GLOBAL
POSITIONING SYSTEM (GPS)
Is
a GLOBAL POSITIONING SYSTEM (GPS) essential to RV travel?
One
thing is sure - it certainly makes life easier. Far
more than a fancy electronic gadget, a GPS system is
a sophisticated electronic device used accurately to
navigate large distances on land, sea and air.
For a relatively small outlay, you can program in your
destination, and get concise driving direction to wherever
it is you want to go. Obviously, they are specially
useful in areas you are visiting for the first time,
making them ideal for RV travel. Handheld models are
great for hiking and mountain biking, and greatly enhance
most outdoor activities.
But
what, actually, is behind this impressive array of electronic
wizardry? How does it all come together in a small,
beneficial and relatively cheap package?
Owned
and operated by the US AIR FORCE, but provided as a
free service to all users around the world, the
GLOBAL POSITIONING SYSTEM, or GPS, is a constellation
consisting of over two dozen satellites orbiting the
Earth. Positioned at an altitude of 12,600 miles (20,200
kms) above the earth, the satellites broadcast precise
timing signals allowing any GPS receiver to accurately
pinpoint its location in terms of latitude, longitude
and altitude around the clock, in any weather, day or
night, anywhere on Earth.
The
wide range of GPS models available have turned the GPS
system into a vital global utility. The system is indispensable
to modern military and civilian navigation on land,
sea and in the air. It is also an extremely important
tool for land surveying and map-making.
In
late 2005, the first in a series of next-generation
GPS satellites was added to the constellation, offering
users several new capabilities. These included a second
civilian GPS signal for enhanced accuracy and reliability.
It is intended that in the near future, additional next-generation
satellites will be added to the constellation, increasing
coverage of the additional civilian channel, as well
as adding a third and fourth civilian signal and advanced
military capabilities to the system.
The
Wide-Area Augmentation System, in use since the year
2000, increases the accuracy of GPS signals to within
6 feet (about 2 meters) for compatible receivers. Differential
GPS techniques have the ability to further increase
accuracy, to about 1/2 inch (about 1 cm).
While
modern GPS systems are extremely accurate, they cannot
replace a proper detailed map and compass for off-road
navigation. You can use the GPS to navigate to a certain
landmark, such as a planned camping spot.
You
can program in routes to follow, positioning in waypoints,
and the system will display a route and even calculate
distance and estimated driving time - but you will still
need a map and compass to navigate successfully.
GPS systems designed for vehicular use, use position
data to locate the user on a road in the unit's map
database. Using this database, the unit can give directions
to other locations along roads also in the database.
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accuracy of GPS can be improved in a number of ways: |
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Differential
GPS (DGPS) can improve the normal GPS accuracy of
4-20 meters to 1-3 meters. DGPS uses a network of
stationary GPS receivers to calculate the difference
between their actual known position and the position
as calculated by their received GPS signal. The
"difference" is broadcast as a local FM
signal, allowing many civilian GPS receivers to
"fix" the signal for greatly improved
accuracy. |
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The
Wide Area Augmentation System (WAAS). This uses
a series of ground reference stations to calculate
GPS correction messages, which are uploaded to a
series of additional satellites in geosynchronous
orbit for transmission to GPS receivers, including
information on ionospheric delays, individual satellite
clock drift, and suchlike. 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).
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The
current WAAS system only works for North America
where the reference stations are located, and due
to the satellite location the system is only generally
usable in the eastern and western coastal regions.
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A
Local Area Augmentation System (LAAS). This is similar
to WAAS, in that similar correction data are used.
But in this case, the correction data are transmitted
from a local source, typically at an airport or
another location where accurate positioning is needed.
These correction data are typically useful for only
about a thirty to fifty kilometer radius around
the transmitter. |
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A
Carrier-Phase Enhancement (CPGPS). This technique
utilizes the 1.575 GHz L1 carrier wave to act as
a sort of clock signal, resolving ambiguity caused
by variations in the location of the pulse transition
(logic 1-0 or 0-1) of the C/A PRN signal. The problem
arises from the fact that the transition from 0-1
or 1-0 on the C/A signal is not instantaneous, it
takes a non-zero amount of time, and thus the correlation
(satellite-receiver sequence matching) operation
is imperfect. A successful correlation could be
defined in a number of various places along the
rising/falling edge of the pulse, which imparts
timing errors. CPGPS solves this problem by using
the L1 carrier, which has a period 1/1000 that of
the C/A bit width, to define the transition point
instead. The phase difference error in the normal
GPS amounts to a 2-3 m ambiguity. CPGPS working
to within 1% of perfect transition matching can
achieve 3 mm ambiguity; in reality, CPGPS coupled
with DGPS normally realizes 20-30 cm accuracy. |
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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. |
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Relative
Kinematic Positioning (RKP) is another approach
for a precise GPS-based positioning system. In this
approach, accurate determination of range signal
can be resolved to an accuracy of less than 10 centimeters.
This is done by resolving the number of cycles in
which the signal is transmitted and received by
the receiver. This can be accomplished by using
a combination of differential GPS (DGPS) correction
data, transmitting GPS signal phase information
and ambiguity resolution techniques via statistical
tests-possibly with processing in real-time (real-time
kinematic positioning, RTK). |
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Many
automobile GPS systems combine the GPS unit with
a gyroscope and speedometer pickup, allowing the
computer to maintain a continuous navigation solution
by dead reckoning when buildings, terrain, or tunnels
block the satellite signals. This is similar in
principle to the combination of GPS and inertial
navigation used in ships and aircraft, but less
accurate and less expensive because it only fills
in for short periods.
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Modern
GPS receivers have been miniaturized to just a few integrated
circuits and so are becoming very economical, making
the technology accessible to virtually everyone.
These
days GPS systems are finding their way into cars, boats,
planes, construction equipment, movie making gear, farm
machinery, even laptop computers. GPS handheld systems
are also becoming increasingly popular.
How
The GPS System Works:
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A
minimum of 24 GPS satellites are in orbit at 12,600
miles (20,200 kilometers) above the Earth. The
satellites are spaced so that from any point on
Earth, at least four satellites will be above
the horizon.
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Each
satellite contains a simple computer, atomic clocks,
and various radios. With an understanding of its
own orbit and the clock, the satellite continually
broadcasts its changing position and time. The
satellites use their on-board atomic clocks to
keep precise time, but are otherwise very simple
and unsophisticated when compared to other modern
spacecraft.
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Several times per day, depending upon various
requirements, the USAF contacts each of the GPS
space vehicles and provides it with a new navigational
upload.
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All
ground-based GPS receivers contain a computer
that can calculate its own position by getting
time signals from three of the four satellites
it can locate, using a process called trilateration
(similar to triangulation). The result is provided
in the form of a geographic position - longitude
and latitude - accurate within 100 meters for
most units. If the receiver is also equipped with
a display screen that shows a map, the position
can be shown on the map.
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If
the unit is able to receive a signal from the
fourth satellite, the GPS receiver can also figure
out the altitude as well as the geographic position.
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If
the GPS receiver is in motion, the unit may also
be able to calculate speed and direction of travel
as well as estimated arrival times at selected
destination
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GPS
Civilian Applications: Navigation
Civilian
applications for the GPS system are as as an international
navigation aid for use in cars, airplanes, and ships.
Personal Navigation Devices (PND) such as hand-held
GPS are used by mountain climbers and hikers. Glider
pilots use the logged signal to verify their arrival
at turn points in competitions. Low cost GPS receivers
are often combined with PDAs, cell phones, car computers,
or vehicle tracking systems. Other civilian applications
include automated agricultural harvesters, location
of stolen vehicles, fleet supervision and more.
Additional
GPS Functions
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Many
systems can give information on nearby points
of interest, such as restaurants, cash machines
and gas stations.
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Some
newer systems can not only give precise driving
directions, they can also receive and display
information on traffic congestion and suggest
alternate routes. This may use either TMC, which
delivers coded traffic information using RDS or
satellite radio, or an Internet link to a provider's
server using technology such as GPRS through the
user's mobile phone
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The
color LCD screens on some automotive navigation
systems can also be used to display television
broadcasts or DVD movies.
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A
few systems integrate with mobile phones for handsfree
talking and SMS messaging.
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GPS
can replace taxi radio-dispatch networks, and
have been applied in some countries. A central
computer tracks all vehicles in the fleet/network,
and automatically dispatches the closest cab within
proximity of the customer's location to answer
the call. To order a cab, the customer can either
talk to an attendant or enter a registered location
code for systematic service. The driver would
enter an ETA (estimated arrival time) on the computer
- which is relayed to the caller by a prerecorded
message - at which point a confirmation can be
made to accept or reject the cab.
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Advanced
car security systems can relay the vehicle's location
via cellular phone services in case of loss or
theft.
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Some navigation devices (map software) also store
the location of known speed traps on its map database,
and can alert the driver in much the same way
as a radar detector as he approaches a speed trap.
GPS may also be integrated into actual radar detection
devices to enhance accuracy, and in some cases,
implement a logic system where the system only
alerts if the driver is raveling above posted
speed limits.
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GPS
Accuracy
The
position calculated by a GPS receiver relies on three
accurate measurements: the current time, the position
of the satellite, and the time delay for the signal.
GPS
receivers vary widely in accuracy, reflected in varying
price levels of different models. Early consumer-grade
receivers typically included six to eight receivers.
As the computer industry has improved the state of the
art in chipmaking, the cost of implementing these receivers
has fallen dramatically, and today even low-cost hand
held receivers typically have twelve receivers. More
expensive units, known as "dual-frequency receivers",
also tune in the L2 signals (Second civilian GPS signal)
in order to correct for ionospheric delays.
Another
major factor in the accuracy of a GPS fix is the amount
of processing applied to the received signals. This
is a function of the performance of the electronics
and the required battery life. Typical in other areas
of applied electronics, these factors have also been
dramatically affected by improved chip making, so that
current low cost receivers vastly outperform much more
expensive earlier models.
GPS
receivers may include an input for differential corrections,
using the RTCM SC-104 format. This is typically in the
form of a RS-232 port at 4,800 bps speed. Data is actually
sent at a much lower rate, which limits the accuracy
of the signal sent using RTCM. Receivers with internal
DGPS receivers can outperform those using external RTCM
data. The cost of implementing these receivers is also
falling dramatically, and even low-cost units are commonly
including WAAS receivers today.
Many
GPS receivers can relay position data to a PC or other
device using the NMEA 0183 protocol. NMEA 2000[12] is
a newer and less widely adopted protocol. Both are proprietary
and are controlled on a for-profit basis by the US-based
National Marine Electronics Association. References
to the NMEA protocols have been compiled from public
records, allowing open source tools like gpsd to read
the protocol without violating intellectual property
laws. Other proprietary protocols exist as well, such
as the SiRF protocol. Receivers can interface with external
devices via a number of means, such as a serial connection,
a USB connection or even a Bluetooth wireless connection.
GPS
Handheld System
The
GPS handheld system is probably the best choice for
outdoor pursuits, since their small screens are more
easily read at arm's length than in a moving car. Most
GPS handheld systems store a pre-loaded data base of
basic maps, many will allow you to download other maps
from your computer.
Comparison
of GPS handheld system units:
There are various types of GPS handheld systems available.
The eTrex Legend is a good no-frills choice. Priced
at about $150, the unit stores a preloaded database
of basic American maps in its 8MB memory, plus allows
you to download other maps (as well as your latest trip
data) from your computer with the supplied serial cable.
Another
good but more expensive handheld option is the Magellan
eXplorist 500. Priced at about $340, the eXplorist has
more features, memory and a color screen. You can also
add more maps with SD memory cards. The Magellan handheld
GPS system is easy to use, and quickly attains a satellite
signal lock. The screen is bright and has a backlight
for easy daylight use. The Magellan GPS also has some
features specific to geocaching, a new hobby which invites
participants to use GPS to find designated treasure
spots.
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