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.
How it works
GPS satellites circle the earth twice a day in a very precise
orbit and transmit signal information to earth. GPS receivers
take this information and use triangulation to calculate the user's
exact location. Essentially, the GPS receiver compares the time
a signal was transmitted by a satellite with the time it was received.
The time difference tells the GPS receiver how far away the satellite
is. Now, with distance measurements from a few more satellites,
the receiver can determine the user's position and display it
on the unit's electronic map.
A GPS receiver must be locked on to the signal of at least three
satellites to calculate a 2D position (latitude and longitude)
and track movement. With four or more satellites in view, the
receiver can determine the user's 3D position (latitude, longitude
and altitude). Once the user's position has been determined, the
GPS unit can calculate other information, such as speed, bearing,
track, trip distance, distance to destination, sunrise and sunset
time and more.
How accurate is GPS?
Today's GPS receivers are extremely accurate, thanks to their
parallel multi-channel design. Garmin's 12 parallel channel receivers
are quick to lock onto satellites when first turned on and they
maintain strong locks, even in dense foliage or urban settings
with tall buildings. Certain atmospheric factors and other sources
of error can affect the accuracy of GPS receivers. Garmin® GPS
receivers are accurate to within 15 meters on average.
Garmin GPS receivers with WAAS
(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 their GPS.
The GPS satellite system
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
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 panels extended.
- Transmitter power is only 50 watts or less.
What's the signal?
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
Ephemeris data tells the GPS receiver where each GPS satellite
should be at any time throughout the day. Each satellite transmits
ephemeris data showing the orbital information for that satellite
and for every other satellite in the system.
Almanac 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 position.
Sources of GPS signal errors
Factors that can degrade the GPS signal and thus affect accuracy
include the following:
- Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses
a built-in model that calculates an average amount of delay
to partially correct for this type of error.
- Signal multipath — This occurs when the GPS signal is
reflected off objects such as tall buildings or large rock surfaces
before it reaches the receiver. This increases the travel time
of the signal, thereby causing errors.
- Receiver clock errors — A receiver's built-in clock is
not as accurate as the atomic clocks onboard the GPS satellites.
Therefore, it may have very slight timing errors.
- Orbital errors — Also known as ephemeris errors, these
are inaccuracies of the satellite's reported location.
- Number of satellites visible — The more satellites a
GPS receiver can "see," the better the accuracy. Buildings,
terrain, electronic interference, or sometimes even dense foliage
can block signal reception, causing position errors or possibly
no position reading at all. GPS units typically will not work
indoors, underwater or underground.
- Satellite geometry/shading — This refers to the relative
position of the satellites at any given time. Ideal satellite
geometry exists when the satellites are located at wide angles
relative to each other. Poor geometry results when the satellites
are located in a line or in a tight grouping.
- Intentional degradation of the satellite
signal — Selective
Availability (SA) is an intentional degradation of the signal
once imposed by the U.S. Department of Defense. SA was intended
to prevent military adversaries from using the highly accurate
GPS signals. The government turned off SA in May 2000, which
significantly improved the accuracy of civilian GPS receivers.