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Global
Positioning System
The Global Positioning System, usually called GPS,
and originally named NAVSTAR, is an intermediate
circular orbit (ICO) satellite navigation system
used for determining one's precise location and providing a
highly accurate time
reference almost anywhere on Earth.
A GPS unit receives time signal transmissions from multiple
satellites, and calculates its position by triangulating this
data. The GPS was designed by and is controlled by the United
States Department of Defense and can be used by anybody for
free. The cost of maintaining the system is approximately $400
million per year. The first of 24 satellites that form the
current constellation of the Global Positioning System (Block
II) was placed into orbit on February
14, 1989.
Technical
description
The system consists of a "constellation" of 24
satellites in 6 orbital
planes. The GPS satellites were manufactured by Rockwell;
the first was launched in February, 1978,
(Block I), and the final, (24th), satellite was launched in 1994.
Each satellite circles the Earth twice every day at an altitude
of 20,200 kilometers (12,600 miles). The satellites carry atomic
clocks and constantly broadcast the precise time according
to their own clock, along with administrative information
including the orbital
elements of their own motion, as determined by a set of
ground-based observatories.
The receiver does not need a precise clock, but does need to
have a clock with good short-term stability and receive signals
from four satellites in order to find its own latitude,
longitude,
elevation,
and the precise time. The receiver computes the distances to the
four satellites by the differences between local time and the
time the satellite signals were sent and it then decodes the
satellites' locations from their radio signals and an internal
database. The receiver should now be located at the intersection
of four spheres,
one around each satellite, with a radius equal to the time delay
between the satellite and the receiver multiplied by the speed
of the radio signals. The intersection point gives the precise
location of the receiver. If elevation information is not
required, only signals from three satellites are needed.
In reality, the four spheres rarely intersect. There are several
causes: The initial local time was a guess; due to the
relatively unprecise clock of the receiver; the radio signals
move more slowly as they pass through ionosphere;
or the receiver may be moving. The receiver then applies an
offset to the local time, and therefore to the spheres' radii,
so that the spheres finally do intersect in one point. Once the
receiver is roughly localized, most receivers mathematically
correct for the ionospheric delay. The delay varies with the
angle to the satellite, which changes the distance that the
radio signal travels through the ionosphere. Some receivers
attempt to fit the spheres to a directed line segment, because
most receivers move.
The receiver contains a mathematical model to account for these
influences, and the satellites also broadcast some related
information which helps the receiver in estimating the correct
speed of propagation. High-end receiver/antenna systems make use
of both L1 and L2 frequencies to aid in the determination of
atmospheric delays. Because certain delay sources, such as the
ionosphere, affect the speed of radio waves based on their
frequencies, dual frequency receivers can actually measure the
effects on the signals.
In order to measure the time delay between satellite and
receiver, the satellite sends a repeating 1,023 bit
long pseudo
random sequence; the receiver knows the seed of the
sequence, constructs an identical sequence and shifts it until
the two sequences match.
Different satellites use different sequences, which lets them
all broadcast on the same frequencies while still allowing
receivers to distinguish between satellites. This is an
application of Code Division Multiple Access, CDMA.
There are two frequencies in use: 1575.42 MHz
(referred to as L1), and 1227.60 MHz (L2). The L1 signal
carries a publicly usable coarse-acquisition (C/A) code as well
as an encrypted P(Y) code. The L2 signal
usually carries only the P(Y) code. The keys required to
directly use the P(Y) code are tightly controlled by the U.S.
government and are generally provided only for military use.
A minor detail is that the atomic clocks on the satellites are
set to "GPS time", which is the number of seconds
since midnight, January 5, 1980. It is ahead of UTC
because it doesn't follow leap
seconds. Receivers thus apply a clock correction factor,
(which is periodically transmitted along with the other data),
and optionally adjust for a local time zone in order to display
the correct time. The clocks on the satellites are also affected
by both special,
and general
relativity, which causes them to run at a slightly faster
rate than do clocks on the Earth's surface. This amounts to a
discrepancy of around 38 microseconds per day, which is
corrected by electronics on each satellite. This offset is a
dramatic test of the theory of relativity in a real-world
system; the offset of which is measured is exactly that
predicted by theory, within the limits of accuracy of
measurement.
The
accuracy of GPS can be improved in a number of ways:
-
Using
a network of fixed ground based reference stations. These
stations broadcast the difference between the location given
by GPS and their real location, and clients can then correct
their position by the same amount. This method is called Differential
GPS or DGPS. The accuracy of DGPS without
degradation of GPS is less than a meter. DGPS was especially
useful when GPS was still degraded (via the "Selective
Availability" described below), since DGPS could
nevertheless provide 5-10 meter accuracy. The DGPS network
has been mainly developed by the Finnish
and Swedish
maritime administrations in order to improve safety in the
archipelago between the two countries.
-
Exploitation
of DGPS for Guidance Enhancement (EDGE) is
an effort to integrate DGPS into precision guided munitions
such as the Joint Direct Attack Munition (JDAM).
-
The
Wide-Area Augmentation System (WAAS). This
uses a number of additional satellites to transmit
correction data, including information on ionospheric
delays, individual satellite clock drift, and suchlike.
Although only a few WAAS satellites are currently available
(in 2002), it is hoped that eventually WAAS will provide
sufficient reliability and accuracy that it can be used for
critical applications such as GPS-based instrument
approaches in aviation (landing an airplane in conditions of
little or no visibility).
-
A
Local-Area Augmentation System (LAAS). This
is similar to WAAS, in that similar correction data is used.
But in this case, the correction data is transmitted from a
local source, typically at an airport or another location
where accurate positioning is needed. This correction data
is typically useful for only about a thirty to fifty
kilometer radius around the transmitter.
-
Wide
Area GPS Enhancement (WAGE) is an attempt
to improve GPS accuracy by providing more accurate satellite
clock and ephemeris (orbital) data to specially-equipped
receivers.
Applications
The primary military purpose is to allow improved command and
control of forces through an enhanced ability to accurately
specify target locations for cruise
missiles or troops. The satellites also carry nuclear
detonation detectors.
The systems is used by countless civilians as well, who can use
the GPS's Standard Positioning Service worldwide free of charge.
Low cost GPS receivers (price $100 to $200) are widely
available. The system is used as a navigation aid in aeroplanes,
ships and cars. Hand held devices are used by mountain climbers
and hikers. Glider pilots use the logged signal to verify their
arrival at turnpoints in competitions.
In the past, the civilian signal was degraded, and a more
accurate Precise Positioning Service was available only to the
United States military and other, mostly government users.
However, on May
1, 2000,
US President Bill Clinton
announced that this "Selective Availability"
would be turned off, and so now all users enjoy nearly the same
level of access, allowing a precision of position determination
of less than 20 meters. For military purposes, "Selective
Deniability" may still be used to, in effect, jam
civilian GPS units in a war zone or global
alert while still allowing military units to have full
functionality.
Military (and selected civilian) users still enjoy some
technical advantages which can give quicker satellite
lock and increased accuracy. Commercial GPS receivers are
also required to have limits on the velocities and altitudes at
which they will report fix coordinates; this is to prevent them
from being used to create improvised cruise missiles or
ballistic missiles.
Many synchronization
systems use GPS as a source of accurate time, hence one of the
commonest applications of this use is that of GPS as a reference
clock for time
code generators or NTP
clocks.
GPS
Jamming
A large part of modern munitions, the so-called "smart
bombs" or precision-guided
munitions, use GPS. GPS jammers are available, from Russia,
and are about the size of a cigarette box. The U.S.
government believes that such jammers were used occasionally
during the U.S.
invasion of Afghanistan. Some officials believe that jammers
could be use to attract the precision-guided munitions towards noncombatant
infrastructure, other officials believe that the jammers are
completely ineffective.
Awards
Two
GPS developers have received the National
Academy of Engineering Charles
Stark Draper prize year 2003:
Other
systems
For
a list of other systems, see satellite
navigation system.
See
also
Air
traffic control, Allan
variance, Degree
Confluence Project, Geocaching,
GMS
localization, Waypoint.
External
links
This
content from Wikipedia
is licensed under the GNU
Free Documentation License.
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