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How Satellite Navigation Works
Global navigation satellites continuously transmit time and distance
information as they orbit the earth in a precise formation. Navigation satellite
receivers use this information to calculate an exact location through
triangulation. Every point on Earth is identified by two sets of numbers called
coordinates. These coordinates represent the exact point where a horizontal
line, known as latitude, crosses a vertical line, known as longitude. The
receiver locks on to at least three satellites and uses the information received
to determine the coordinates of the device.
By comparing the time the signals were transmitted from the satellites and
the time they were recorded, the receiver calculates how far away each satellite
is. The distance of the receiver from three or more satellites reveals its
position on the surface of the planet. With these distance measurements, the
receiver might also calculate speed, bearing, trip time, distance to
destination, altitude and more.
The satellite navigation device may display its position as
longitude/latitude, Universal Transverse Mercator (UTM), Military Grid (MG) or
simply as a point on an electronic map. Many Thales Navigation receivers provide
comprehensive mapping data, making satellite navigation an easy tool to enhance
your recreational and industrial activities.
Line of Sight
Satellite navigation receivers operate by line of sight with global positioning
satellites. This means that at least three satellites must be in
"view" of a receiver in order to calculate longitude and latitude. A
fourth satellite must also be within line of sight to calculate altitude. On
average, eight satellites are continuously within line of sight of every
position on Earth; the more satellites in view, the more accurate the
positioning.
Though the radio signals of navigation satellites will pass through clouds,
glass, plastic and other lightweight materials, satellite navigation receivers
will not work underground or in other enclosed spaces.
Precision
On average, a satellite navigation receiver is accurate to within 15 meters.
Thales Navigation employs several technologies to increase the accuracy of their
Ashtech and Magellan receivers. An accuracy of 3 meters or better is achieved
using correction signals from satellite navigation augmentation systems. In the
U.S., an accuracy of 3 meters is achieved using signal corrections from a
network of ground stations and fixed position satellites known as WAAS (Wide
Area Augmentation System). Throughout Europe a similar system provides the same
accuracy; EGNOS (European Geostationary Navigation Overlay System). In Asia,
satellite navigation signal correction is provided by MSAS (Multifunctional
Transport Satellite-based Augmentation System). Other ways to increase the
accuracy of satellite navigation include the use of DGPS (Differential Global
Positioning System); ground relay stations, set at known positions, that
transmit corrected satellite navigation signals. Various methods and
applications of DGPS can increase satellite navigation accuracy from a few
meters to within a few millimeters. Using DGPS requires a differential beacon
receiver and antennae in addition to a satellite navigation device. Accuracy can
also be increased using an RTK (Real-Time Kinematic) satellite navigation
system. This is a receiver capable of transmitting a phase-corrected signal from
a known position to one or more rover receivers.
A number of positioning errors can occur, limiting accuracy to within 15 to
25 meters. These errors are monitored and compensated for in a number of ways:
- Orbiting errors - Occasionally a satellite's reported position does not
match its actual trajectory. In the U.S., the Department of Defense
continuously monitors each satellite, making orbital corrections with
onboard booster rockets.
- Poor geometry - If all the satellites within line of site of a receiver
are clustered closely together, or lined up relative to the position of the
receiver, the geometric calculations necessary for triangulating a position
become difficult and less reliable. The use of differential correction
signals from satellite-based augmentation systems or DGPS can compensate for
both orbital errors and poor geometry.
- Multi-path signals - Signals may be reflected off tall buildings or other
obstructions before reaching the receiver, increasing the distance a signal
travels, reducing accuracy.
Thales Navigation receivers make a number of complex mathematic calculations
to effectively compensate for other potential errors in positioning:
- Atmospheric delay - Satellite navigation signals slow as they pass through
the Earth's atmosphere. Thales Navigation receivers calculate the average
delay in nanoseconds to compensate.
- Clock errors - The clock built into a receiver is not as accurate as the
atomic clock on a navigation satellite, which is accurate to one second
every million years. Each Thales Navigation receiver compensates for time
differentials by comparing the time signals of several satellites and
adjusting its calculations and its clock to match.
Used by permission. 2002 Thales Navigation
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