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Appendicies
Appendix B
What is GPS?
The global positioning system is a satellite-based navi-
gation system consisting of a network of 24 orbiting NAV-
STAR satellites that are eleven thousand nautical miles in
space and in six different orbital paths. The satellites are
constantly moving, making two complete orbits around
the Earth in less than 24 hours.
A Little Satellite Info:
• The fi rst GPS satellite was launched in February, 1978.
• Each satellite weighs about 2,000 pounds and is 17
feet across with the solar panels extended.
• Transmitter power is only 50 watts, or less!
• Each satellite transmits two signals, L1 and L2. Civilian
GPS uses the ‘L1’ frequency of 1575.42 MHz.
• Each satellite is expected to last approximately 10
years. Replacements are constantly being built and
launched into orbit. The GPS program is currently
funded with replacements through 2006.
The orbital paths of these satellites take them between
roughly 60 degrees North and 60 degrees South latitudes.
What this means is you can receive satellite signals any-
where in the world, at any time.
One of the biggest benefi ts over previous land-based
navigation systems is GPS works in all weather condi-
tions. No matter what your application, when you need it
the most, your GPS receiver will keep right on working.
So what information does a GPS satellite transmit?
The GPS signal contains a ‘pseudo-random code’, ephem-
eris (pronounced: ee-fem-er-is) and almanac data. The
pseudo-random code identifi es which satellite is transmit-
ting—in other words, an I.D. code. We refer to satellites
by their PRN (pseudo-random number), from 1 through
32, and this is the number displayed on a GPS receiver
to indicate which satellite(s) we are receiving. Ephemeris
data contains important information such as satellite
status (healthy or unhealthy), and the current date and
time. The almanac data tells the GPS receiver where each
GPS satellite should be at any time throughout the day.
Each satellite transmits almanac data that provides orbital
information for all of the satellites in the GPS network.
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