Transponder and prefix codes

Starfleet vessels transmit a transponder signal that can be used to identify them. Prefix codes prevent unauthorized personnel from taking control of the ships computers. Starfleet uses a system of transponder codes to identify and locate its starships across the vast reaches of space. All Federation vessels are equipped to emit a transponder code. his is a kind of IFF signal, which stands for Identification: Friend or Foe?, a phrase that originates with the early black-box transponders installed in aircraft on Earth in the 20th century
Transponders can prove extremely useful if a ship is damaged or destroyed. In these circumstances, the ship's flight recorder transmits a transponder signal across subspace allowing it to be located and retrieved as soon as possible. Transponders are invaluable in this kind of situation, because if a ship is severely damaged it may not be generating any kind of energy signatures that can be used to locate vessels.
== Instant Recognition ==
Transponder codes in the 24th century transmit basic information about a vessel. This often includes the ship's name, political allegiance, and its class. Because transponder codes are normally broadcast over subspace, they can be used to locate a ship over very long distances, and therefore overcome many of the limitations of sensor systems.
Transponder codes can be extremely useful when a group of ships are acting together, since they enable the fleet's commander to instantly identify each vessel under his or her command and to see exactly how they are deployed. Federation vessels automatically cross-reference transponder codes with onboard computer records and sensor data to gain a detailed picture of an approaching vessel.
The use of transponder codes is widespread among other races, but for tactical reasons the codes are not broadcast openly. Transponders are not intended to allow anybody and everybody to locate a ship; cloaked vessels, for example, do not transmit them.
Transponder codes are normally only transmitted over subspace in code. This prevents enemy ships from using them to track vessels indiscriminately and ensures that military vessels retain a tactical advantage.
== Sensor identification ==
Of course, transponder codes are not the only way to identify a vessel. 24th century sensor systems are extremely sophisticated and, at close range, can gather enough data to identify any known vessel. If a ship enters a heavily developed area, a network of sensors on board ships, space stations, and planetary installations can be used to relay information to a tactical operations center. All this effort achieves the same result as a single subspace receiver picking up a coded transponder signal.
=== Sensor ghosts ===
However, it is not impossible to deceive sensors. One of the primary means of identifying a ship is by using its warp signature, and ships can be modified to emit fake ones.
== Prefix codes ==
Federation vessels use another code which is vital to the ship's security. This code is known as the prefix code and is designed to prevent an enemy vessel from remotely taking control of the Federation vessel's computer systems. Every Starfleet vessel is assigned its own unique code when it is launched. All Starfleet ships carry a record of all the prefix codes, and in an emergency they can be used to take control of another vessel. The prefix codes are seldom used, but there have been instances where knowledge of a ship's prefix code has been instrumental in winning a battle or preventing a war.