Gps
What is GPS?
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, th
e 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.
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, th
e 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.
satellites to calculate a 2D position (latitude and longitude)
and track movement. With four or more satellites in view
, he 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.
Newer 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 th
e corrected signal, users must have a differential beacon receiver and beacon antenna in addition
to their GPS.
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 th
e 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.
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.
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 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 receiving.
Ephemeris data, which is constantly transmitted by each satellite, contains
important
important
information about the status of the satellite (healthy or unhealthy), current
date and time
date and time
. This part of the signal is essential for determining a position.
The almanac data tells the GPS receiver where each GPS satellite should
be at any time
be at any time
throughout the day. Each satellite transmits almanac data showing the
orbital information
orbital information
for that satellite and for every other satellite in the system.
Sources of GPS signal errors
Factors that can degrade the GPS signal and thus affect accuracy include the
following:
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.
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