Summary
Communication satellites (COMSats) are small, automated satellites placed in orbit around UNSC colonies and space to relay COM signals.
They are found in orbit above several major UNSC worlds, including Earth, Reach, and Sigma Octanus IV. They can be accessed through the planetary network, and AIs such as Cortana have done so for various purposes.
Cortana once hijacked COMsat 419 during the MJOLNIR shields test at Reach.
Trivia
"419" is also seen as "Echo 419", Foehammer's call sign. These numbers (4, 1, 9) add up to 14 = 7 + seven.