Io Station

Io Station was a United Nations Space Command military platform, believed to be in orbit around Io, the third largest moon of Jupiter.

Presumably, it picked up a communications fragment (located near Io) from the Prophet of Regret's fleet before it arrived in the Sol System.

It contacted John-117 on the Forerunner Dreadnought before the Spartan contacted Cairo Station on Earth during the Second Battle of Earth.