The Sentinel is used to detect whether information being exchanged through satellite is Fairy relevant. Most of the time, it's someone playing a fantasy video game. Each fairly-related call is given an urgency rating, and the more fairy words mentioned, the higher the rating and severity.
In the Eternity Code, Artemis Fowl made a phone call listing all the fairy-related names he knew, knowing that this would be picked up by the Sentinel with an extremely high rating and arouse great suspicion. He deducted that this would result in a member of the LEP venturing above ground to his location. Artemis needed a fairy to heal Butler after a fatal bullet wound nearly killed him, therefore, he figured, a well trained LEP member would be the best to heal his bodyguard.
In this case, instead of the Sentinel being used to insure the safety of the People, it was used merely to call for help by the genius Artemis Fowl.