How to Choose a GPS Receiver

[Adriel Guerra]

How to Choose a GPS Receiver

PS stands for Global Positioning System, which pinpoints locations on land and sea using a network of 24 orbiting satellites built by the United States Department of Defense.

Originally intended as a navigational aid for the US military, GPS technology is now used for everything from tracking ships on the ocean to helping backpackers find their favorite campsites, and can be found factory-built directly into cars, watches, two-way radios, PDAs and more.

With all these options, how do you know which unit is right for your needs? Let's take a look.



Satellites are data transmitters. On their own they don't do much more than beam waves of information down to earth. GPS technology captures a combination of three or more satellite-transmitted waves to triangulate, or trilaterate, the GPS receiver's position on earth.
In 2003, the United States government flipped the switch on the latest advancement in ensuring the accuracy of GPS technology—Wide Area Augmentation System (WAAS). This can give your WAAS-enabled GPS unit a position accuracy of better than 3 meters 95% of the time.
GPS technology works day or night, rain or shine, making it relatively easy to find your way through an old-growth forest in the dead of night. But batteries do run out, and electronic devices can break, so don't throw away that compass and map just yet.
When comparing GPS units, think toward the future. Will you want a unit that will adapt, especially when it comes to memory size and mapping capabilities? Imagine the scenario in which you'll be using your unit. How's your eyesight? Displays vary in color and size. And if you find the controls too complicated, your GPS won't do much good relegated to a kitchen drawer.
GPS units can be fun as well as useful. Check out geocaching (gee-oh-cashing). A high-tech version of hide-and-seek, this game involves using your GPS to find the locations around the world of hidden caches placed by other geocachers!
Want immediate GPS recommendations based on your needs? Use our handy GPS Finder.


Navigation Features

Recording waypoints
One of the most useful aspects of GPS receivers is their ability to record and store information about specific locations, allowing you to record your favorite spots as you visit them, so your GPS receiver can lead you back to them on later trips.
"Waypoints" (recorded points along your route between your starting point and your destination) make it easier to navigate from point to point and stay on track. Because you can't always travel from point A to point B in a straight line, waypoints tell you where to turn along the way.
In general, the more storage space a GPS receiver has for waypoints and route information, the better. Built-in memory can vary from 1 MB to 115 MB, the number of waypoints from 250 to 1,000.

Route mapping
Some GPS units allow you to save waypoints in ordered sets. This means you can input information about entire routes from start to finish, then follow them by asking your GPS receiver to point from one waypoint to the next as you travel.
Some, but not all, GPS receivers allow you to follow these saved waypoint sets in either direction. This means you can:

Store waypoints during the outbound portion of your trip (as you travel), so your GPS can lead you (waypoint by waypoint) back home again.
Input waypoints for an upcoming trip before you leave home (by taking the coordinates from your map), so your GPS unit can lead you to the right destination as you go.

Internal maps
One of the biggest variables between units (and prices) is how much preloaded map info they contain. You can look at a map of your general location right on your GPS screen, then use it to orient yourself to your surroundings. The choices are almost endless. A unit might have a basemap of the Americas, all countries bordering the Atlantic Ocean, or backcountry details of major rivers.
Certain GPS models can be augmented by downloading topographic or road maps from CD-ROMS (sold separately) via your computer.

The Big Easy

Usability
Many of the most popular units now have "simulation" modes that allow you to familiarize yourself with how they operate, even if you're inside an REI store where satellite signals can't be picked up.
Take a test drive, pushing the GPS buttons should become second nature: Are the directions simple to understand? If you lost the instruction manual, could you figure out the basics by yourself?
Try getting from one point on a map to another. How does the unit tell you which direction to go? On some models, it's as easy as keeping one line on the screen on top of another.
Look for other features such as an altimeter, a magnetic or digital compass, or weather-related data.
Keep your end use in mind—you may be selecting and ordering your GPS unit online from the comfort of your home office, but are looking forward to using your GPS for that saltwater fly-fishing trip on the Merrimack River. Put yourself there and imagine how you and your GPS will interact.

The screen
Color screens can be important, especially in maritime environments. You'll want that lump on the screen to quickly distinguish between land mass and water depth. Other watersport enthusiasts find that less-expensive grayscale (black-and-white) screens are just as suitable.
For automobile navigation, a large screen is for many the deciding factor when choosing a GPS unit. Sometimes less is more—a screen too cluttered with an excess of information can be a dangerous distraction.
Backcountry hikers often look for illumination options. Can it be read in a variety of light conditions? Does it light up for nighttime or low-light use?

Antenna configuration
The debate about the "best" type of antenna rages on. Simply put, there is no best. Each type and style has its pros and cons—determining which works best for your planned adventures should be the deciding factor.
Some say a Quadrifilar Helix (Quad Helix) antenna, the most common in today's models, is the best for use under tree cover. (Think of an egg-beater, whose spiral blade configuration is capable of creating greater splatter coverage than say, a wooden spoon.) But when compared to the flat patch antenna, a popular alternative, reception quality between the two types can be similar, provided the antennas have the same aperture. Currently, improvements in semiconductor technology, rather than antenna configuration, seem to be driving advancements in increased reception.
In the meantime, keep in mind a general rule of thumb: a physically larger GPS receiver is likely to have higher-quality reception under forest canopy than a more compact unit, regardless of antenna type.
There are other antenna issues to consider as well:

Built-in antennas are the least likely to break off or get damaged during normal use.
External antennas are less protected, but they can be re-positioned and manipulated to improve a receiver's pickup in some situations.
Optional plug-in antennas are great when you want to use your GPS in a car or boat. They allow your GPS to pick up signals even if you're seated under a metal roof or hunkered down belowdecks and those satellite signals can't get through.

Performance and Power


Speed
GPS units search for and acquire location information constantly. But some units process data and provide updates far more quickly than others. In general, the more channels a GPS unit has, the faster it will compute and refresh its on-screen information. (When comparing models head-to-head, remember that a unit's performance in "simulation" mode may be far faster than its speed in the wilderness.) And it takes some units longer than others to re-acquire satellite signals once they've been turned off and back on. Look for Time To First Fix (TTFF) data when comparing models.

Power supply
Keep in mind what type of batteries the unit will use and ask yourself the following: Will it work with more than one type? How many extra batteries will I have to pack in (and out) if I head out on a long trip?

As with digital cameras, the battery life of a GPS unit depends on how many features it has eating up the supply. To combat the notoriously ravenous appetite of eletronics, many GPS units have a "sleep" function that conserves energy when the GPS is not being used. Some simpler units lose temporarily stored information when the unit is turned off.
Most modern GPS receivers have built-in back-up batteries to keep your stored information safe when the main internal battery runs out. This internal battery can be replaced by the manufacturer who is able to save and reload your hard-won navigation information.

Weigh Your Options


Size
Today's smallest GPS units are no bigger than a cell phone and fit in the palm of your hand. In general, the lighter and smaller a GPS unit is, the simpler it will be to operate. But weight and space savings often come at a cost, usually either a higher sticker price or fewer features. Some models compensate with a well-chosen feature. An oversized screen perhaps, on an undersized unit.

Shape
In evaluating the physical design of a unit, also consider its durability. Keep your eyes open for pieces that can break off, buttons that can jam easily or outer casings that might crack when dropped. Most GPS units are waterproof rated, and some even float.

Cost
When comparing options, ask yourself what you want your GPS to do for you. You may be perfectly happy with a less-expensive model that includes only basic features. And remember that add-ons—cases, cords, CD-ROMS, mounting brackets, etc.—will add to the price as well as to the versatility of the unit you choose. In answer to the expanding needs of the GPS user, many GPS brands are now "bundling" their most popular units with the accessories most commonly asked for, taking the guesswork out of add-ons.

[Adriel Guerra]

GPS Glossary

or something not much bigger than a deck of cards, GPS terminology can be infinitely more perplexing than learning the rules of Pinochle.

Some terms, such as "trilaterate," have been invented just for the need to describe the unique navigational features of today's electronics.

Look in the previous millennium's dictionaries and you'd be hard pressed to find many of the terms and acronyms below. It's certainly not a new phenomenon, however. The now-common word "radar" was created by combining elements of the words "radio," "detecting," and "ranging."

Some manufacturers of top-name GPS units have developed terms to better define special features. Garmin, for example, uses AutoLocate® to indicate their technology which displays satellite data that is based on a last-known fix when not enough data is available to collect and calculate a current position fix.

Here's a primer of the most-often used terms to help you navigate the language of your GPS unit. Keep in mind though, that new words are being coined every day!

Glossary | A-C | D-F | G-M | N-S | T-Z



A-C


2D Operating Mode
A two-dimensional GPS position fix that includes only horizontal coordinates (no GPS elevation). It requires a minimum of three visible satellites.

3D Operating Mode
A three-dimensional GPS position fix that includes horizontal coordinates, plus elevation. It requires a minimum of four visible satellites.

Accuracy
A measure of how close an estimate of a GPS position is to the true location.

Acquisition Time
The time it takes a GPS receiver to acquire satellite signals and determine the initial position.

Active Antenna
An antenna that amplifies the GPS signal before sending it to the receiver.

Active Leg
The segment of a route currently being traveled. A "segment" is that portion of a route between any two waypoints in the route.

Almanac Data
Information transmitted by each satellite on the orbits and state (health) of every satellite in the GPS constellation. Almanac data allows the GPS receiver to rapidly acquire satellites shortly after it is turned on.

Altimeter
An instrument for determining elevation, especially an aneroid barometer used in aircraft that senses pressure changes accompanying changes in altitude.

Analog Signal
The principal feature of analog signals is that they are continuous. In contrast, digital signals consist of values measured at discrete intervals.

Anti-Spoofing
Encryption of the P-code to protect the P-signals from being "spoofed" through the transmission of false GPS signals by an adversary.

Atomic Clock
A very precise clock that operates using the elements cesium or rubidium. A cesium clock has an error of one second per million years. GPS satellites contain multiple cesium and rubidium clocks.

Azimuth
The horizontal direction from one point on the earth to another, measured clockwise in degrees (0-360) from a north or south reference line. An azimuth is also called a bearing.

Basemap
Some units come with permanently built-in basemaps, which typically include coverage of oceans, rivers, and lakes; principal cities, smaller cities, and towns; interstates, highways, and local thoroughfares; and railroads, airports, and political boundaries. Basemaps are available in a variety of global coverage areas, depending on the user's needs.

Beacon
Stationary transmitter that emits signals in all directions (also called a non-directional beacon). In DGPS, the beacon transmitter also broadcasts pseudo-range correction data to nearby GPS receivers for greater accuracy.

Bearing
The compass direction from a position to a destination, measured to the nearest degree (also call an azimuth). In a GPS receiver, bearing usually refers to the direction to a waypoint.

C/A Code
See Coarse/Acquisition Code.

Carrier Frequency
The frequency of an unmodulated output of a radio transmitter. The GPS L1 carrier frequency is 1575.42 MHz.

Cartography
The art or technique of making maps or charts. Many GPS receivers have detailed mapping-or cartography-capabilities.

CDI
See Course Deviation Indicator.

CDMA
See Code Division Multiple Access.

Code Division Multiple Access (CDMA)
A method whereby many radios use the same frequency, but each one has a unique code. GPS uses CDMA techniques with codes for their unique cross-correlation properties.

Clock Bias
The difference between the indicated clock time in the GPS receiver and true universal time (or GPS satellite time).

Clock Offset
A constant difference in the time reading between two clocks, normally used to indicate a difference between two time zones.

CMG
See Course Made Good.

Coarse/Acquisition Code (C/A Code)
The standard positioning signal the GPS satellite transmits to the civilian user. It contains the information the GPS receiver uses to fix its position and time, and is accurate to 100 meters or better.

COG
See Course Over Ground.

Cold Start
The power-on sequence where the GPS receiver downloads almanac data before establishing a position fix.

Control Segment
A worldwide chain of monitoring and control stations that control and manage the GPS satellite constellation.

Coordinates
A set of numbers that describes your location on or above the earth. Coordinates are typically based on latitude/longitude lines of reference or a global/regional grid projection (e.g., UTM, MGRS, Maidenhead).

Coordinated Universal Time (UTC)
Replaced Greenwich Mean Time (GMT) as the world standard for time in 1986. UTC uses atomic clock measurements to add or omit leap seconds each year to compensate for changes in the rotation of the earth.

Course
The direction from the beginning landmark of a course to its destination (measured in degrees, radians, or mils), or the direction from a route waypoint to the next waypoint in the route segment.

Course Deviation Indicator (CDI)
A technique for displaying the amount and direction of crosstrack error (XTE).

Course Made Good (CMG)
The bearing from the 'active from' position (your starting point) to your present position.

Course Over Ground (COG)
Your direction of movement relative to a ground position.

Course To Steer
The heading you need to maintain in order to reach a destination.

Course Up Orientation
Fixes the GPS receiver's map display so the direction of navigation is always "up."

Crosstrack Error (XTE/XTK)
The distance you are off the desired course in either direction.


D-F


Datum
A math model which depicts a part of the surface of the earth. Latitude and longitude lines on a paper map are referenced to a specific map datum. The map datum selected on a GPS receiver needs to match the datum listed on the corresponding paper map in order for position readings to match.

Desired Track (DTK)
The compass course between the "from" and "to" waypoints.

DGPS
See Differential GPS.

Differential GPS (DGPS)
An extension of the GPS system that uses land-based radio beacons to transmit position corrections to GPS receivers. DGPS reduces the effect of selective availability, propagation delay, etc. and can improve position accuracy to better than 10 meters.

Dilution Of Precision (DOP)
A measure of the GPS receiver/satellite geometry. A low DOP value indicates better relative geometry and higher corresponding accuracy. The DOP indicators are GDOP (geometric DOP), PDOP (position DOP), HDOP (horizontal DOP), VDOP (vertical DOP), and TDOP (time clock offset).

Distance
The length (in feet, meters, miles, etc.) between two waypoints or from your current position to a destination waypoint. This length can be measured in straight-line (rhumb line) or great-circle (over the earth) terms. GPS normally uses great circle calculations for distance and desired track.

DOD
The United States Department of Defense. The DOD manages and controls the Global Positioning System.

DOP
See Dilution Of Precision.

Downlink
A transmission path for the communication of signals and data from a communications satellite or other space vehicle to the earth.

DTK
See Desired Track.

Elevation
The distance above or below mean sea level.

Ellipsoid
A geometric surface, all of whose plane sections are either ellipses or circles.

Ephemeris Data
Current satellite position and timing information transmitted as part of the satellite data message. A set of ephemeris data is valid for several hours.

EPE
See Estimated Position Error.

Estimated Position Error (EPE)
A measurement of horizontal position error in feet or meters based upon a variety of factors including DOP and satellite signal quality.

Estimated Time Enroute (ETE)
The time it will take to reach your destination (in hours/minutes or minutes/seconds) based upon your present position, speed, and course.

Estimated Time Of Arrival (ETA)
The estimated time you will arrive at a destination.

ETA
See Estimated Time Of Arrival.

ETE
See Estimated Time Enroute.

Frequency
The number of repetitions per unit time of a complete waveform, as of a radio wave (see L1 and L2 frequencies in this glossary).


G-M


Geocaching
A high-tech version of hide-and-seek. Geocachers seek out hidden treasures utilizing GPS coordinates posted on the Internet by those hiding the cache.

Geodetic Datum
A math model representing the size and shape of the earth (or a portion of it).

Geographic Information System (GIS)
A computer system or software capable of assembling, storing, manipulating, and displaying geographically referenced information (i.e., data identified according to their location). In practical use, GIS often refers to the computer system, software, and the data collection equipment, personnel, and actual data.

Geosynchronous Orbit
A specific orbit around where a satellite rotates around the earth at the same rotational speed as the earth. A satellite rotating in geosynchronous orbit appears to remain stationary when viewed from a point on or near the equator. It is also referred to as a geostationary orbit.

GIS
See Geographic Information System.

Global Positioning System (GPS)
A global navigation system based on 24 or more satellites orbiting the earth at an altitude of 12,000 statute miles and providing very precise, worldwide positioning and navigation information 24 hours a day, in any weather. Also called the NAVSTAR system.

Glonass
The Global Orbiting Navigational Satellite System; the Russian counterpart to the United States' GPS system.

GMT
See Greenwich Mean Time.

GoTo
A route consisting of one leg, with your present position being the start of the route and a single defined waypoint as the destination.

GPS
See Global Positioning System.

Greenwich Mean Time (GMT)
The mean solar time for Greenwich, England, which is located on the Prime Meridian (zero longitude). Based on the rotation of the earth, GMT was used as the basis for calculating standard time throughout most of the world. UTC (see Coordinated Universal Time), replaced Greenwich Mean Time (GMT) as the world standard for time in 1986.

Grid
A pattern of regularly spaced horizontal and vertical lines forming square zones on a map used as a reference for establishing points. Grid examples are UTM, MGRS, and Maidenhead.

Heading
The direction in which a vehicle is moving. For air and sea operations, this may differ from actual Course Over Ground (COG) due to winds, currents, etc.

Healthy
A term used when an orbiting GPS satellite is suitable for use. "State" is also used to refer to satellite health.

Input/Output (I/O)
The two-way transfer of GPS information with another device, such as a nav plotter, autopilot, or another GPS unit.

Initialization
The first time a GPS receiver orients itself to its current location and collects almanac data. After initialization has occurred, the receiver remembers its location and acquires a position more quickly because it knows which satellites to look for.

Ionosphere
A region of the earth's atmosphere where ionization caused by incoming solar radiation affects the transmission of GPS radio waves. It extends from a height of 50 kilometers (30 miles) to 400 kilometers (250 miles) above the surface.

Invert Route
To display and navigate a route from end to beginning for purposes of returning to the route's starting point.

L1 Frequency
One of the two radio frequencies transmitted by the GPS satellites. This frequency carries the Coarse Acquisition Code (C/A code), P-Code, and the nav message, and is transmitted on a frequency of 1575.42 MHz.

L2 Frequency
One of the two radio frequencies transmitted by the GPS satellites. This frequency carries only the P-Code, and is transmitted on a frequency of 1227.6 MHz.

L Band
The radio frequencies that extend from 390 MHz to 1550 MHz. The GPS carrier frequencies are in the L band (1227.6 MHz and 1575.42 MHz).

LAAS
See Local Area Augmentation System.

Latitude
A position's distance north or south of the equator, measured by degrees from zero to 90. One minute of latitude equals one nautical mile.

LCD
See Liquid Crystal Display.

Leg (Route)
A portion of a route consisting of a starting (from) waypoint and a destination (to) waypoint. A route that is comprised of waypoints A, B, C, and D would contain three legs. The route legs would be from A to B, from B to C, and from C to D.

Lithium Battery
A battery using the soft, silvery, highly reactive metallic element named lithium; good for uses where weight and cold weather conditions are concerns.

Line Of Sight (LOS) Propagation
Of an electromagnetic wave, propagation in which the direct transmission path from the transmitter to the receiver is unobstructed. The need for LOS propagation is most critical at GPS frequencies.

Liquid Crystal Display (LCD)
A display circuit characterized by a liquid crystal element sandwiched between two glass panels. Characters are produced by applying an electric field to liquid crystal molecules and arranging them to act as light filters.

Local Area Augmentation System (LAAS)
The implementation of ground-based DGPS to support aircraft landings in a local area (20-mile range).

Longitude
The distance east or west of the prime meridian (measured in degrees). The prime meridian runs from the north to south pole through Greenwich, England.

LORAN
Loran, which stands for LOng RAnge Navigation, is a grid of radio waves in many areas of the globe that allows accurate position plotting. Loran transmitting stations around the globe continually transmit 100 kHz radio signals. Special shipboard Loran receivers interpret these signals and provide readings that correspond to a grid overprinted on nautical charts. By comparing signals from two different stations, the mariner uses the grid to determine the position of the vessel.

Magnetic North
Represents the direction of the north magnetic pole from the observer's position. The direction a compass points.

Magnetic Variation
In navigation, at a given place and time, the horizontal angle (or difference) between true north and magnetic north. Magnetic variation is measured east or west of true north.

Map Display
A graphic representation of a geographic area and its features.

Mean Sea Level
The average level of the ocean's surface, as measured by the level halfway between mean high and low tide. Used as a standard in determining land elevation or sea depths.

Multipath Error
An error caused when a satellite signal reaches the GPS receiver antenna by more than one path. Usually caused by one or more paths being bounced or reflected. The TV equivalent of multipath is "ghosting."

Multiplexing Receiver
A GPS receiver that switches at a very rapid rate between satellites being tracked. Typically, multiplexing receivers require more time for satellite acquisition and are not as accurate as parallel channel receivers. Multiplexing receivers are also more prone to lose a satellite fix in dense woods than parallel channel GPS receivers.

[Adriel Guerra]

N-S

Nautical Mile
A unit of length used in sea and air navigation, based on the length of one minute of arc of a great circle, especially an international and U.S. unit equal to 1,852 meters (about 6,076 feet).

Navigation
The act of determining the course or heading of movement. This movement could be for a plane, ship, automobile, person on foot, or any other similar means.

Navigation Message
The message transmitted by each GPS satellite containing system time, clock correction parameters, ionospheric delay model parameters, and the satellite's ephemeris data and health. The information is used to process GPS signals to give the user time, position, and velocity. Also known as the data message.

NAVSTAR
The official U.S. Government name given to the GPS satellite system. NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging. NMEA (National Marine Electronics Association) A U.S. standards committee that defines data message structure, contents, and protocols to allow the GPS receiver to communicate with other pieces of electronic equipment aboard ships.

NMEA Standard
A NMEA standard defines an electrical interface and data protocol for communications between marine instrumentation.

North Up Orientation
Fixes the GPS receiver's map display so north is always fixed at the top of the screen.

Parallel Channel Receiver
A continuous tracking receiver using multiple receiver circuits to track more than one satellite simultaneously.

P-Code
The precise code of the GPS signal typically used only by the U.S. military. It is encrypted and reset every seven days to prevent use from unauthorized persons.

Pixel
A single display element on an LCD screen. The more pixels, the higher the resolution and definition.

Position
An exact, unique location based on a geographic coordinate system.

Position Fix
The GPS receiver's computed position coordinates.

Position Format
The way in which the GPS receiver's position will be displayed on the screen. Commonly displayed as latitude/longitude in degrees and minutes, with options for degrees, minutes and seconds, degrees only, or one of several grid formats.

Prime Meridian
The zero meridian, used as a reference line from which longitude east and west is measured. It passes through Greenwich, England.

Pseudo-Random Code
The identifying signature signal transmitted by each GPS satellite and mirrored by the GPS receiver in order to separate and retrieve the signal from background noise.

Pseudo-range
The measured distance between the GPS receiver and the GPS satellite using uncorrected time comparisons from satellite-transmitted code and the local receiver's reference code.

Quadrifilar Helix Antenna
Also known as Quadhelix Antenna. A type of antenna in which four spiraling elements form the receiving surface of the antenna. Unlike Patch Antennas which are flat, Quadrifilar Helix Antennas are encased either in a plastic cylinder or within the GPS unit itself, resulting in a GPS unit that is capable of getting the best reception possible but can also result in a slightly larger size unit. This doesn't necessarily guarantee reception in every situation, however. Areas where you may still have difficulty garnering reception are very dense forests, slot canyons, caves or downtown Manhattan.

RS-232
A serial input/output standard that allows for compatibility between data communication equipment made by various manufacturers. Radio Technical Commission For Maritime Services (RTCM) Special Committee 104 A committee created for the purposes of establishing standards and guidance for interfacing between radio beacon-based data links and GPS receivers, and to provide standards for ground-based differential GPS stations.

RAIM
Receiver Autonomous Integrity Monitoring; A GPS receiver system that would allow the receiver to detect incorrect signals being transmitted by the satellites by comparing solutions with different sets of satellites.

Route
A group of waypoints entered into the GPS receiver in the sequence you desire to navigate them.

SA
See Selective Availability.

Search The Sky
A message shown when a GPS receiver is gathering satellite almanac data. This data tells the GPS receiver where to look for each GPS satellite.

Serial Communication
The sequential transmission of the signal elements of a group representing a character or other entity of data. The characters are transmitted in a sequence over a single line, rather than simultaneously over two or more lines, as in parallel transmission. The sequential elements may be transmitted with or without interruption.

Selective Availability (SA)
The random error, which the government can intentionally add to GPS signals, so that their accuracy for civilian use is degraded. SA is not currently in use.

SOG
See Speed Over Ground.

SONAR
A system using transmitted and reflected underwater sound waves to detect and locate submerged objects or measure the distance to the floor of a body of water.

Space Segment
The satellite portion of the complete GPS system.

Speed Over Ground (SOG)
The actual speed the GPS unit is moving over the ground. This may differ from airspeed or nautical speed due to such things as head winds or sea conditions. For example, a plane that is going 120 knots into a 10-knot head wind will have a SOG of 110 knots. Also known as Speed Above Ground (SAG).

Spread Spectrum
The received GPS signal is wide bandwidth and low power. The L-band signal is modulated with a pseudo-random noise code to spread the signal energy over a much wider bandwidth than the signal information bandwidth. This provides the ability to receive all satellites unambiguously and to give some resistance to noise and multipath.

Statute Mile
A unit of length equal to 5,280 feet or 1,760 yards (1,609 meters) used in the U.S. and some other English-speaking countries.

Straight-Line Navigation
The act of going from one waypoint to another in the most direct line and with no turns.


T-Z


Time To First Fix (TTFF)
If you have not used your GPS unit for several months, the almanac data for the satellites may be out of date. The unit is capable of recollecting this information on its own, but the process can take several minutes. Time to First Fix (TTFF) is the time it takes a GPS receiver to find satellites after the user first turns it on (when the GPS receiver has lost memory or has been moved over 300 miles from its last location).

Track Up Orientation
Fixes the GPS receiver's map display so the current track heading is at the top of the screen.

Track (TRK)
Your current direction of travel relative to a ground position (same as Course Over Ground).

Transducer
A device, much like a microphone, that converts input energy of one form into output energy of another. Fishfinders separate and enhance the information received from a transducer to show underwater objects.

Triangulation
A method of determining the location of an unknown point, as in GPS navigation, by using the laws of plane trigonometry.

Trilaterate
A simple mathematical principle similar to triangulation and using a series of circles for two-dimensional (2D) problem solving, and spheres for three-dimensional (3D) problem solving.

TRK
See Track.

TRN
See Turn.

Troposphere
The lowest region of the atmosphere between the surface of the earth and the tropopause, characterized by decreasing temperature with increasing altitude. GPS signals travel through the troposphere (and other atmospheric layers).

True North
The direction of the north pole from your current position. Magnetic compasses indicate north differently due to the variation between true north and magnetic north. A GPS receiver can display headings referenced to true north or magnetic north.

TTFF
See Time To First Fix.

Turn (TRN)
The degrees which must be added to or subtracted from the current heading to reach the course to the intended waypoint.

Universal Transverse Mercator (UTM)
A nearly worldwide coordinate projection system using north and east distance measurements from reference point(s). UTM is the primary coordinate system used on U.S. Geological Survey topographic maps.

Uplink
A transmission path by which radio or other signals are sent from the ground to an aircraft or a communications satellite.

User Interface
The way in which information is exchanged between the GPS receiver and the user. This takes place through the screen display and buttons on the unit.

User Segment
The segment of the complete GPS system that includes the GPS receiver and operator.

UTC
See Coordinated Universal Time.

UTM
See Universal Transverse Mercator.

Velocity Made Good (VMG)
The rate of closure to a destination based upon your current speed and course.

WAAS
See Wide Area Augmentation System.

Waterproof
Many GPS units are waterproof in accordance with IEC 529, a European system of test specification standards for classifying the degrees of protection provided by the enclosures of electrical equipment. An IEC 529 IPX7 designation means the GPS case can withstand accidental immersion in one meter of water for up to 30 minutes. An IEC 529 IPX8 designation is for continuous underwater use.

Wavelength
The distance between points of corresponding phase of two consecutive cycles of a wave.

Waypoints
Waypoints are locations or landmarks worth recording and storing in your GPS. These are locations you may later want to return to. They may be check points on a route or significant ground features. (e.g., camp, the truck, a fork in a trail, or a favorite fishing spot). Waypoints may be defined and stored in the unit manually by taking coordinates for the waypoint from a map or other reference. This can be done before ever leaving home. Or more usually, waypoints may be entered directly by taking a reading with the unit at the location itself, giving it a name, and then saving the point. Waypoints may also be put into the unit by referencing another waypoint already stored, giving the reference waypoint, and entering the distance and compass bearing to the new waypoint.

Wide Area Augmentation System (WAAS)
A system of satellites and ground stations that provide GPS signal corrections for better position accuracy. A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time. WAAS consists of ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message.

WGS-84
World Geodetic System, 1984. The primary map datum used by GPS. Secondary datums are computed as differences from the WGS 84 standard.

Y-Code
The encrypted P-Code.

XTE/XTK
See Crosstrack Error.

[Adriel Guerra]

GPS Frequently Asked Questions (FAQs)

From its beginnings as space-age technology more than three decades ago, GPS technology has progressed from electronic novelty to the point where, for many, it's an indispensable requirement of daily life. Indeed, GPS technology may one day be built into almost every electronic unit we operate.

Don't throw out that GPS receiver you bought back in 1997 just yet though. Far from being obsolete, that unit's basic function—a compass that works day or night every day of the year-still works just fine (provided the batteries are fresh). But newer generation GPS units can do some amazing things. In fact, the future of GPS technology and its uses is limited only by man's imagination.

If you're new to the world of Global Positioning, these five often-asked questions will help acquaint you with this now-ubiquitous technology:

Q. What does the acronym GPS stand for?
A. Global Positioning System.

Q. How does GPS work?
A. GPS technology is a global navigation system based on two dozen satellites that orbit the earth at an altitude of 12,000 statute miles and provide very precise, worldwide positioning and navigation information 24 hours a day, in any weather. GPS technology captures a combination of three or more satellite-transmitted waves to triangulate, or trilaterate, your GPS receiver's position on earth.

Q. Who maintains the satellites?
A. Built by the United States Department of Defense (DOD) at a cost of $12 billion and intended primarily for military use, the DOD manages and controls the two dozen satellites which make up the Global Positioning System. The system includes three backup satellites. The United States Government has officially named this satellite system NAVSTAR (NAVigation Satellite Timing And Ranging).

Q. How accurate is GPS technology?
A. Initially, the signals of the GPS satellites contained random errors, known as Selective Availability (SA). The United States government disabled SA in May 2000, releasing the proper transmission of these satellite signals for public use. This made GPS-secured locations ten-times more accurate than they were prior to 2000 and aids not only outdoor adventurers, but public-service agencies such as fire and police departments as well.

In 2003, the United States government flipped the switch on the latest advancement in ensuring the accuracy of GPS technology—Wide Area Augmentation System (WAAS). From earth, data that has been corrected is collected from stations across the United States and uploaded to a geostationary satellite, which in turns transmits the data back to a WAAS-enabled GPS receiver. This data can give your WAAS-enabled GPS unit a position accuracy of better than three meters 95% of the time, and can achieve sub-meter results—a far cry from the football field-sized positioning provided by the first GPS units of the 1980s.


Differential GPS (DGPS) is an extension of GPS that uses land-based radio beacons to transmit satellite position corrections to your DGPS receiver. Approximately 60 broadcast stations, including two control centers, have been established by the United States Coast Guard. DGPS stations recalculate the effect of random error, propagation delay, ionosphere and troposphere irregularities, etc. and can improve position accuracy to between 1 and 3 meters. Most recreation-grade GPS units require the purchase an additional receiver antenna to access DGPS signals.

Q. Do I have to subscribe to a service in order to use my GPS unit?
A. The DOD Navstar GPS is free. However, there are some augmented systems, such as DGPS, that require receiver parts which may not be part of your GPS unit and will have to be purchased separately. The Coast Guard maritime DGPS service, for example, became fully operational in March 1999 and as of 2004 supports approximately 1.5 millions users. There are other competing positioning systems, both ground-based and satellite-based, that provide correction signals for a subscription fee. These are accessed primarily by commercial concerns such as surveyors and oil-rigging companies. Additional business-related advancements in GPS technology on the horizon include Local Area Augmentation System (LAAS), which will provide GPS support for automatic landings of airplanes at commercial airports.