Introducing Geographic Information Systems with ArcGIS® Third Edition by Michael Kennedy

Author:Michael Kennedy
Language: eng
Format: epub
Publisher: John Wiley & Sons
Published: 2013-03-03T16:00:00+00:00

Ground-Based Stations

While the GPS satellites are free from drag by the atmosphere, their tracks are influenced by the gravitational effects of the moon and sun, and by the solar wind. Further, they are crammed with electronics. Thus, both their tracks and their innards require monitoring. This is accomplished by four ground-based stations near the equator, spaced around the world. Each satellite passes over at least one monitoring station twice a day. Information developed by the monitoring station is transmitted back to the satellite, which in turn rebroadcasts it to GPS receivers. Subjects of a satellite’s broadcast are the health of the satellite’s electronics, how the track of the satellite varies from what is expected, the current almanac3 for all the satellites, and other, more esoteric subjects that need not concern us. Other ground-based stations exist, primarily for uploading information to the satellites. The GPS Master Control Station at Schriever Air Force Base near Colorado Springs, Colorado.

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