The Glossary Of GPS(4)

The Glossary Of GPS Between G-L

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.