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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.

The accuracy of GPS can be improved in a number of ways:
* 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.
* 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).
* 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.
* 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.
* 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.
* 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.
* 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).
* 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.

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

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.

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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