GLOBAL
POSITIONING SYSTEM (GPS)
Is
a GLOBAL POSITIONING SYSTEM (GPS) essential to
RV travel?
One
thing is sure - a Global Positioning System certainly
makes outdoor life easier. Far more than a fancy
electronic gadget, a GPS 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.
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