The correlative update system is a program within DHDs that tells when a Stargate has changed its position. Because of stellar drift, the update system will cause a Stargate to randomly dial the other Stargates to update its location to inform them of where they are.
Ba'al used the update system to send Avenger to the other Stargates, effectively shutting down the Stargate Network. Nerus later discovered a means of disabling the correlative update system, which Ba'al then intended to use to rebuild his empire. (SG1: "Avenger 2.0", "Off the Grid")
Fan Thoughts Edit
Some fans have wondered if after this was discovered why it was never used to find gate addresses. The thought being you would send out a program to one DHD gate that is programmed to go to the other updated gates with the correlative update system and it would find the gate address dialing the next gate and every 5 gates it would dial back to the original gate and upload an updated copy of gate addresses before resuming. within a matter of hours it would in theory have every gate address in the milky way or pegasus galaxy(with the assumption the pegasus gates use the correlative update system)