GNSS – Global Navigation Satellite System / GPS, GLONASS, Galileo & BeiDou
Global Positioning System (GPS), officially NAVSTAR GPS, is a global satellite-based positioning system for positioning and timing. The GPS system itself is operated by the U.S. Department of Defense for both military use and use by the general public and was officially put into operation in 1995. Using the difference in the radio signal propagation times of at least three or more of the 32 GPS satellites, a GPS receiver can accurately determine its position worldwide to within a few meters using trilateration.
A-GPS (Assisted GPS) is mainly used in mobile terminals and in cellular networks to enhance performance by means of faster localization and higher accuracy. A-GPS uses the coordinates of the base station serving the radio cell in which the mobile terminal is located and transmits this information to the mobile terminal via an assisted channel.
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. For the ranging codes and navigation message to travel from the satellite to the receiver, they must be modulated onto a carrier frequency. In the case of the original GPS design, two frequencies are utilized; one at 1575.42 MHz called L1 for civilian use; and a second at 1227.60 MHz, called L2 for military use. GPS uses a code division multiple access technique (CDMA), meaning each of the 32 satellites uses a different code on the same frequency to differentiate from one another. The receiver must therefore know all codes used by each satellite in order to determine its position.
GLONASS, an acronym for Globalnaya navigatsionnaya sputnikovaya sistema, is a satellite-based positioning system comparable to GPS, also using trilateration to determine the current position. Run by the Russian Aerospace Defence Forces, GLONASS consists of 24 satellites of which 21 are used for transmitting signals and three being spares. These satellites are positioned in three orbital planes and provide full global coverage since October 2011. GLONASS provides similar accuracy to GPS, in order to get a position fix one must receive signals from at least four satellites.
As for the differences between GLONASS and GPS, GLONASS uses a frequency division multiple access method (FDMA), meaning each satellite uses the same code but transmits on its very own frequency within the so called L1 band (1602.5625 MHz to 1615.5 MHz).
Galileo, named after Galileo Galilei, is the European alternative satellite-based positioning system and will be under civilian control. Galileo will be put into full operation 2020 at the earliest. First services based on the 6 satellites (today) will be offered in 2016. Until 2020 the full set of 30 (of which 6 will be spares) satellites will be deployed, at a higher altitude (23222 km above the Earth) than the GPS and GLONASS Satellites. (Galilei is to be the most precise positioning system.)
Galileo will offer 5 basic services, called Open Service, Commercial Service, Safety-of-Life, Public Regulated Service and Search And Rescue. The European positioning system uses a code division multiple access technique (CDMA) and operates in the L1 (like GPS), L5 and E6 (1278,75 MHz) frequency bands.
BeiDou-2/COMPASS is the Chinese satellite-based positioning system and will consist of 35 satellites covering the globe entirely.
The predecessor BeiDou-1, utilizing 4 satellites (3+1 spare) in geostationary orbit, is established for test purposes with limited service. The benefit using the geostationary orbit is a smaller constellation of satellites. At the same time the coverage is limited to certain areas. As for BeiDou-2, 16 satellites have been launched until 2012, 14 of them being in service. The Frequencies bands used are E1, E2, E5B and E6, while E6 is also used by Galileo. BeiDou-2 uses a code division multiple access technique (CDMA). Contrary to the other satellite-based positioning systems not only medium Earth orbit (MEO) satellites (30), but also 5 geostationary orbit (GEO) satellites (as in BeiDou-1) will be utilized.