Global Positioning Satellite (GPS) navigation has become a household term. The first GPS satellites were launched in 1973, and public access was available in 1994. GPS is maintained by the United States Air Force, but competing systems include the Russian GLONAS system, which works on the same principles.
The GPS system works via continuously broadcasted signals from a constellation of satellites. Each signal contains the satellite’s orbital information and a time stamp from a very carefully calibrated atomic clock. Because the signal broadcasts at the speed of light, the discrepancy in time stamps and orbital information represents a sphere originating from the broadcast point. Three overlapping spheres have only a single point of overlap on the surface of the Earth, providing locational information. A fourth overlapping broadcast sphere will yield a unique point of overlap in three dimensions, and thus, gives altitude information.
The GPS system is a constellation of 24 active satellites, each with an orbit height of 20,200 km, at an inclination of 55 degrees relative to the Earth’s equator. Circular orbits with this parameter have an orbital period of 12 hours and an orbital velocity of 14,000 km/hour, or 3888.89 meters per second. The constellation will have a minimum of six satellites overhead from nearly any point on the surface of the earth. Satellite sightlines are a little harder to get near the North or South Poles. The current US constellation has 28 satellites, including four older satellites that have been put on standby mode as a source of ready spares in orbit. Additional in-orbit spare satellites may exist, but the Air Force has not revealed their presence yet.
Commercial and Military Precision
Military GPS systems also have a fixed point receiver within the operating area that acts as a constant reference point. With this additional fixed point, Military GPS systems can get positional reads with accuracy less than 30 centimeters (one foot). Civilian systems don’t have an additional fixed receiver for differential GPS. As a result, they are limited to an accuracy of 10 to 20 meters. Prior to their removal in 2000, civilian GPS systems had integrated timing errors limiting precision to no more than 90 to 100 meters.
GPS fails when the receiver cannot generate a fix on the satellite. For civilian GPS, this is probably because the receiver hasn’t been exposed to enough of the sky, or the path is obscured by trees or rain. In the realm of military vulnerabilities, there have been attempts at GPS jammers (radios broadcasting on the same frequency that the GPS satellites use with the intent to overwhelm GPS signals), and GPS spoofers, which try to insert correctly formatted GPS signals with incorrect data. It takes two incorrect signals out of four to throw off the accuracy tremendously. GPS satellites are also vulnerable to anything that would prevent their signal from broadcasting through the ionosphere and solar storms, which damage satellites.
Satellite Manufacturing and Launch Information
GPS satellites are currently being upgraded and replaced as the budget permits; at any given time the US constellation has satellites from multiple generations of the service. Lockheed Martin received a 1.4 billion dollar contract to augment and update the satellites in the existing GPS constellation; this has resulted in launches of the Block IIF intermediate satellite, which is expected to have a service life of 12 years. Block IIIA satellites, with more powerful transmitters and communications denial capabilities, are slated to launch in 2014, according to AmericaSpace.