The Global Positioning System (GPS) is a space-based global navigation satellite system that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible by anyone with a GPS receiver. GPS was created and realized by the U.S. Department of Defense (DOD) and was originally based on and run with 24 satellites. It was established in 1973 to reduce the large number of navigation aids and to overcome the limitations of previous navigation systems.