GPS satellite system
Wide Area Augmentation System) capability can improve accuracy to less
than three meters on average. No additional equipment or fees are
required to take advantage of WAAS. Users can also get better accuracy
with Differential GPS (DGPS), which corrects GPS signals to within an
average of three to five meters. The U.S. Coast Guard operates the most
common DGPS correction service. This system consists of a network of
towers that receive GPS signals and transmit a corrected signal by
beacon transmitters. In order to get the corrected signal, users must
have a differential beacon receiver and beacon antenna in addition to
The Global Positioning System (GPS) is a satellite-based navigation
system made up of a network of 24 satellites placed into orbit by the
U.S. Department of Defense. GPS was originally intended for military
applications, but in the 1980s, the government made the system available
for civilian use. GPS works in any weather conditions, anywhere in the
world, 24 hours a day. There are no subscription fees or setup charges
to use GPS.
The 24 satellites that make up
the GPS space segment are orbiting the earth about 12,000 miles above
us. They are constantly moving, making two complete orbits in less than
24 hours. These satellites are travelling at speeds of roughly 7,000
miles an hour.
GPS satellites are powered by
solar energy. They have backup batteries onboard to keep them running in
the event of a solar eclipse, when there's no solar power. Small rocket
boosters on each satellite keep them flying in the correct path.
Here are some other interesting
facts about the GPS satellites (also called NAVSTAR, the official U.S.
Department of Defense name for GPS):
- The first GPS satellite was
launched in 1978.
- A full constellation of 24
satellites was achieved in 1994.
- Each satellite is built to
last about 10 years. Replacements are constantly being built and
launched into orbit.
- A GPS satellite weighs
approximately 2,000 pounds and is about 17 feet across with the solar
- Transmitter power is only 50
watts or less.
GPS satellites transmit two low
power radio signals, designated L1 and L2. Civilian GPS uses the L1
frequency of 1575.42 MHz in the UHF band. The signals travel by line of
sight, meaning they will pass through clouds, glass and plastic but will
not go through most solid objects such as buildings and mountains.
A GPS signal contains three
different bits of information — a pseudorandom code, ephemeris data and
almanac data. The pseudorandom code is simply an I.D. code that
identifies which satellite is transmitting information. You can view
this number on your Garmin GPS unit's satellite page, as it identifies
which satellites it's receiving.
Ephemeris data, which is
constantly transmitted by each satellite, contains important information
about the status of the satellite (healthy or unhealthy), current date
and time. This part of the signal is essential for determining a
The almanac data tells the GPS
receiver where each GPS satellite should be at any time throughout the
day. Each satellite transmits almanac data showing the orbital
information for that satellite and for every other satellite in the
GPS receivers have been miniaturized to just a few integrated
circuits and so are becoming very economical. And that makes the
technology accessible to virtually everyone.
These days GPS is finding its way into cars, boats, planes,
construction equipment, movie making gear, farm machinery, even laptop
Soon GPS will become almost as basic as the telephone. Indeed, at
Trimble, we think it just may become a universal utility.
A standard GPS receiver will
not only place you on a map at any particular location, but will also
trace your path across a map as you move. If you leave your receiver on,
it can stay in constant communication with GPS satellites to see how
your location is changing. With this information and its built-in clock,
the receiver can give you several pieces of valuable information:
- How far you've traveled
- How long you've been
- Your current speed
- Your average speed
- A "bread crumb" trail
showing you exactly where you have traveled on the map
- The estimated time of
arrival at your destination if you maintain your current speed
To obtain this last piece of
information, you would have to have given the receiver the coordinates
of your destination, which brings us to another GPS receiver capability:
inputting location data.
The position information in a
Magellan GPS receiver may be displayed as longitudeAatitude, Universal
Transverse Mercator, Military Grid or other system coordinates.
Information must be received from three satellites in order to obtain
two-dimensional (latitude and longitude) fixes, and four satellites are
required for three-dimensional (latitude, longitude and altitude)
Each satellite continuously
broadcasts two signals, L1 and L2. The L1 frequency contains the C/Acode
which provides Standard Positioning Service (SPS) for worldwide civilian
use. The encrypted P-code is broadcast on both the L1 and L2 frequency,
resulting in the Precise Positioning Service (PPS) for military use. The
SPS signal will provide a civilian user an accuracy of better than 25
meters. Because they are so accurate, civilian GPS receivers using the
SPS signal are sometimes subjected to Selective Availability (SA)
interference by the United States Government, to maintain optimum
military effectiveness of the system. When engaged, SA inserts random
errors in the data transmitted by the satellites. As a result, SPS
signal accuracy can be reduced to 100 meters.
The Global Positioning System is a constellation of
satellites which orbit the earth twice a day, transmitting precise time
and position information. With a GPS receiver, users can determine their
location anywhere on earth. Position and navigation information is vital
to a broad range of professional and personal activities, including
boating, surveying, aviation, national defense, vehicle tracking and
navigation, and more.