The Global
Positioning System (GPS) is a world-wide 24-hour
navigation positioning system operated by the US
Department of Defence. It consists of a Ground Control
Segment, a Space Segment and a User Equipment Segment.
The User equipment segment is what is commonly known as a
GPS receiver.
24 earth-orbiting satellites in six different orbits
form the Space Segment . (There are also 3 or 4
operational spares in orbit at any one time.) Each
satellite circles 10,900 nautical miles above the earth
in orbits inclined at an angle of 55 degrees to the
equator. Each satellite transmits precision timing
signals (derived from onboard atomic clocks) on two
frequencies, L1 and L2. A separate channel on each
frequency is dedicated to each satellite.
The navigation messages broadcast on the L1 frequency
contain two codes, one for civilian use, and another
encrypted code for military use. The L2 broadcast
contains a second set of navigational messages, which
when combined with the encrypted code in the L1
frequency, can resolve positions to less than 20 meters.
Known as the Precise Positioning Service (PPS), this
service is available only to the US military, and its
allies.
The non-encrypted codes in the L1 frequency, (available
to civilian users), provide the Standard Positioning
Service (SPS). When GPS was in its initial testing phases
it was found that this service provided position fixes
that were far more accurate than was originally intended,
so SPS accuracy was intentionally degraded by the
introduction of random errors in the timing signal--reducing
the position fixing accuracy of GPS to 100 meters 98% of
the time. This intentional degradation of the timing
signal was known as Selective Availability
(SA), and constituted over half the total GPS error prior
to May 1, 2000. (The satellite clock need only be "dithered"
by a few millionths of a second to create the desired
effect. That is why, in spite of SA, GPS time is the most
accurate clock you will have on board your vessel.)
However, recognizing the importance of GPS to the
civilian economy, the United States Government removed
Selective Availability on May 1, 2000. Now the single
largest contributor to GPS error is interference with the
broadcast signals caused by the ionosphere (a shell of
electrically charged particles that surrounds the earth.)
Now a GPS position is expected to be accurate within 20
meters.
Each satellite also broadcasts "Almanac"
and "Ephemeris" messages. Your
earthbound GPS receiver uses the almanac
to determine which satellites are above the horizon and
what channels they are broadcasting on. The receiver then
locks on to the most appropriate satellites for fixing a
position.
