buffer, then compare the average of those combined images to determine if it was similar to the data in its small memory system. The data for the flight path was very low resolution in order to free up memory to be used for high resolution data about the target area. The guidance data was computed by a mainframe computer which took spy satellite photos and estimated what the terrain would appear like during low level flight. Since this data would not match the real terrain exactly, and since terrain changes seasonally and with changes in light quality, DSMAC would filter out differences between maps and use the remaining similar sections in order to find its location regardless of changes in how the ground appeared. It also had an extremely bright strobe light it could use to illuminate the ground for fractions of a second in order to find its position at night, and was able to take the difference in ground appearance into account. Tomahawk Block III introduced in 1993 added time-of-arrival control and improved accuracy for Digital Scene Matching Area Correlator (DSMAC) and jam-resistant GPS, smaller, lighter WDU-36 warhead, engine improvements and extended missile's range. Tactical Tomahawk Weapons Control System (TTWCS) takes advantage of a loitering feature in the missile's flight path and allows commanders to redirect the missile to an alternative target, if required. It can be reprogrammed in-flight to attack predesignated targets with GPS coordinates stored in its memory or to any other GPS coordinates. Also, the missile can send data about its status back to the commander. It entered service with the US Navy in late 2004. The Tactical Tomahawk Weapons Control System (TTWCS) added the capability for limited mission planning on board the firing unit (FRU). Tomahawk Block IV introduced in 2006 adds the strike controller which can change the missile in flight to one of 15 preprogrammed alternate targets or redirect it to a new target. This targeting flexibility includes the capability to loiter over the battlefield awaiting a more critical target. The missile can also transmit battle damage indication imagery and missile health and status messages via the two-way satellite data link. Firing platforms now have the capability to plan and execute GPS-only missions. Block IV also has an improved anti-jam GPS receiver for enhanced mission performance. Block IV includes Tomahawk Weapons Control System (TTWCS), and Tomahawk Command and Control System (TC2S). On