Global Positioning System - GPS
The global positioning system is a location
determination network that uses satellites to act as reference points for
the calculation of position information. These man-made reference points
can be viewed as aerial lighthouses that are visible to user equipment and
can also transmit additional information that can provide extremely
accurate location information to the GPS function within location
determination devices.
Several orbiting satellite systems are used for
global positioning, which include:
The
USA GPS System
The USA Global Positioning System (GPS) is a
location determination system that was developed by the Department of
Defense's (DOD) Ivan Getting, and Massachusetts Institute of Technology
(MIT).
This system, which consisted of eleven
satellites, was called NAVSTAR (Navigation System with Timing and Ranging)
and launched between 1978 and 1985. By 1980, 18 satellites were part of
the NAVSTAR constellation and used by the military for GPS.
In 1983, President Ronald Regan declassified GPS
technology, allowing for public use. This was prompted by the take down of
Korean Airline 007 by Russian Military jets when the commercial airliner
drifted into Russian airspace. Since then, public use of the satellites
for commercial purposes was allowed and by July of 1995, 24 satellites
were in place, completing the full system. It was off limits to the public
between 1990 and 1993, during the first Gulf War, to allow for exclusive
use by the military. Although the constellation currently has 29
satellites in orbit, only 24 are required for normal operation leaving
five spares in case of lost operation.
GLONASS
The GLONASS system is a global
positioning system that is operated by the Russian Republic, (State
Unitary Enterprise of Applied Mechanics). The system has continued to
evolve, providing more accuracy and precision, and much like the US
system, it also has 24 satellites. Unlike the US satellites, which are
launched individually, GLONASS satellites can be launched in groups of
three. Additionally, the GLONASS system operates at a higher altitude than
the US system. There are some technical trade offs to this approach such
as better accuracy in the northern hemisphere at the cost of less accuracy
in the southern hemisphere. Since GLONASS serves the Russian Republic,
this is in their favor.
European
Galileo System
The European Galileo System is
a global navigation system similar to the US GPS system, which was started
in May 2003 by the European Union and the European Space Agency The system
was to be built between 2006 and 2008 at the cost of 3 billion Euros, and
is expected to be complete by 2013 with a cost of $3.4 Billion Euro, as of
this printing. This system will use 30 satellites for a complete
constellation and will be for civilian use only. China, Korea, Israel and
other non-EU countries are invested in the project and will use the system
as well. The EU system aims to be the most accurate of the world’s
systems.
Beidou
Satellite Navigation System
The Beidou System is a Chinese
regional satellite positioning system. It is a two-way satellite system
that allows mobile devices to request position information from the
satellite network.
Terrestrial and
Other Positioning Systems
Terrestrial positioning
systems are position location systems that use land-based transmitters or
reference points to act as reference points for the calculation of
position information. Some of the terrestrial based positioning systems
include long range aid to navigation (LORAN), dead reckoning (DR), and
inertial navigation systems (INS).
How GPS Works
Recall that there are 24
satellites being used in the US GPS system at all times. These satellites
orbit the earth in such a way that at any given time and location, at
least four satellites are visible to a GPS driven device. Each satellite
is equipped with an extremely accurate atomic clock so the satellite is
always aware of the current time on Earth at the Prime Meridian. The
satellites are also aware of their own positions with the assistance of
ground stations that give continuous updates. The 24 satellites orbit the
earth transmitting their time and position. These pieces of data are
received by the antennas attached to radio receivers inside a GPS device.
As a GPS device starts up, it
must scan its radio tuner for very faint GPS satellite signals. Once it
has collected data (the position of a satellite and the time the satellite
sent the position) from at least three satellites, a location fix can be
made.
Differential time of arrival
and triangulation are the methods used to determine location in a GPS
system. Dead reckoning may also be used when satellites are not visible.
Differential
Time of Arrival
Differential time of arrival
is the method used to determine how far each satellite is from a GPS
device. Although each satellite transmits its position and the time it was
at that position, it takes time for that signal to reach the Earth. The
receiver contains a very accurate clock, which can determine the
difference in time between the current time and when the satellite sent
the signal. With this differential time and the speed of radio
waves, the distance from each of the three satellites can be determined
using the simple formula:
Rate x Time =
Distance
Trilateration
Trilateration is a method that
is used to determine position on Earth in three dimensions. GPS deals with
three-dimensions rather than two. Since the distance from the Earth to a
satellite results in a sphere rather than a flat circle, the calculation
is a bit complex.
Using trilateration, rather
than draw circles to determine position we need to draw spheres. For
example, if the first acquired satellite is 25,000 miles from position one
cannot simply draw a circle around that satellite and determine a position
25,000 miles from it. A sphere must be plotted, extending toward Earth and
away from Earth. A second satellite is calculated to be 25,001 miles from
position, resulting in another sphere. The two spheres intersect, creating
a perfect circle. A circular plane now exists, extending down through the
earth and out into space. A large number of potential positions have
now been eliminated, but there is not yet an exact location. Many
potential positions still exist and a third satellite is needed to define
a sphere that intersects with the two current spheres resulting in two
points that define possible position. One point is in space and one is on
earth. Since the world is roughly a sphere, the point in space can be
eliminated and the approximate position of the GPS receiver is located on
Earth. A fourth satellite is necessary to account for altitude and provide
an exact fix of the location. The plotting of a fourth sphere provides the
exact location and altitude of the receiver at the time the four
measurements were taken.
This figure shows a global positioning satellite
(GPS) system. This diagram shows how a GPS system receives and compares
the signals from orbiting GPS satellites to determine its geographic
position. Using the precise timing signal based on a very accurate clock,
the GPS receiver compares the signals from 3 or 4 satellites. Each
satellite transmits its exact location along with a timed reference
signal. The GPS receiver can use these signals to determine its distance
from each of the satellites. Once the position and distance of each
satellite is known, the GPS receiver can calculate the position where
these distances cross. This is the location. This information can be
displayed in latitude and longitude form or a computer device can use this
information to display the position on a map on a computer display.
Global Positioning System
Operation - GPS Diagram
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