GNSS turns orbiting clocks into a global grid
Satellite navigation uses spacecraft to fix positions on Earth. A GNSS offers worldwide coverage; regional augmentation systems such as Japan's QZSS, India's GAGAN, and Europe's EGNOS sharpen accuracy for aviation and surveying. Relativity matters too: GPS satellite clocks run ahead of ground clocks by about 38 microseconds a day.
Satellite navigation lets a receiver find its longitude, latitude, and altitude, often to within centimetres or metres, by timing radio signals sent along a line of sight from satellites. Four global systems now operate: America's GPS, Russia's GLONASS, China's BeiDou, and the European Union's Galileo. Each covers the planet with 18 to 30 satellites in medium Earth orbit, spread over several orbital planes tilted more than 50 degrees, circling about 20,000 kilometres up roughly every twelve hours.
Regional add-ons improve the picture. Augmentation services such as WAAS in the United States, EGNOS in Europe, MSAS in Japan, GAGAN in India, and SDCM in Russia correct the global signals, while India's NAVIC and Japan's QZSS are regional navigation systems. On the ground, networks of reference stations, down to a single station sending real-time kinematic corrections, sharpen positions further. Because many systems share frequencies near the L1 band, multi-GNSS receivers can use several at once.
Before satellites, longwave radio chains such as DECCA, LORAN, GEE, and Omega sent a pulse from a master station followed by pulses from slave stations, and the delays revealed position. The first satellite system, Transit, was deployed by the U.S. military in the 1960s and relied on the Doppler effect: the frequency heard on the ground shifted as each satellite passed along its known path.
Modern satellites broadcast their orbital data, a rough almanac of the whole fleet plus a precise ephemeris for themselves, together with the exact moment of transmission. Each timed signal puts the receiver somewhere on a sphere around that satellite, and where several spheres meet gives a fix. Einstein's general relativity has to be built into the timing, since clocks aboard GPS satellites gain about 38 microseconds a day on clocks at the surface.
Source: Satellite navigation
