Transporter lock
From Memory Alpha, the free Star Trek reference
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A transporter lock is a status condition in transporter systems indicating that the targeting scanner has located the coordinates of a person or object to be beamed. Obtaining or maintaining a transporter lock lets the transporter operator to know the subject's location, even in motion, allowing the beaming process to start more quickly. This is an essential safety precaution when a starship away team enters a potentially dangerous situation that would require an emergency beam-out.
A transporter lock is usually maintained by tracing the homing signal of a communicator or combadge. When there is a risk that such devices would be lost in the field or are otherwise unavailable, personnel may be implanted with a subcutaneous transponder before an away mission to still provide a means to maintain a transporter lock. Alternatively, sensors may be used to scan for the biosign or energy signature of a subject, which can then be fed into the transporter's targeting scanner for a lock.
