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GPS Navigation System Receivers
Portable and Car Receivers

In the car or in the field, on land, sea or air, the GPS navigation system receiver is your key to a safe, enjoyable trip.

The GPS Global Positioning System is a satellite system used for navigation. With a GPS receiver, you can keep track of your location, all around the world, 24 hours a day, no matter what the weather.

The GPS system is a group of 24 satellites that circle the earth and beam radio signals back to the earth痴 surface. The GPS receiver you buy is an electronic device that detects these radio signals and calculates the position of your receiver on the earth. They are useful in many occasions. There is a GPS receiver in most cell phones, so that when you call 911 to report an emergency, your position is also sent to the operator. GPS receivers are in many cars. They can be mounted on bicycles, ATVs, quads, trucks and even worn on your wrist. The newest technology includes downloadable maps, street-by-street directories, and topographic maps for the outdoor adventurer. There are GPS receivers in computers, PDAs and Pocket PCs.

The GPS hand-held receivers and vehicle receivers are helpful on every trip. There are also other interesting applications for GPS receivers. They are used to track the movement of tectonic plates under the San Andreas Fault to predict earthquakes. Sailing vessels are made aware of the location of underwater rocks and hazards that might damage the ship. GPS is invaluable to keep track of search and rescue operations. If your GPS receiver has an outside antenna, it can become an educational and amusing toy for children in the backseat on your trip.

How the GPS Navigation System Works

The GPS Navigation System was undertaken in 1960 as part of a military project to make inter-continental ballistic missiles more accurate. The working of the GPS system is amazing. The space satellites beam radio signals that contain an almanac and an electronic code. The almanac is the current position of each satellite around the world. The code allows the receiver to determine its distance from the satellite. Five ground control stations, operated by the U.S. Department of Defense, track the satellites, monitor their condition and make adjustments to the atomic clock on each. There is no limit on the number of receivers the GPS system can accommodate. The United States has the only fully functional GPS system, but other countries have systems in development.

How the GPS Navigation System Receiver Works

At least four of the GPS satellites can be detectable from any spot on the globe at any time. Your GPS receiver monitors them and chooses the four satellites closest and in the best positions to provide accurate data. To start, your GPS device receives the system time. Three satellites transmit their sky location codes at exactly the same time. The speed of the signal is known. The time from the signal transmission to the time it is received tells your receiver how far it is from each of the three satellites. Then it can calculate your longitude, latitude and altitude. The distance the signal travels must be adjusted for the effect of the ionosphere, which slows down the radio signals. There are several additional adjustments calculated to take this into consideration.

Military Use of the GPS Navigation System

The GPS satellites transmit two sets of information, the CA code and the P code. Civilian receivers can only access the CA code. The more accurate P code is used only by military receivers. The ionosphere slows radio signals at different rates, depending on the frequency of the signal. So for military P code, the satellite transmits simultaneously on two frequencies. The military receiver records the time delay between the two sets of frequency data and calculates the effect of the ionosphere on its radio signals. Until 2000, small random errors were deliberately included in satellite data on the civilian CA code. This strategy, called Selective Availability, was undertaken for security purposes, and is not presently in use.

Accuracy of the GPS Navigation System

Accuracy of the GPS receivers was improved with the Wide Area Augmentation System, WAAS. WAAS is a pair of addition satellites over the equator, plus 38 land stations, that provide data to reduce the error in the GPS codes. WAAS is able to correct for most of the ionosphere signal delay. If your receiver is WAAS enabled, and you are in an area with WAAS coverage, your receiver can take advantage of improved accuracy. Without WAAS capability, your receiver is accurate to 49.2 feet horizontally and 62.3 feet vertically. With WAAS, your receiver gives you horizontal accuracy of 9.8 feet and vertical accuracy of 19.7 feet, 95% of the time. Choose a receiver with WAAS capability if at all possible.

Your GPS navigation receiver provides you with the estimated position error, EPE, to tell you that satellite geometry is favorable and the computed position is accurate. When satellites are not in a position relative to your location to provide accurate date, your receiver will register an outage. As the satellites move into a better position a few minutes later, the receiver will begin working again.

A receiver uses signals from four satellites to calculate a 3D position with latitude, longitude and altitude. If only three satellites are available in your location, the receiver will calculate a 2D position with latitude and longitude, and accuracy is reduced to between 500 and 5000 feet. The sensitivity of the antenna in your GPS receiver determines how useful it will be in travels to remote areas when there is minimum satellite coverage.

Features of your GPS Navigation Receiver: The Antenna

The antenna, required to detect the signals, can be located inside or outside the GPS receiver. If the inside antenna is a quadrifilar antenna, point it toward the sky for best reception. If the inside antenna is a patch microstrip antenna, hold the antenna parallel to the sky. Most GPS receivers with an internal antenna will have to remain on the dashboard of the car to pick up the satellite signals through the windshield.

External antennas are connected to the receiver by a cord. They are useful when the receiver does not have a clear view of the sky. Active external antennas will amplify the signal, while passive external antennas simply detect and transfer the signal. External antennas are waterproof and will mount to the roof of a vehicle either magnetically or with suction cups.

Features of the GPS Navigation Receiver Maps

Your GPS receiver has a built-in map to display on the screen along with your present position and route. The base map shows major highways, while the detailed map includes minor streets and points of interest such as hotels, restaurants, hospitals, airports, museums and gas stations. With the detailed map, the receiver can search for locations by address, street intersection, street name and location name. Detailed maps are generally downloaded onto a memory card that is inserted into the receiver. The receiver designed for use in a car provide gives you point-by-point directions that follow streets and highways shown in the build-in map. For adventure travel, downloadable topographical maps show trailheads and other useful data. If you plan to use downloadable maps, select a GPS receiver with some type of memory card, such as SD cards. To store all the road maps of great Los Angeles requires 21 megabytes of memory. The entire road map for North America can fit into 2 gigabytes of memory.

For many remote areas of the world, electronic maps are not yet available. In these areas, you can use your GPS receiver in conjunction with an old-fashioned paper map. You may also sometimes prefer a detailed paper map to the small-screen map on your receiver. As you know, a system of grids is drawn over your paper map and your electronic GPS map with reference coordinates. The most popular grid systems are Universal Transverse Mercator, called UTM, and Latitude/Longitude, which cover most of the paper maps used in the world. Select a GPS receiver that supports one or both of these grids. There are also the British Grid OSGB, the Maidenhead Grid used by amateur radio operators, the Military Grid Reference System, called MGRS and the Universal Polar Stereographic UPS.

Every map has a single reference point from which all other locations on the map are measured. The reference point of a map is called the map datum. The two most common datum in North America are North American Datum 1927, referred to as NA27, and World Geodetic System 1984, referred to as WGS84. If you travel internationally, you will need other datum, possibly a different datum for each country you visit. Before you use the receiver, you must tell it the datum of the map you are using.

Your GPS Navigation Receiver Computer Access

Your GPS receiver should also offer two-way communication with your computer. The industry standard protocol for this communication with your computer is called National Maritime Electronics Association NMEA 183 version 3.01. You can use it to store trip information and to download maps and waypoints.

Your GPS Navigation Receiver Compass

A compass needle moves and continuously points to the magnetic pole. If you have an electronic compass built into your GPS receiver, it can detect the earth痴 magnetic field and determine your direction even when you are not moving. When you move at a quick page, the GPS receiver will be able to calculate your bearing. But when you move slowly, you値l need an electronic compass or a manual one. Both the electronic compass in your receiver and a manual compass must be held parallel to the ground to take a reading.

Features of Your New GPS Navigation Receiver Altimeter

Although your GPS receiver can calculate your altitude from the satellite signals, a built in altimeter is more accurate. The built-in altimeter uses atmospheric pressure to calculate altitude. Air pressure is highest at sea level and lower at the top of a mountain. A change in the weather will affect the accuracy of the altimeter.

Your New GPS Navigation Receiver Batteries

Most GPS receivers use conventional batteries. However some have rechargeable batteries similar to cell phones. If you use your GPS in a car, you can recharge the batteries in the cigarette lighter.

If you use your GPS in the backcountry, you値l want a receiver with a smaller screen which uses less power. In the backcountry, you値l want to use conventional batteries, and carry a supply of replacements. No matter if the batteries go dead, your GPS receiver does not lose the stored data. It uses a small internal battery, or else flash memory, to maintain data even during battery failure. You値l save batteries by using external power like a cigarette lighter whenever it is available. You can conserve battery power by not using the receiver night light, called a screen backlight.

Using Your New GPS Navigation Receiver ~ Waypoints

The coordinate of a location is called a waypoint or landmark. These are your destinations. On the keypad, you enter the destination map coordinates or street address. The typical GPS receiver can store 500 or more waypoints. The receiver lets you assign a name and a graphical symbol to each waypoint, be it a transportation hub, signs or points of interest. When you select a waypoint, the receiver displays it and calculates your route. To start your trip, you select a waypoint, using either a built-in map, or an alphabetical list. Then press the Goto button for directions. Before you buy a GPS receiver, be sure to try out its waypoint access features for convenience. Your GPS receiver can automatically reverse the travel route for your return trip home.

Using Your New GPS Navigation Receiver ~ Routes

Suppose you want to travel past several waypoints on your trip, or you want to take the scenic roads to the destination. Your GPS receiver lets you set up a route, but earmarking a number of waypoints. When you arrive at one waypoint, the receiver directs you to the next waypoint automatically. Some GPS receivers automatically receive traffic updates by FM radio, and can alert you to traffic delays and alternate routes.

Take your Bearing with the GPS Navigation Receiver

Bearing is the compass direction of the route you are traveling, expressed in degrees. Think as if you are standing in the center of a circle. If you travel due east, your bearing is 90 degrees; due south, 180 degrees; due west 270 degrees and due north 0 degrees. Your automatic compass can use either the North Pole, or else the magnetic north pole in northern Canada, to calculate your bearing. You can choose the setting you want to use. Most maps use the North Pole, called true north, and show true north at the top of the map. However, a magnetic compass uses magnetic north, and you値l want the GPS receiver set to magnetic north also. The difference between magnetic north and true north, measured in degrees, is called the declination, and depends on your location.

More Trip Information from your GPS Navigation Receiver

Your GPS screens will show voluminous useful information: Distance, Speed, the Bearing of your desired course, your current course Heading, the CrossTrack Error, sometimes called the Course Deviation Indicator, the Course to Steer, Estimated Time for the Trip, Estimated Time of Arrival, Current Time, Distance to the Next Waypoint, Maximum and Minimum Elevation reached, total Ascent and total Descent during the trip. It will also show you a variety of speed statistics. All this information is reported in the measurement system you select, metric, nautical or Statute. The Statute system is the standard US system of miles, etc. The GPS receiver will tell you if you are off course, and the change needed to get back on course. In addition, there is a multitude of other data output. If you leave the GPS receiver on during the trip, it will record a track log of the route you actually travel, which can be saved as a route for later trips.

Navigation Screens on the GPS Receiver

The data from your GPS receiver is shown on various screens. The Position screen tells you about your present location. The Steering screen is used to show waypoints when you are driving on the Goto or Route mode. The Compass Navigation Screen, also called the pointer screen, is useful for hikers using a compass. The Highway Navigation Screen shows you the straight line course to your destination. The Map Screen shows the roads on the built-in map, along with the route and turns to get to your destination. It can be zoomed in and out to control the amount of detail shown. You can orient the map screen to show your destination at the top of the screen, or to show true north at the top of the screen.

I hope life brings you much success. I wish you a very happy day.
-----     Surfer Sam  

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