The Development of the GPS satellite system      

What is a GPS unit and why do you need one?

The GPS (Global Positioning System) is a "constellation" of 26 satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location by a system of triangulation. It also gives you a highly accurate time reference thanks to the atomic clock on board each satellite. The GPS system was built and controlled by the United States Department of Defense, but it can be used by anyone with a GPS receiver unit, free of charge. Most general-use GPS receivers can locate a user with an accuracy of better than 10 metres.

Following the launch of the first satellite in 1978, GPS equipment was largely used in military and scientific applications. The primary military purpose of the GPS is to be able to tell exactly where the troops are, and for accurate targeting of smart bombs, cruise missiles, or other weapons. The satellites also carry nuclear detonation detectors - a major part of the United States Nuclear Detonation Detection System.

Scientists, on the other hand, rely on GPS systems to provide accurate data that was never available previously. This has enormous benefits in geological studies such as measuring the movement of the Earth's tectonic plates (seismology) and arctic ice flows, along with volcanic activity.

Outside the scientific arena, everyday commercial applications for GPS receivers are expanding. The system is used as a navigation aid in aeroplanes and ships, plus computer-controlled machinery such as harvesters, mine trucks and other vehicles. In recent years, GPS technology has entered the consumer market, with lower costs enabling everything from boat and car-based systems, down to pocket-sized personal models (an essential item for would-be woodsmen). GPS units are also commonly bundled into mobile phones, and personal GPS units are almost de rigueur for any serious outdoor adventuring, such as bushwalking, cross-country skiing, ballooning, sailing and flying. There are also a range of personal GPS receivers that can turn a regular smart phone into a fully fledged navigational unit.

GPS units are also often standard equipment on many cars, trucks and commercial vehicles. In these conveyances, the features go far beyond replacing the ever-handy street directory. Today's GPS receivers cannot only tell you where you are and direct you where to go, they can also display a range of valuable information like traffic and weather details, "black spots", fixed speed cameras and red light cameras. And avoiding one of those even once can just about pay for the system!

"Big Brother" is now watching

GPS is all about being able to tell where things (or people) are. So it is no surprise that many commercial applications have developed for vehicle monitoring. Many companies now use GPS systems to monitor their vehicle fleets, giving them detailed reports on where their vehicles are and for how long. This application, however, is starting to get into the realm of "big brother" (no, not the reality TV show - the Orwellian idea of constant monitoring of individuals for control purposes).

Using this system you could monitor the vehicle of a sales rep, for instance, and tell when and where he was at all times; when the car was stopped, idling or mobile; if it strayed out of his "territory" and even if he made an unauthorised stop at McDonalds!

While sales reps may not applaud the idea of such close surveillance, a GPS unit in a luxury car is becoming a must-have feature - not only to assist drivers to find their way around, but also to help prevent theft and assist in vehicle recovery. An anti-theft GPS unit can tell if a car is operated by an unauthorised user (GPS-speak for "stolen"). The unit emits a "vehicle tamper" notification which is received at a central monitoring base and police or private response teams are able to track the vehicle. Some can then immobilise the vehicle at the discretion of the pursuers. This type of system can even incorporate a "panic button" which can be used in the event of car-jacking; and incorporate an anti-tow sensor, so the vehicle can't even be picked up and carried away.

GPS tracking can also be applied to people. In other "big brother" news, it was reported recently in New Scientist that kids in Japan will soon be wearing GPS transceivers in their school blazers, allowing parents to track their whereabouts on a notebook!

Meanwhile, wearable GPS bracelets and anklets have been developed for tracking prisoners on probation or under house arrest. A similar device was reportedly pilfered by a burglar, who didn't know what it was but thought the techno-looking device must be valuable. Police wasted no time in tracking the device and educating the unwary thief.

How the Global Positioning System works

GPS is an acronym for Global Positioning System. As the name suggests, it is a device that is designed to give you a precise location on the earth's surface. For the purpose of mapping, the earth's surface is divided into a grid of meridian lines called longitude (running from pole to pole) and latitude (running around the earth). These divisions are marked off in degrees. Longitude circles the globe from 0 degrees, which runs through Greenwich in England to 180 degrees West in one direction and 180 degrees East in the other, roughly dividing the world through the Bering straights separating North America and Russia. Latitude rings the globe from 0 degrees at the equator to 90 degrees North and South at each pole. Each degree is further broken down into smaller measures of minutes and seconds, enabling your precise position on the earth's surface to be plotted in relation to the grid.

In order for your GPS unit to tell you where you are standing, it needs to communicate with satellites circling the earth. Currently the GPS system uses 29 satellites (including three spares in case of failure) in intermediate orbits circling the earth twice a day at an altitude of 20,200 kilometres. The satellites are aligned so that at least four are within line of sight from virtually any place on earth. These satellites are monitored from five ground stations and atomic clocks on board are monitored and synchronised to within one millisecond of each.

Your GPS receiver receives a signal from four satellites that are currently in line of sight and accurately calculates the precise distance from each of the satellites by measuring the time delay between when the satellite sent the signal and the local time it was received. The time delay multiplied by the speed of light gives the distance to the satellite. The satellite also sends data that identifies its orbital position and the receiver can, by comparing the satellite's position and range, determine its own location. The local GPS receiver "imagines" the satellite is located on a sphere some 65-85 light-milliseconds distance. When it reads the information from four satellites it is able to locate its positioned based on where the four imaginary spheres intersect. Accuracy of location is determined by some technical calculations that enable the local clock in the GPS receiver to recalibrate according to the extremely accurate time setting in the satellites' atomic clocks.

This time-position information can then be translated to the grid system used to map the earth's surface, and therefore give you a precise location in real time, or calculate distance travelled over time or calculate distance between two fixed points, including altitude.

In order for a GPS receiver to work however, its aerial must have a clear sky view. Dense bush and tall buildings hamper the effectiveness of a GPS receiver. The use of a GPS receiver may also be limited in areas that are involving military operations. Military operations can locally deny GPS to hostile and suspected hostile users without affecting the rest of the world or its own applications using a technique of local jamming.

As noted before, there are  26 satellites making up the GPS space segment orbiting at about 12,000 miles above the Earth. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of about 7,000 miles an hour.

GPS satellites are powered by solar energy collected from large solar panels. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep them flying in the correct path.

Facts about the GPS satellite system (also called NAVSTAR, the official U.S. Department of Defense name for GPS)

• Transmitter power is only 50 watts or less.
• The first GPS satellite was launched in 1978.
• A full constellation of 24 satellites was achieved in 1994.
• Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
• A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.

What Signal do GPS Satellites Transmit?

GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

A GPS signal contains three different bits of information - a pseudorandom code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on your Garmin GPS unit's satellite page, as it identifies which satellites it's receiving.

Ephemeris data tells the GPS receiver where each GPS satellite should be at any particular time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system.

Almanac data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.

Sources of GPS signal error

Factors that can degrade the GPS signal and thus affect accuracy include the following:

• Ionosphere and troposphere delays - The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
• Signal multipath - This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors. Satellite signals cannot penetrate solid matter, so the system emits multipule signals to get around solid mass like city buildings, mountains and rocky outcrops.
• Receiver clock errors - A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
• Orbital errors - Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
• Number of satellites visible - The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
• Satellite geometry/shading - This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
• Intentional degradation of the satellite signal - Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The US government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

Glossary of Terms

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. The Garmin® eTrex® Vista and Summit models contain a basic GPS with a built-in barometric altimeter.

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.

AutoLocate®
This is a proprietary feature of Garmin GPS receivers. A Garmin unit displays the "AutoLocate" status when it is looking for and collecting data from satellites that were visible at its last known or initialized position (almanac data), but it has not collected enough data to calculate a position fix.

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
Garmin mapping 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 pseudorange 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.

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.

DCG®
See Depth Controlled Gain.

Depth Controlled Gain (DCG)
A Garmin proprietary technology that automatically adjusts fishfinder sensitivity according to depth, not echo intensity. The result is a much more detailed and accurate picture of bottom structure.

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).

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 statue miles and providing very precise, worldwide positioning and navigation information 24 hours a day, in any weather. Also called the NAVSTAR system. For more information, see About GPS.

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 is used as the basis for calculating standard time throughout most of the world.

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 soft, silvery, highly reactive metallic element that is used in batteries 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.

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.
PanTrack^TM
A Garmin-proprietary feature that allows the user to move the pointer and pan a track in either direction, then select a location along the track to start a TracBack® or GoTo, or to mark a waypoint.

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.

Pseudorange
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
A type of GPS antenna in which four spiraling elements form the receiving surface of the antenna. For GPS use, quadrifilar antennas are typically half-wavelength or quarter-wavelength size and encased in a plastic cylinder for durability.

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.

See-Thru® Technology
A Garmin exclusive technology which allows the various Garmin fishfinders to hear both weak and strong signals simultaneously so as to identify fish returns under the toughest conditions: suspended in thermoclines or even hiding near structures.

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. This technology is used in Garmin fishfinders and sounder products.

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.

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.

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).

TracBack
The proprietary Garmin feature which takes your current track log and converts it into a route to guide you back to a starting position.

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.

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
Most Garmin GPS units are waterproof in accordance with IEC 529 IPX7. IEC 529 is a European system of test specification standards for classifying the degrees of protection provided by the enclosures of electrical equipment. An IPX7 designation means the GPS case can withstand accidental immersion in one meter of water for up to 30 minutes. An 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. (At this time, the system is still in the development stage and is not fully operational.) WAAS consists of approximately 25 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.

For more information, see What is WAAS?, or visit the FAA's website.

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.