The Directional Unit is part of the Main Time Mechanism. It takes the Space-Time Coordinates for departure and arrival/tv] 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. A fully functional and precisely tuned TARDIS is accurate to 1 millisecond and 1 meter. But in practice the Directional Unit is accurate down to about 3 meters or so. The "closer" a TARDIS is to Gallifrey's Time Zone (3.5 billion years ago and the centre of the Mutter's Stellian Spiral) the easier it is to navigate the Vortex. A TARDIS's computers require 12 minutes to re-orientate after materialization.
A fully functional Directional Unit is essential for precise navigation and landings. If the TARDIS hasn't been recently overhauled then the operator might need to changed the relativity unit before attempting a space-time jump. If this isn't done the TARDIS could end up temporally overshooting its intended time zone. A TARDIS that miss-jumps often finds itself at a Temporal Nexus Point (a time zone where history is easily changed). There are some locations that can only be reached by travelling 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 than 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).
The following color code is used:
- Black: For information from the TV Series, including Dimensions in Time, and 1996 TV Movie.
- Blue: For information from the Novels and Audios including Target, Virgin, BCC, and Big Finish.
- Green: For information from 'licensed' reference sources such as the Technical Manual, Doctor Who Magazine, and the Role Playing Games.
- Red: For information from unofficial sources -The Faction Paradox series, behind the scenes interviews, author's speculation, and popular fan belief.
The TARDIS Technical Index is copyright Will B Swift.