GPS GLOSSARY | A-C | D-F | G-M | N-S | T-Z | A - C Acquisition Time The time it takes for your GPS unit to acquire a lock onto enough satellites (three for a 2D and four for a 3D) position fix. Active Leg The segment of a route currently being traveled. Almanac Data Constellation information (including location and health of satellites) transmitted by the satellites and collected by your receiver. Almanac data allows your receiver to quickly acquire satellites as soon as it is turned on. Azimuth The angle of measurement between the horizon and a satellite or other object. Bearing (BRG) The compass direction from your position to a destination, measured to the nearest degree. C/A Code (Course Acquisition Code) The standard civilian GPS code - which is subject to degradation by Selective Availability. Cold Start Powering up a unit after it has been turned off for an extended period of time and no longer contains current ephemiris data. Control Segment A world-wide chain of monitoring and control stations that ensure the accuracy of GPS satellite clocks and their positions. Coordinates A unique description of a geographic position using numeric or alphanumeric characters. Course The direction from the beginning landmark of a course to its destination (measured in degrees, radians, or mils). 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. Crosstrack Error (XTE/XTK) The distance you are off the desired course in either direction. D - F Data Message An electronic message giving the satellite's location, clock corrections and health included in the GPS signal received from each satellite. Datum A geographic model obtained by referencing the earth's sea level surface area and applying theoretical mathematical calculations. Map datums may vary from chart to chart. The map datum for your GPS unit should match the datum listed on the corresponding chart. Refer to the map legend to find the specific datum for your chart. Desired Track (DTK) The compass course between the "from" and "to" waypoints. Differential GPS (DGPS) A system of beacons which broadcasts signals to help increase the accuracy of GPS positioning. Dilution of Precision (DOP) A multiplicative factor caused by geometry that modifies ranging between the user and his set of satellites. Also referred to as GOP or GDOP. Distance To Waypoint A measurement in statute/nautical/or metric increments from your current position to your current destination waypoint. Elevation Distance above or below average sea level. Ephemeris Current satellite position and timing information transmitted as part of the satellite data message. 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 left to your destination based upon your present speed and course. Estimated Time of Arrival (ETA) The time of day of your arrival at a destination. Fast-Multiplexing Receiver A GPS receiver using 1 or 2 channels which rapidly switch from satellite to satellite to recover the data message. Fast-Switching Channel A channel which samples a number of satellite ranges in rapid order. "Fast" is categorized as quick enough (2 to 5 milliseconds) to recover the data message. G - M Geometric Dilution of Position (DOP) (See Dilution of Precision.) Geometric Quality The probable accuracy of a position fix found by taking into consideration the relative position of satellites. 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. Heading The direction of intended movement. This may differ from actual COG due to winds, sea conditions, etc. I/O (Interface Option) The one-way or 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. After initialization has occurred, the receiver remembers its location and acquires a position more quickly because it doesn't need a large amount of satellite information. Invert Route To display and navigate a route from end to beginning for purposes of returning back to the route's starting point. Latitude A position's distance north or south of the equator measured by degrees from 0 to 90. 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. Longitude A position's distance east or west of the prime meridian (usually measured in degrees) which runs from the North to South Pole through Greenwich, England. Magnetic North Represents the direction of the north magnetic pole from the observer's position. Magnetic Variation Errors in magnetic compass readings caused by variance in the earth's magnetic field at different locations on the planet. Navigational charts list the variation and a yearly level of increase. Multipath Error An error caused when a signal reaches the receiver antenna by more than one path. Usually caused by one or more paths being bounced or reflected. Multiplexing Channel A GPS receiver channel that can track a variety of satellite signals in sequential order. N - S 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. NOAA National Oceanic and Atmospheric Administration. NOAA is responsible for the production of navigational charts and for monitoring and informing mariners of weather conditions. Parallel Channel Receiver A receiver employing multiple receiver circuits, each dedicated to receiving and processing the signal from a single satellite. Pixel A single display element of an LCD screen. The more pixels, the higher the resolution and definition. Position A geographic location on the earth, commonly measured in latitude and longitude. 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 degrees and minutes, with options for degrees, minutes, and seconds, degrees only, or one of several grid formats. 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 A distance measurement using uncorrected time comparisons from satellite transmitted code and the local receiver's reference code. Route A planned course of travel that is defined by a sequence of waypoints. SA See Selective Availability Satellite Constellation The placement in space of a group of satellites. Search The Sky A message shown when a GPS receiver is gathering data from satellites to compute a position, without almanac data. Selective Availability (SA) The random error which the government intentionally adds into GPS signals so that their accuracy, for civilian use, is degraded. The level of SA is subject to accuracy degradation to 100m 2DRMS. 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 sea conditions or head winds. For example a plane that is going 120 knots into a 10 knot head wind may have a SOG of 110 knots. Straight Line Navigation The act of going from one waypoint to another in the most direct line and with no turns. T - Z TracBack The GARMIN feature which takes your current track log and converts it into a route of up to 30 waypoints to guide you back to a starting position. Track (TRK) Your current direction of travel relative to a ground position (same as COG). True North The direction of the North Pole from your current position. Magnetic compasses are slightly incorrect due to effects of the Earth's magnetic field. GPS units correct for magnetic influences. Turn (TRN) The degrees which must be added to or subtracted from the current heading to reach the course to the intended waypoint. Universal Time Coordinated (UTC) A universal time standard, referencing the time at Greenwich, England. Also referred to as GMT or Zulu time. Universal Transverse Mercator (UTM) A worldwide coordinate projection system utilizing north and east distance measurements from reference point(s). UTM is the primary coordinate system used on United States Geological Survey topographic maps. 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. Velocity Made Good (VMG) The rate at which you are approaching a destination, based upon your current speed and course. Waypoint A permanently stored and named position in the GPS receiver's memory.