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The GPS implements two major corrections to its time signals for relativistic effects: one for relative velocity of satellite and receiver, using the special theory of relativity, and one for the difference in gravitational potential between satellite and receiver, using general relativity.
GPS transmits 2 signal types: military and a commercial. The accuracy of the military signal is classified but can be assumed to be well under 0.5 meters. The GPS system space segment is composed of 24 to 32 satellites in medium Earth orbit at an altitude of approximately 20,200 km (12,600 mi).
GPS encodes this information into the navigation message and modulates it onto both the C/A and P(Y) ranging codes at 50 bit/s. The navigation message format described in this section is called LNAV data (for legacy navigation). The navigation message conveys information of three types: The GPS date and time, and the satellite's status.
The United States' Global Positioning System (GPS) consists of up to 32 medium Earth orbit satellites in six different orbital planes. The exact number of satellites varies as older satellites are retired and replaced. Operational since 1978 and globally available since 1994, GPS is the world's most utilized satellite navigation system.
When GPS was first being put into service, the US military was concerned about the possibility of enemy forces using the globally available GPS signals to guide their own weapon systems. Originally, the government thought the "coarse acquisition" (C/A) signal would give only about 100- metre (330 ft ) accuracy, but with improved receiver ...
The GPS alternatives rely on signals that can be measured locally (for instance, motion or magnetic fields as used in a compass), so a vessel can navigate even when GPS is unavailable or untrusted.
The Military Grid Reference System (MGRS) [1] is the geocoordinate standard used by NATO militaries for locating points on Earth. The MGRS is derived from the Universal Transverse Mercator (UTM) grid system and the Universal Polar Stereographic (UPS) grid system, but uses a different labeling convention.
A Selective Availability Anti-spoofing Module (SAASM) is used by military Global Positioning System receivers to allow decryption of precision GPS observations, while the accuracy of civilian GPS receivers may be reduced by the United States military through Selective Availability (SA) and anti-spoofing (AS). [1]