Passage: A new atomic clock being developed for navigation satellites will perform better than previous devices. The clock will use a new microwave cavity design to provide a compact and lightweight package and new electronic techniques to maintain long-term stability. The clock can provide precise navigation information because it is stable to one second in three million years. The difference in the time when signals from four satellites arrive at one location can be used to calculate that position to within a few yards.

According to the passage, signals from how many satellites will be used to calculate a position?