Geography Reference
In-Depth Information
Earth-Circling Satellites
The United States GPS design calls for a total of at least 24 and up to 32 solar-powered radio transmitters,
forming a constellation such that several are “visible” from any point on Earth at any given time. The
first one was launched on February 22, 1978. In mid-1994 twenty-four were broadcasting. The minimum
“constellation” of 24 includes three “spares.” As many as 31 have been up and working at one time.
The GPS satellites are at a “middle altitude” of about 11,000 nautical miles (nm), or roughly 20,400 kilometers
(km) or 12,700 statute miles above the Earth's surface. This puts them above the standard orbital height
of the erstwhile space shuttle, most other satellites, and the enormous amount of space junk that has
accumulated. They are also well above Earth's air, where they are safe from the effects of atmospheric
drag. When GPS satellites “die,” they are sent to orbits about 600 miles further out, where they will remain
virtually forever.
GPS satellites are below the geostationary satellites, usually used for communications and sending TV,
telephone, and other signals back to Earth-based fixed antennas. These satellites are 35,763 kilometers
(which is 19,299 nautical miles or 22,223 statute miles) above the Earth, where they hang over the
equator, relaying signals from and to ground-based stations.
The NAVSTAR satellites are neither polar nor equatorial, but slice the Earth's latitudes at about 55
o
,
executing a single revolution every 12 hours. Further, although each satellite is in a 12-hour orbit, an
observer on Earth will see it rise and set about four minutes earlier each day. (For an explanation use
your browser to search for solar and sidereal days.) There are four to six satellites in slots in each of six
distinct orbital planes (labeled A, B, C, D, E, and F) set 60 degrees apart. The orbits are almost exactly
circular.
GPS satellites move at a speed of 3.87 kilometers per second (8,653 miles per hour). Different versions
of the satellites have evolved over the years. They weigh 1100 to 2200 kilograms (1 or 2 tons) and have a
width of about 11.6 meters (about 38 feet) with the solar panels extended. Those panels generate roughly
1000 watts of power. The radio on board broadcasts with about 40 watts of power. (Compare that with
your maximum permitted FM station with 50,000 watts.) The radio frequency used for the civilian GPS
signal is called “GPS L1” and is at 1575.42 megahertz (MHz). Each satellite has on board four atomic
clocks (either cesium or rubidium) that keep time with 3 billionths of a second or so, allowing users on
the ground to determine the current time to within about 40 billionths of a second.
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
almanac
3
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.
3
An almanac is a description of the predicted positions of heavenly bodies.
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