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GPS Basics

Author: ThePhantum

What is GPS?

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 (or more) satellites. These satellites were placed into orbit by the U.S. Department of Defense as the system was originally intended for military applications. However, in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

The 24+ satellites that make up the GPS system are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours (the satellites are traveling at speeds of roughly 7,000 miles an hour). These satellites are equipped with atomic clocks and send out radio signals as to the exact time and their location in space. The radio signals from the satellites are picked up by the GPS receiver. Once the GPS receiver locks on to four or more of these satellites, it can triangulate its location from the known positions of the satellites. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from four or more more satellites, the receiver can determine the user's position. This is a very simple explanation, but unless you are a surveyor or engineer who needs to understand how to use GPS to locate within fractions of an inch, this is all you really need to know.

GPS Uses

Through the use of these receivers, you can navigate to a predetermined location (or back to a starting point) without the use of maps or any other equipment. When used with accurate maps like ones provided by the USGS (United States Geological Service), you can navigate to identified locations on maps or take readings from a location that you are at or have been at and plot those locations on a map.

For Basic GPS use, you only need to understand the most basic of the receiver’s functions...i.e. - Set-up and initialization functions, saving a waypoint, and using the GOTO feature. For this level of use, you don’t need to understand the coordinate grid systems, datums or how to use maps. Therefore, a new GPS user can make good use of the unit with a minimal amount of experience by simply reading the manual and working the keypad.

Those who need to (or want to) refer to maps and use them to plot and read position coordinates must understand at least one of the coordinate grid systems. Once the grid system is understood and its coordinate references can be identified on maps, it is a simple matter to learn to use the tools and techniques to properly identify coordinates.

Grid Systems

While the topic here is GPS basics and not general mapping techniques, the two are closely interrelated. Thus, a brief mention of the most common grid systems is necessary. The two most common grid systems in use in North America (and referenced on maps) are the Latitude Longitude (Lat/Long) grid and the Universal Transverse Mercator (UTM) grid. Both of these grid systems consist of reference points, units of measurement, and some designation of direction to clearly identify a position.

The Lat/Long grid has been in use for centuries and most people have heard of the basic terms of latitude or longitude.

The UTM grid is a metric grid system based on 60 grid zones around the globe and a set of values in meters from reference points of the grid. Despite some of the basic terminology being foreign to most, there are some aspects to this grid that make it very simple and easy to use.

More information on both of these grid systems (and their relationship to GPS) can be found at

GPS features and options

Like any other form of electronics, there are many options and prices. The first consideration should be "How will the GPS equipment be used?". The type of equipment needed all depends on its intended use. It is as difficult to read a small handheld screen while driving down a bumpy dirt road as it is to take a laptop computer on a hiking trip.

Like any other form of electronics, there are many options and prices. The first consideration should be "How will the GPS equipment be used?". The type of equipment needed all depends on its intended use. It is as difficult to read a small handheld screen while driving down a bumpy dirt road as it is to take a laptop computer on a hiking trip.

Since modern GPS receivers share many of the same features (regardless of price), understanding all the features available will let you to make a more informed purchase and allow you to use the equipment to its full potential. Items bracketed with asterisks are features that are either highly desirable (IMHO) and/or need to be considered during the "How will the GPS be used?" phase.

** 12 Parallel Channels ** - Make sure your receiver is capable of 12 Parallel Channels. Most units sold after 1997 are equipped with this feature, however, the first receivers available were all single channel. Some of these older units have similar model numbers with marketing stating they can scan 12 satellites. This is not the same as a 12 Parallel unit that can process data from 12 satellites through 12 channels all at the same time. The older single channel equipment processes information one satellite at a time through a single channel, making them much slower.

Accuracy - As long as the antenna has a clear view of the sky, accuracy is consistent in most receivers. Regardless of the style or cost, accuracy averages within 15 meters anywhere in the world. Accuracy can be improved within three meters with WAAS.

Address Finder - Allows an exact address to be located within a basemap database.

Alarms - An alarm that notifies the user of an approaching waypoint. Text Alarms flash a message on the screen, audible alarms sound a tone...some of the higher priced units even talk to you.

Altimeter - A 3-D, four-satellite fix provides elevation information. However, satellite based altimeters are not known for being highly some units provide a built in barometric altimeter for accurate elevation readings independent of a satellite connection.

** Antenna ** - The options available are built-in, detachable or external. The antenna option is important because it is a determining factor on how the equipment can be used. Receivers with built-in antennas are more durable for hiking, but are restricted in their use. Detachable antennas are ideal for receivers used in and out of vehicles. External is for vehicle applications where equipment is mounted with no clear view of the sky. This allows the GPS to be mounted inside a vehicle.

Auto Routing - Provides turn-by-turn directions to a waypoint. Directions may be in the form of arrows and/or automated voice commands. Usually requires basemaps and/or the ability to transfer maps to the unit via a computer connection.

** Basemap ** - Most recent model receivers include a map database stored within their memory. Basemaps include general information on cities, roadways and waterways. The maps are not always highly detailed, but are still impressive, considering they typically include such large geographic areas like North America or Europe. This is highly recommended. The additional cost is marginal and the increase in the equipment’s usefulness is substantial.

Clock & Timer - Receivers provide precise atomic time in either a 12 or 24 hour display. Various timer features include date, time traveled and estimated time of arrival.

Compass Data - Receivers provide a compass direction if the unit is moving approximately ten MPH. Stationary, they will provide a compass bearing from a current location to any other location. A receiver can be programmed to display compass data in either true or magnetic north. A pointer is also provided to help you maintain the correct bearing. Some of the most recent units to hit the market include a built-in digital compass to provide direction while the receiver is stationary.

** Computer Interface ** - Data in/out capability allows the unit to receive, (upload), data from a computer, or send, (download), data to a computer. This information includes digital maps, track logs, waypoints and routes.

Coordinates Displayed - Most receivers provide the option to display mapping coordinates in multiple international formats. The primary two are latitude/longitude and UTM, Universal Transverse Mercator.

Cursor Arrow Data Field - Receivers with a rocker keypad can scroll a cursor arrow on a map page. Data displayed in a Data Field includes the coordinates of the cursor arrow’s position, as well as the distance from the current location.

DGPS or WAAS Ready - These receivers are capable of accepting radio signals that can increase accuracy to within three meters. These accuracy-improving systems are primarily used in aviation and marine applications.

** Memory ** - For receivers with a basemap, memory is used to store additional mapping data. GPS manufacturers provide greater detailed maps on CD-ROM exclusively for their brand. Mapping detail, especially topo contour lines use a great deal of memory. Two megabytes will allow the storage of a few small areas. Eight megabytes may allow the storage of the primary areas of a home state. Ideally, a receiver will have a minimum of eight megabytes of memory. Some models use memory cards allowing virtually unlimited storage with high capacity cards.

** Power Source ** - Handheld units run on batteries. Vehicle mounted units (usually) run on 12V. A unit that has the option using of a 12V source allows you to use a cigar lighter power cable. Battery life is important for extended use when no external power source is available. Receivers are rated for battery life duration for both continuous use and power saver modes.

** Rocker Keypad ** - Using a receiver without a rocker keypad is like using a computer without a mouse.

Routes - A series of waypoints listed in sequence from start to finish. Designed to guide to a destination, they can also be inverted or reversed, to track back from the destination to the starting point. Routes typically contain up to 30 waypoints...but the more waypoints a route can contain, the longer and more complex those routes can be.

Satellite Status Page - Information includes the number of satellites being locked onto with a signal strength bar for each.

** Screen size ** - For visual ease of operation, use a receiver with the largest screen that can be realistically carried.

Sun Position - Provides sun and moon positions including sunrise and sunset.

Track Log - Plots an electronic "breadcrumb" trail as a sequence of dots or trackpoints, showing a path traveled.

Travel Data - Includes distance and time to next waypoint, current speed, average speed and trip odometer.

Water Resistance - Receivers are rated for their resistance to water. Water-resistant usually means the equipment can be splashed or briefly dunked. Waterproof means the equipment can be submerged to a specific depth for a specific amount of time before damage occurs. Regardless of rating, use a watertight box or bag if used around water. Saltwater can kill electronics instantly.

Waypoints - Specifically recorded locations stored within a receiver’s memory. Saved waypoints allow the return to exact locations. Most modern receivers have the ability to store at least 300 waypoints.

GPS Receiver Types

While there can be some 'crossover' in regards to some specific models (hybrids, if you will), GPS Receivers are grouped into the four following basic categories.

Hand Held without Mapping - These basic receivers are about the size of a TV remote control, include a built in antenna and an approximate two-inch view screen. They usually run off of batteries and some have the ability to run from 12v power sources. They include many of the features of more expensive units, but without a basemap database.

Hand Held with Mapping - These are the same as the above units, except they include a basemap database

Vehicle Mount - The term vehicle is universal for practically anything: ATV’s, cars, trucks, race cars, boats, planes, whatever. The units are the size of a fish-finder, with a much easier to read four-inch plus sized screen. Batteries are replaced by 12-volt wiring and antennas are mounted externally.

GPS Receiver kits for PDA and Laptop computer - Handheld computers can be converted into GPS receivers through a combination of operating software and an antenna. Handheld personal digital assistants “PDA’s,” use an adapter sleeve with an antenna that plugs into an expansion slot. Laptops use a dash mounted remote antenna.

GPS Computer Interface

GPS units that have basemap data, are usually capable of interfacing with a PC or laptop. Software can then be used to transfer data back and forth between the GPS unit and the computer. The data can be maps (or mapset info), waypoints, track logs, routes, etc. The software and mapsets are usually proprietary to the manufacturer, so they cannot be shared among the different brands. However, waypoints and certain aspects of tracklogs & routes (since they are both just a collection of waypoints) can be shared with between different GPS brands, provided the same grid system is selected.

Common GPS related Terms and Acronyms

The following is a listing of common terms and acronyms used in relation to GPS and GPS receivers.

Active Log - The segment of a route currently being traveled.

Backtrack - Navigation of a route in reverse order from the last position fix. Basically, the reverse of navigating a route in normal sequence.

Bearing - For G.P.S. navigation, it generally refers to the compass reading to navigate from one position to another measured to the nearest degree. (In traditional nautical navigation, it is referenced in degrees from quadrants of the compass)

Coordinates - A set of numbers that describes a given position for a given coordinate grid system. Examples of coordinate grids are Latitude/Longitude and Universal Transverse Mercator (UTM).

Course - The direction in degrees between two waypoints or the course indicated by a G.P.S. receiver when the "GOTO" function is activated.

COG - Course Over Ground: The actual direction you (G.P.S. receiver) are traveling.

Datum - A reference system for vertical and horizontal positions. Different datums have different positions for the physical location of their origins, thus different datums will represent coordinates in different positions. Differences can be as much as a mile. All reliable maps that show coordinate systems provide Datum information.

Declination - The angular difference between True North and some other reference for north such as Grid North or Magnetic North.

Differential G.P.S. - A means of compensating for Selected Availability error in G.P.S. locating using radio transmissions.

Elevation - The distance above mean sea level usually in meters or feet.

Equator - The Latitude reference point for the Lat/Long grid system. Other positions of latitude are referenced as degrees of North Latitude if they are north of the equator and degrees of South Latitude if they are south of the equator. Also see Latitude.

ETA - Estimated Time of Arrival: The estimated time of your arrival computed at your present speed toward the destination.

ETE - Estimated Time En route: The projected travel time it will take to arrive at your destination or waypoint.

GIS - Geographic Information System: A category of computer programs and applications that are used to organize, analyze and display geographic information.

GMT - Greenwich Mean Time: Time at 0° or the Prime Meridian passing through Greenwich England. Also known as Universal Time Coordinated (UTC) and, in aviation, Zulu Time.

G.P.S. - Global Positioning System: A generic term for satellite based positioning system.

Grid - A system of horizontal and vertical lines used to chart specific position coordinates i.e. the Latitude/Longitude Grid system.

Grid North - The direction that north/south lines of a grid point.

Grid Zone - One of the 60 zones covered by the Universal Transverse Mercator (UTM ) grid system of navigation.

Heading - The direction that you are facing or traveling see Bearing.

Initialization - The initial orienting process that a G.P.S. receiver does when it is first turned on. If it is the first time the receiver has been turned on or if the receiver is more than a few hundred miles from the last location it was used, it will take some time or help from the operator by inputting or selecting from a menu the approximate location.

Landmark - A specific location or identifiable natural or man-made geographic feature. Also referred to as a waypoint in G.P.S. navigation.

Latitude - The angular distance north or south of the equator when using the Latitude/Longitude grid system. These latitude lines are parallel to the equator and often referred to as "Parallels".

Legend - Information on a map for such things as scale, datum, and other details.

MGRS - Military Grid Reference System: A metric grid system that is a subset of the UTM system. It uses letter pairs to represent 10km squares.

Magnetic Declination - The difference between True North and the direction that a magnetic compass points for a given location. Magnetic declination varies from place to place and can change as much as a degree in a year.

Magnetic North - The direction that a magnetic compass will point. Can be 10°, 15° or even more different than True North.

Mean Sea Level - The average level of the ocean.

Meridian - A line of longitude going north and south from pole to pole measured in degrees from the Prime Meridian or 0°. See Longitude.

MOB - Man Over Board: A feature in G.P.S. receivers to quickly mark a given location as you are traveling. Some receivers then activate a GOTO navigation function to that waypoint.

NAD27 - North American Datum of 1927: The datum used on most large scale USGS topographic maps.

NAD83 - North American Datum of 1983: A newer datum than the NAD27. Almost identical to WGS84.

NAVSTAR - The name of the United States Department of Defense satellite navigation system.

Northing - A term used to denote positions north of the origin in the UTM coordinate grid system.

Parallel - A line of latitude. See latitude.

Position Fix - The calculated position of the current location by a G.P.S. receiver.

Prime Meridian - The reference line for Longitude in the Lat/Long grid system. All other meridians of longitude are designated in degrees East or West of the Prime Meridian up to 189°. The Prime Meridian is 0° Longitude.

Route - A series of progressive waypoints stored in a G.P.S. receiver which can be activated and navigated.

Scale - The ratio between actual distance and that same distance represented in a map.

Selected Availability - The intentional error introduced in G.P.S. signals by the government to degrade accuracy.

SOG - Speed Over Ground: The speed the G.P.S. receiver is traveling.

Topographic Map - A map that gives not only locations of roads, landmarks and other features, but also gives contour lines at different elevations to show valleys, hills and other topography of the area.

True North - The geographic north of the globe and represented on the Lat/Long grid by the lines of longitude or meridians.

USGS - United States Geologic Survey: A part of the U.S. Department of Interior and the primary mapping agency for the U.S. government.

UTC - Universal Time Coordinated: The time on which all G.P.S. signals are synchronized.. The same as Greenwich Mean Time.

UTM - Universal Transverse Mercator: A metric grid system consisting of 60 zones to cover the earth’s surface.

VMG - Velocity Made Good: The speed at which you are progressing toward the destination or the GOTO waypoint.

Waypoint - A location designated by a set of coordinates and stored in a G.P.S. receiver to be later used as a GOTO destination or as part of a route.

XTE - Cross Track Error: The amount deviated to the left or right from the true course.

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