Filter
Top Products
C-2
Rev 2 Apr/2004
KMD 250 Pilot's Guide
Appendix C
GPS Primer
quickly lock on to the satellites in view. Otherwise, the receiver would
have to look for each of the 24 satellites to determine which ones could
be used.
Almanac data is very crude data which describes the approximate orbital
position of the satellites. Each of the 24 satellites transmits the almanac
data for all satellites, so a GPS receiver has only to listen to one satellite
in order to know which satellites are “visible” (in the sky) at that particular
time. Almanac data is good for about six months, so when you turn the
receiver off, then back on a month later, it will know what satellites to
look for.
Ephemeris data is very precise data which each satellite transmits to tell
the GPS receiver exactly where it is and what its orbital parameters will
be for about the next four hours. Each satellite transmits its own unique
ephemeris data.
GPS SYSTEM SEGMENTS
The GPS system is composed of three segments: the Space Segment,
the Control Segment, and the User Segment.
The Space Segment consists of the 24 NAVSTAR satellites which orbit
the earth at an altitude of 10,898 nautical miles. The satellite orbits are
very precisely planned so that the entire surface of the earth may use the
GPS system 24 hours a day, every day. There are almost always more
than six satellites in view from anywhere on Earth.
The Control Segment consists of a network of ground-based monitoring
and control stations. The Master Control Station is located in Colorado
Springs, Colorado. All satellite data which is collected by the other
ground stations is assimilated and analyzed at Colorado Springs. Based
on these analyses, ephemeris updates (such as system clock correc-
tions) are sent (uplinked) to the satellites through radio transmitters at
the ground stations. These ground stations are located at Kwajalein
(west of Hawaii in the central Pacific Ocean), Diego Garcia (in the Indian
Ocean), and Ascension (in the south Atlantic Ocean).
As an owner of a GPS system, you can now claim to be a certified
member of the GPS User Segment! GPS has many users and uses,
and more are being dreamed up all the time. Not only are aircraft using
GPS navigation, so are military systems and personnel, boaters, hikers,
and surveyors. Personal automobiles and transport trucks use Intelligent
Transportation Systems, or ITS, to find their destinations, and track their
movements. Some biologists attach GPS receivers to animals to mon-
itor their movement and migration patterns. Geologists even use GPS to
track the movement of glaciers and to analyze plate tectonics (move-
ments of the Earth’s crust).
GPS Primer
Appendix C 8/13/07 10:02 AM Page C-2