GEOG 482
The Nature of Geographic Information

15. Space Segment

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The space segment of the Global Positioning System currently consists of approximately 30 active and spare NAVSTAR satellites (new satellites are launched periodically, and old ones are decommissioned). "NAVSTAR" stands for "NAVigation System with Timing And Ranging." Each satellite circles the Earth every 12 hours in sidereal time along one of six orbital "planes" at an altitude of 20,200 km (about 12,500 miles). The satellites broadcast signals used by GPS receivers on the ground to measure positions. The satellites are arrayed such that at least four are "in view" everywhere on or near the Earth's surface at all times, with typically up to eight and potentially 12 "in view" at any given time.

GPS satellites and their paths around Earth
Figure 5.15.1 The constellation of GPS satellites.
Illustration © Smithsonian Institution, 1988. Used by Permission.

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The U.S. Coast Guard's Navigation Center publishes status reports on the GPS satellite constellation. Its report of August 17, 2010, for example, listed 31 satellites, five to six in each of the six orbits planes (A-F), and one scheduled outage, on August 19, 2010. You can look up the current status of the constellation here.

NAVSTAR satellite
Figure 5.15.2 Artist's rendition of a NAVSTAR satellite
NAVSTAR GPS Joint Program Office, n.d.