The Directional Unit is part of the Main Time Mechanism. It takes the Space-Time Coordinates for departure and arrival and converts them into Epsilon Coordinates for travel through the Space-Time Vortex. For this reason it is essential to accurately know your current location to ensure a successful flight. The shorter the trip, the simpler it is to calculate the Epsilon Coordinates. Executing a short hop, however, has other difficulties. The resulting Epsilon Coordinates serve as a path through the Space-Time Vortex allowing a TARDIS to avoid any unknown regions or areas of intense time spillage. In practice the Directional Unit is accurate down to about 3 meters or so.

A fully functional Directional Unit is essential for precise navigation and landings. If the Unit's Linear Calculator is not serviced every 500 years it can cause missjumps. A TARDIS that missjumps often finds itself at a Temporal Nexus Points (a time zone where history is easily changed). There are some locations that can only be reached by traveling through dangerous areas of the Vortex. Using Manual Override, a course can be plotted to these locations.

A directional unit from a Mark IV TARDIS Console requires more power then one installed on a Mark I. If a Mark IV Directional Unit is used on a Mark I Console it will burn itself out after one use to prevent the destruction of the central column. Even using the Fast Return Switch, a TARDIS with fused coordinates can only travel back and forth from its current location to its previous landing point. (with a leeway of about 18 months in either direction).