From Halopedia, the Halo wiki
Slipspace guidance beacons are slipstream space navigational aid devices used by the UNSC. Dropped into slipspace by another vessel such as a prowler, they are intended to assist other vessels in performing short in-system jumps with more accuracy than otherwise possible, something that can be of considerable advantage in a space engagement. Even with the use of guidance beacons however, human slipspace technology is not perfect, especially when the jump is performed near a significant gravity well. This was demonstrated during the Battle of Psi Serpentis in 2543, when a part of Battle Group India reappeared from an in-system jump scattered from the main group despite guidance beacons planted by the prowler UNSC Wink of an Eye.
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