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GPS receivers use almanac data to predict which satellites are nearby when they're looking for GPS signals. Almanac data includes a set of parameters for each GPS satellite that can be used to calculate its approximate location in orbit.
Using almanac data saves time by letting the receiver skip looking for satellites that are below the horizon.
GPS satellites include almanac data in the signals they transmit to GPS receivers. Although variations in satellite orbits can accumulate with time, almanac data doesn't need to be highly accurate to be useful. Data collected before your receiver was last switched off may remain usable for weeks or months.
A 2-dimensional position fix that only includes the X (longitude) and Y(latitude) coordinates.
No altitude is calculated. To calculate a 2D fix you need at least 3 visible satellites.
A 3-dimensional position fix that only includes the X (longitude), Y(latitude) and Z(altitude) coordinates.
To calculate a 3D fix you need at least 4 visible satellites.
AGPS is short for Assisted GPS. Assisted GPS is used to speed up start-up times of GPS equipment. GPS may have problems getting a lock when the signal is weak and in such a case Assisted GPS would assist in getting a lock. An example of Assisted GPS is downloading almanac data using internet to shorten the time to first fix.
A characteristic of a map feature described as a text or number. Attributes are often stored in a tabular format. An example of an attribute, is a value indicating the number of residents in a city or area.
CAD stands for Computer Aided Design. An automated system for the design, drafting and display of graphically oriented information.
A centroid is the x,y coordinate of the center of a one or two dimensional shape.
Course Made Good
Course Over Ground
DGPS is short for Differential GPS. Differential GPS is an extension of the GPS system that uses land-based radio beacons (Long Wave, UHF or R.D.S.) 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.
Another example of DGPS is WAAS/EGNOS, which is using GPS satellites to broadcast position corrections. WAAS/EGNOS is less accurate the land based radio beacons, because a correction for a larger area has to be sent.
The course is defined as the direction between two points. The course is expressed in degrees or radians.
EGNOS stands for European Geostationary Navigation Overlay Service. Like WAAS in North America, EGNOS is an enhancement to GPS in Europe, that can allow specially equipped GPS receivers to more accurately calculate their position.
EGNOS uses a network of ground-based stations that compare their precisely known location with locations calculated from GPS satellite signals. Any differences found can be used to create correction data that's broadcast from EGNOS satellites.
EGNOS can help correct for error caused by distortion of GPS signals as they pass through the ionosphere as well as clock and orbital variations associated with individual GPS satellites.
Estimated Position Error. The estimated position error in meters calculated by the GPS receiver based on the GPS satellite positions.
EUREF Permanent Network. European network of 140 permament GPS reference stations in more then 30 European countries.
Estimated Time of Arrival. Time at which you reach the end of the planned route.
Estimated Time of Enroute. Time after which you reach the end of the planned route.
European Terrestrial Reference System. European Reference System based on the ITRS system. Also called ETRS89.
Europe's own satellite navigation system, also known as GNSS (Global Navigation Satellite System). The system is expected to work in 2012.
A geofence is a virtual boundary around a geographical space.
Geographic Information System
Global Navigation Satellite System. GLONASS is the Russian global positioning system that's similar to the United States' Navstar GPS system. The first satellites for GLONASS were launched in the early 1980s. Today, GLONASS has fewer satellites and is not as widely used as the United States' GPS system.
Global Positioning System. 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
GPS Exchange Format. XML format used to exchange GPS data (track, waypoints and routes) between GPS mapping software programs.
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.
In mapping, a layer is a collection of features with the same feature type. An example of a layer, is a collection of line features (ways, streams, contours), a collection of area features (lakes, towns, land coverage) or a collection of points (cities, sightings, heights).
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.
A map feature is a representation of a real world object on a map. Features can be represented as points, polygons, lines, texts, or a cell or pixel in a raster format. A feature can contain coordinates and attributes (optional).
The name given to the satellites of the GPS system. NAVSTAR stands for Navigation Satellite Timing and Ranging.
National Marine Electronics Association. An 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. Also called NMEA-0183. The latest version of NMEA is called NMEA2000 which uses a bus system instead of serial communications.
Networked Transport of RTCM via Internet Protocol. Industry standard on how to transmit RTCM data over an IP based network.
Receiver Independend Exchange format. Raw data used to post process data received from a GPS receiver. All satellite information (timings, positions etc) is stored in this file.
A serial communications standard to transfer data between various communication equipment. This protocol is used by PC serial ports.
Radio Technical Commision for Maritime Service. Standard used to exchange data between GPS devices and correction signal receivers or datalinks.
Real Time Kinematic. GPS method where a base station or datalink is used which sends corrections to the mobile GPS receiver. Using this method it is possible to measure positions realtime and with very high accuracy.
Selective Availability. Selective Availability Selective Availability is a feature of the U.S. GPS system that enables the Department of Defense (DOD) to purposely degrade the accuracy of signals that are used for civilian purposes. This system is no longer active after it has been turned off in the year 2000.
Topology is the mathematics of relationships. In the world of GIS, it refers to such relationships as adjacency (e.g. two polygons next to each other), connectivity (i.e. line joins) and spatial overlay. We can overlay a polygon layer (e.g. park) with a distribution of animal sightings (e.g. points) and determine how many and which sightings occur within the park.
Time to First Fix (TTFF) describes the time and process required for a GPS device to acquire enough usable satellite signals and data to provide accurate navigation.
Universal Transverse Mercator. UTM is a map projection based on the Transverse Mercator projection which can be used anywhere around the globe. To use the UTM projection the globe is divided in 60 zones.
Virtual Reference System
Wide Area Augmentation System. WAAS is an enhancement to GPS in North America that can allow specially equipped GPS receivers to more accurately calculate their position.
WAAS uses a network of ground-based stations that compare their precisely known location with locations calculated from GPS satellite signals. Any differences found can be used to create correction data that's broadcast from WAAS satellites.
WASS can help correct for error caused by distortion of GPS signals as they pass through the ionosphere as well as clock and orbital variations associated with individual GPS satellites.
A waypoint is a reference point, or set of coordinates that precisely identify a location.