In a resupply crate on the left when leaving the Presidential metro.
This is Paladin Tristan.
If you're listening to this, then you've made it through the Presidential Metro and Adams Air Force Base is all that stands in your way. Our scouts report that the fire controls for the orbital missile satellite are located in the main tower of the Enclave Mobile Platform. The only way into the platform is by way of a ramp that lowers from a watch post the Enclave set up in the old air traffic control tower. When you opened the crate it triggered a beacon. We'll pick up the signal and start the assault to cover your entry.
Good luck, and Godspeed.