Halo 4 Campaign Mission 3

Master Chief has now started to pick up the UNSC Infinity’s signal, but it is reading from the interior of the planet. Cortana and Chief decide to find the mass transit system to reach the interior of the planet. They reach the interior to only find a satellite in the direct center of the planet.

Cortana then realizes that it can be used to send a signal to Infinity to warn them about the gravity well, but two different energy pylons are scrambling the message. They then precede through a portal to the first pylon where they find various Prometheans guarding it. After defeating the enemies, they shut down the pylon, boosting both the signal and the ability to send the message.

They then precede to the next pylon, only to find that Covenant cruisers have made their way inside and are fighting the Prometheans. The two shut down the second pylon and race to the satellite to send the message. They reach the controls and try to send the message, but instead release the Didact . (Source)