From Halopedia, the Halo wiki

REACH DOC NET was a computer system on the planet Reach, presumably to keep tabs on military personnel and matériel around the planet. On August 30, 2552, the AI Doppler alerted FLEETCOM that Covenant data invasion packets were penetrating firewalls protecting REACH DOC NET, forcing counter-intrusion security to be enacted.[1]

During the Fall of Reach, Doppler also detected that the UNSC Circumference had been disconnected from REACH DOC NET, resulting in Doppler's failure to recognize its intact navigation database. Three SPARTAN-IIs were subsequently dispatched from UNSC Pillar of Autumn to destroy it.[1]

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