The NAV Marker Transmitter[1][2] is a small ground tracking device of the United Nations Space Command.
They are small devices, relatively the size of a tick, and can be discreetly attached to an object. The transmitter will then broadcast the location of that object to a nearby UNSC Heads-Up Display calibrated to their frequency.
NAV Marker Transmitters have been used by Spartans during their operations, such as the raid on the URF's Jefferson Station.