The Global Positioning System (GPS) is the only Allied support power. It allows a player to see all enemy movement on the map, with the exception of submarines.
The Allied tech center article contains information about obtaining the GPS.
The GPS grants a significant advantage to the player, this makes the Allied tech center a popular target. In particular, when the GPS is on, artillery can be aimed at enemies without the need to scout, making it more dangerous. As of the most recent update, artillery no longer auto-targets enemies that can only be seen via GPS.