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| About GPS |
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GPS stands for Global Positioning System, and is a way of locating
a GPS receiver in anywhere on the Earth, and even in orbit. Global
Positioning System satellites transmit signals to equipment on the
ground. GPS receivers only receive satellite signals and they do not
transmit. There are at least position 24 operational GPS satellites
at all times. The satellites orbit with a period of 12 hours. Ground
stations are used to precisely track each satellite's orbit.
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| How GPS works? |
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| GPS operations depend on a very accurate time reference, which is
provided by atomic clocks. Each GPS satellite has atomic clocks on
board, which transmits data that indicates its location and the current
time. All GPS satellites synchronize operations so that these signals
are transmitted at the same instant. The signals arrive at a GPS
receiver at slightly different times because some satellites are
farther away than others. The distance to the GPS satellites can
be determined by estimating the amount of time it takes for their
signals to reach the receiver. A GPS receiver obtain its position
in three dimensions (latitude, longitude and altitude ) by estimating
the distance to at least four GPS satellites.
A GPS receiver knows the location of the satellites, because that
information is included in satellite transmissions. By estimating
how far away a satellite is.
The accuracy of a position determined with GPS depends on the type
of receiver. Most hand-held GPS units have about 10-20-meter accuracy.
Other types of receivers use a method called Differential GPS (DGPS)
to obtain much higher accuracy. DGPS requires an additional receiver
fixed at a known location nearby. Observations made by the stationary
receiver are used to correct positions recorded by the roving units,
producing accuracy greater than 1 meter.
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| GPS PRODUCTS |
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| There are several solutions available to consumers, from sophisticated
in-car GPS systems, which provide voice guiding feature, very powerful
dynamic route calculation, and the option to update the map from a
CD. Handheld GPS devices that are widely available from different
manufacturer especially Garmin and Magellan are designed for outside
use, which store a map internally.
They are not usually too sophisticated to provide dynamic route
calculation since that require road information.
A more recent solution, which uses the power of a PDA to run GPS
software, has appeared on the market by different manufacturer,
and uses the interface functionality to attach a GPS receiver to
the unit.
Finally, marine users need sea bed information , navigable channels,
and other maritime data that enables them to navigate safely.
Fishermen also use marine GPS known as fishfinders, A fishfinder
comprises GPS and also sonar, along with advanced tracking functions
and storage for various kinds of fishing and maritime information.
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| What is WAAS? |
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The Wide Area Augmentation System (WAAS) uses a system of satellites
and ground stations that provide GPS signal corrections, giving you
even better position accuracy. How much better? Try Position an average
of up to five times better.
A WAAS-capable receiver can give you a position accuracy of better
than three meters 95 percent of the time. And you don't have to purchase
additional receiving equipment or pay service fees to utilize WAAS. |
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| The origins of WAAS |
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| The Federal Aviation Administration (FAA) and the Department of
Transportation (DOT) are developing the WAAS program for use in precision
flight approaches. Currently, GPS alone does not meet the FAA's navigation
requirements for accuracy, integrity, and availability. WAAS corrects
for GPS signal errors caused by ionospheric disturbances, timing,
and satellite orbit errors, and it provides vital integrity information
regarding the health of each GPS satellite. |
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| How it Works |
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| WAAS consists of approximately 25 ground reference stations positioned
across the United States that monitor GPS satellite data. Two master
stations, located on either coast, collect data from the reference
stations and create a GPS correction message. This correction accounts
for GPS satellite orbit and clock drift plus signal delays caused
by the atmosphere and ionosphere. The corrected differential message
is then broadcast through one of two geostationary satellites, or
satellites with a fixed position over the equator. The information
is compatible with the basic GPS signal structure, which means any
WAAS-enabled GPS receiver can read the signal. |
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| Who benefits from WAAS? |
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Currently, WAAS satellite coverage is only available in North America.
There are no ground reference stations in South America, so even though
GPS users there can receive WAAS, the signal has not been corrected
and thus would not improve the accuracy of their unit. For some users
in the U.S., the position of the satellites over the equator makes
it difficult to receive the signals when trees or mountains obstruct
the view of the horizon. WAAS signal reception is ideal for open land
and marine applications. WAAS provides extended coverage both inland
and offshore compared to the land-based DGPS (differential GPS) system.
Another benefit of WAAS is that it does not require additional receiving
equipment, while DGPS does.
Other governments are developing similar satellite-based differential
systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation
System (MSAS), while Europe has the Euro Geostationary Navigation
Overlay Service (EGNOS). Eventually, GPS users around the world will
have access to precise position data using these and other compatible
systems. |
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100 meters: Accuracy of the original GPS system, which was subject
to accuracy degradation under the government-imposed Selective Availability
position (SA) program.
15 meters: Typical GPS position accuracy without SA.
3-5 meters: Typical differential GPS (DGPS) position accuracy.
< 3 meters: Typical WAAS position accuracy. |
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