NAVSAT and NAVSTAR
In 1964 the U.S. Department of Defense launched the first GPS satellites. They made up the Transit Navigation System, or NAVSAT. It was designed to guide Polaris submarines. In 1967 the U.S. opened NAVSAT signals to civilian use. Many ships still use it today.
NAVSAT positioning uses the Doppler effect. As a satellite approaches, the frequency of its radio waves shifts upward. As the satellite recedes, the frequency shifts downward. NAVSAT receivers use this information to determine their position relative to the satellites above them. NAVSAT pinpoints a position within 50 yards (46 meters).
NAVSAT led to NAVSTAR (or GPS), the global positioning system we know today. NAVSTAR satellites orbit 10,898 miles (17,539 kilometers) above Earth. They transmit digital radio signals from atomic clocks. These signals include both time and location data. The GPS receiver compares the time the signal was sent to when it was received. This enables it to determine its location.
In all, 24 GPS satellites transmit signals 24 hours a day. In doing so, they reach all points on Earth. Any given spot receives at least four signals. Using three signals, the receiver calculates latitude and longitude. The fourth signal provides altitude. The latter is important for airline pilots and mountain climbers.
Like NAVSAT, NAVSTAR was designed for military use. As a military system, it enables missiles to find enemy targets. Originally, NAVSTAR GPS provided two levels of service. The U.S. military reserved use of its Precise Positioning Service. It provides positioning accuracy within a 24-yard (22-meter) range. A process called selective availability (SA) was used to limit the system's capabilities for Standard Positioning Service. It was used by the public. The U.S. Defense Department has always provided its GPS signals free of charge. But it reserves the right to shut down or block the system at any time.
In 2000 the U.S. government turned off SA. Instead, the current system limits GPS for civilians in sensitive areas. Today, GPS is accurate to within approximately 25 feet (8 meters). But weather and the landscape can affect coverage. The quality and type of receiver matters as well.
GPS for Everyone
GPS has blossomed into a multibillion-dollar industry. Much of the growth has been in civilian use of smartphone apps. Most modern smartphones have GPS capabilities. They use apps to provide directions and track distances for running and other exercises. They can even locate the phone if it is lost. Many cars are now equipped with GPS devices as well. They provide drivers with a map and turn-by-turn directions. Systems built into cars can also track vehicles that have been stolen or involved in an accident.
Specialized GPS receivers provide access to databases of descriptive data. This may be cross-referenced with location information. This is especially helpful in land management and forestry.
Navigation around the World
GPS is not the only global satellite navigation system. Russia's military runs the Global Navigation Satellite System (GLONASS). It has 24 satellites. India and China have regional systems as well.
The European Union (EU) began operating its own satellite navigation system in 2016. It is called Galileo. The system has 18 satellites in space. Six more will be launched by 2020. Galileo will then be complete. It is expected to be accurate to within 3 to 6 feet (1 to 2 meters).
