any required corrections to the missile via a radio signal, often using the radar tracking the missile as the radio signal. As this system is simple to build, it was used as the basis for many early surface-to-air missile (SAM) systems. However, it has a significant drawback, especially for long-range fire. Radar signals spread out in space, similar to the cone-like beam of a flashlight, with typical beam spreads on the order of around 5 degrees. This means that at longer ranges, the target's location is only known within a gross value, perhaps on the order of several kilometers. Various signal encoding techniques can be used to narrow this down to something on the order of 0.1 degrees, but at long range, this still provides accuracy only on the order of hundreds of meters. This inaccuracy demands a huge warhead to ensure the destruction of the target. This problem is avoided in the semi-active radar homing (SARH) concept. In these systems, the ground station still illuminates the target with its radar, but the receiver is on the missile. The reflection of the original signal off the target produces another cone-like beam, but one that is narrowest at the target. The receiver on the missile uses this signal to guide on, thus becoming increasingly accurate as it flies toward the target. There are a number of minor issues that result in a maximum accuracy (for early designs) on the order of