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The original GPS design contains two ranging codes: the coarse/acquisition (C/A) code, which is freely available to the public, and the restricted precision (P) code, usually reserved for military applications. The current SVNs and PRN numbers for the GPS constellation may be found at NAVCEN. A single SV ID/PRN number may have been used by several satellites at different points in time and a single satellite may have used different SV ID/PRN numbers at different points in time. At any point in time, any SV ID/PRN number is in use by at most a single satellite. Unlike SVNs, the SV ID/PRN number of a satellite may be changed (also changing the ranging codes it uses). There is a fixed one-to-one correspondence between SV identifiers and PRN numbers described in the interface specification. In addition, all operating satellites are numbered with a space vehicle identifier (SV ID) and pseudorandom noise number (PRN number) which uniquely identifies the ranging codes that a satellite uses.
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Satellites are uniquely identified by a serial number called space vehicle number (SVN) which does not change during its lifetime. However, unlike typical QAM systems where a single bit stream is split in two half-symbol-rate bit streams to improve spectral efficiency, in GPS signals the in-phase and quadrature components are modulated by separate (but functionally related) bit streams.įurther information: List of GPS satellites § PRN to SVN history Some satellites transmit several BPSK streams at the same frequency in quadrature, in a form of quadrature amplitude modulation. The ranging codes are also called chipping codes (in reference to CDMA/ DSSS), pseudorandom noise and pseudorandom binary sequences (in reference to the fact that it is predictable, but statistically it resembles noise).
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Only a limited number of central frequencies are used satellites using the same frequency are distinguished by using different ranging codes in other words, GPS uses code division multiple access. The GPS satellites (called space vehicles in the GPS interface specification documents) transmit simultaneously several ranging codes and navigation data using binary phase-shift keying (BPSK).
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The format of civilian signals is described in the Interface Specification (IS) which is a subset of the ICD. The interface to the User Segment ( GPS receivers) is described in the Interface Control Documents (ICD). Some limited use of restricted signals can still be made by civilians without decryption this is called codeless and semi-codeless access, and is officially supported. In addition, there are restricted signals with published frequencies and chip rates but encrypted coding intended to be used only by authorized parties. L2C, L5 and L1C are modernized signals, are only broadcast by newer satellites (or not yet at all), and as of January 2021, none are yet considered to be fully operational for civilian use. L1 C/A is also called the legacy signal and is broadcast by all currently operational satellites. In order of date of introduction, these are: L1 C/A, L2C, L5 and L1C. There are four GPS signal specifications designed for civilian use. The navigation messages include ephemeris data, used to calculate the position of each satellite in orbit, and information about the time and status of the entire satellite constellation, called the almanac. GPS signals include ranging signals, used to measure the distance to the satellite, and navigation messages. The GPS satellite constellation is operated by the 2nd Space Operations Squadron (2SOPS) of Space Delta 8, United States Space Force. Receivers on or near the Earth's surface can determine location, time, and velocity using this information. GPS signals are broadcast by Global Positioning System satellites to enable satellite navigation. Civilian GPS receiver (" GPS navigation device") in a marine application