From Halopedia, the Halo wiki
UNSC Astronavigation, also known as ASTRONAV, is a branch of the United Nations Space Command. It deals with the creation and maintaining of navigational data, such as the location of human colonies.
Because of the sensitive nature of this information, all ASTRONAV personnel were secretly implanted with subdural pulse-coded emitters so ONI field officers could track them in the event of Covenant invasion. In this situation, ASTRONAV personnel were to be taken to secure locations, and if that was not possible, they were to be terminated.
A good Astronavigator could identify and triangulate a very precise point in space using just a few known XNAV Pulsar signatures and any consumer-grade spectrometer or field kit.
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