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Q. Where are the GPS satellites?

A. The GPS satellites are in a Medium Earth Orbit (MEO), sometimes called Intermediate Circular Orbit (ICO). This is the region of space around the earth above low earth orbit (2,000 kilometers (1,243 mi)) and below geostationary orbit (35,786 kilometers (22,236 mi)). The most common use for satellites in this region is for navigation, such as the GPS (20,200 kilometers (12,552 mi)), Glonass (19,100 kilometers (11,868 mi)) and Galileo (23,222 kilometers (14,429 mi)) constellations. The orbital periods of MEO satellites range from about 2 to 24 hours, the GPS satellites orbit the earth two times every sidereal day (a sidereal day is 3 minutes and 56 seconds shorter than a solar day). The GPS constellation was originally conceived as 6 planes of 4 satellites each with an inclination of 55° w.r.t. the equator and a separation of 60° right ascension. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. As of March 2008 there were 31 active satellites in the constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a non-uniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail. L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future Block III satellites.