From Halopedia, the Halo wiki
The Tripoli was a remote station held by the UNSC. On November 3, 2552, it was the meeting place for Admiral Carl Patterson's battle group that had been assigned to go to Onyx for the aid of Dr. Catherine Halsey and the UNSC forces trapped on the planet. After returning to Earth from Installation 05, the prowler UNSC Dusk was ordered by Fleet Admiral Hood to follow the slipspace wake of the Covenant destroyer Bloodied Spirit, hijacked by Spartan-II Blue Team, until it reached Tripoli, where the Dusk was to rendezvous with Admiral Patterson's task force.
The station was named after Tripoli, the capital of Libya.