The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Air Force. It is a global navigation satellite system that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. Obstacles such as mountains and buildings block the relatively weak GPS signals.
For example, the quality of the GPS receiver, the position of the GPS satellites at the time the data was recorded, the characteristics of the surroundings (buildings, tree cover, valleys, etc) and even the weather. Although the US military no longer routinely degrades the quality of GPS signals, and announced in September 2007 that it would be removing Selective Availability altogether from future versions of GPS satellites, currently it can still nobble the system anytime it pleases. Using multiple systems also promises to make satellite navigation much faster: if more satellites are “in view,” the so-called Time-to-First-Fix (TTFF) —the initial delay before your satnav locks onto satellites, downloads the data it needs, and is ready to start calculating your position—is reduced.
In time, civilian GPS will become increasingly accurate, especially as more satellites (and more different satellite systems) are added, but it’s likely that military systems will always have an advantage, for one reason or another. Even civilian SPS receivers are now officially accurate to within “13 meters (95 percent) horizontally and 22 meters (95 percent) vertically”, though a variety of different errors (caused by the atmosphere, obstructions blocking line of sight to satellites, signal reflections, atmospheric delays, and so on) can compound to make them very much less accurate at times. The ionosphere and troposphere distort and delay satellite signals in quite complex ways, for quite different reasons that we won’t go into here, and GPS receivers have to compensate to ensure they can make accurate measurements of distance.
The receiver “listens out” for these signals and, if it can pick up signals from three or four different satellites, it can figure out your precise location (including your altitude). As a result of these shortcomings, the United States military developed another system: Navstar (Navigation Satellite for Time and Ranging) Global Positioning System. When fully operational, it consists of 24 satellites and 6 spare satellites that orbit the earth in an altitude of about 23260 km. In the data list, the satellites are named “GSAT” followed by the launch number.
Many receivers can receive both GPS and GLONASS signals, and sometimes also signals from Galileo. GPS IIF satellites are three-axis stabilized with a zero momentum system that enables the vehicles to fly in an Earth-oriented position with spacecraft nadir pointed to the sun. The Global Position System is operated by the US Air Force and the Block IIF Satellites are part of its maintenance and modernization process as the new generation of Navigation Satellites are being used to replace aging satellites in the existing GPS fleet.
F900, Satellite GPS Timekeeping Technology with Worldwide Reception, Time Adjustment Available in 27 Cities (40 Time Zones), Satellite GPS Timekeeping System with Worldwide Reception Area, World’s Fastest Timekeeping Signal Reception Speed From GPS Navigation Satellites – As Quick as 3 Seconds. In a nutshell, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy. The satellite navigation system calculates a user’s location by measuring tiny differences in the arrival time of electromagnetic pulses from several positioning probes in the sky, but a new atomic clock on Beidou-3 could reduce the margin of error to a few millimetres.
Two Beidou-3 satellites are expected to be launched on September 29, the website reported, with developers saying the system was 10 times more precise than GPS. Given that information and a time stamp from the satellite, the GPS receiver of the customer can calculate the position of the satellite and then use that information from multiple satellites to then calculate the position of the GPS receiver. The system offers a standard C/A positioning and timing service giving horizontal position accuracy within 180 feet (55 meters) and vertical position within 230 feet (70 meters) based on measurements from four satellite signals.