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Glossary
- absolute accuracy
- the closeness of an estimate (or measurement) to the truth with respect to the earth's reference frame
- absolute positioning
- the determination of the coordinates of a receiver with respect to the earth's reference frame by intersection of the signals from four or more GPS satellites (also referred to as single point positioning or simply point positioning)
- accuracy
- the closeness of an estimate (or measurement) to its true (but unknown) value
- alerts
- a table which indicates the positions of the satellites in view to an observer on the earth's surface at a designated location and time
- almanac files
- data files which contain parameters describing the location of the GPS satellites with respect to time and which are needed to compute predictions of satellite availability, visibility and geometry
- ambiguity
- the integer number of carrier cycles between a satellite and receiver
- anti-spoofing (AS)
- the denial of access of the P code to civilian users
- azimuth
- the horizontal angle clockwise from north to the location of the satellite in the sky
- baseline
- a pair of stations for which simultaneous GPS data has been collected
- blunders
- errors which result from some equipment malfunction or observer's mistake (also referred to as gross errors)
- broadcast ephemerides
- a set of parameters which describe the location of satellites with respect to time, and which are transmitted (broadcasted) from the satellites
- C/A code
- the GPS code which is freely available to civilian users, is referred to as the coarse acquisition code and is modulated on the L1 carrier at a frequency of 1.023 MHz
- carrier frequencies
- continuous electromagnetic radiation of constant amplitude and frequency emitted by a radio transmitter; GPS satellites emit two carrier frequencies: L1 at 1575.42 MHz and L2 at 1227.60 MHz, which are biphase modulated by code and satellite messages
- continuous kinematic
- type of GPS positioning where at least one GPS receiver continuously tracks GPS satellites while moving, with the objective of providing the position of the trajectory of the receiver (also referred to as pure kinematic)
- conventional static GPS positioning
- method of relative positioning whereby two or more GPS receivers, each set up over a station, collect data from the same satellites simultaneously, for at least one half hour and more commonly at least an hour; it is by using this type of relative positioning with carrier measurements that the greatest potential accuracies may be achieved with GPS positioning
- conventional terrestrial coordinate system
- an earth-centred coordinate system with the Z axis directed towards the north pole, the X axis passing through the plane which contains the Greenwich Meridian and the Y axis perpendicular to the X and Z axes to form a right handed system; can be used to describe the location of points on the earth or satellites in space
- cutoff angle
- the elevation angle below which GPS signals are not recorded due to an option set in the GPS receiver or GPS processing software (also referred to as mask angle)
- cycle slip
- an unknown jump in the number of carrier cycles resulting from failure to maintain continuous lock on a satellite
- datum
- a point, line, surface or set of quantities used as a reference upon which measurements are based
- differential positioning
- a type of relative positioning whereby measurements made at a known monitor point are used to correct measurements at an unknown rover point (occasionally in literature differential positioning is used interchangeably with the more generally defined relative positioning)
- dilution of precision (DOP)
- a numerical indicator of the geometric strength of the satellite constellation in relation to positioning at a certain location and time
- dual frequency receiver
- a receiver which is capable of tracking both L1 and L2 GPS carrier frequencies
- elevation angle
- the angle from the GPS receiver's antenna between the horizontal and the line of sight to the satellite
- ellipsoid
- a smooth mathematical surface which resembles a squashed sphere and is used to represent the earth's surface
- geodetic coordinate system
- an earth-centred coordinate system where latitude is the positive angle from the centre of the earth northwards from the equator and longitude is the positive angle from the centre of the earth eastwards from the Greenwich Meridian
- geoid
- the equipotential surface (i.e. the surface on which the gravity potential is constant) which best approximates mean sea level
- geoid model
- describes the pattern of geoid undulations over the earth's surface as a function of latitude and longitude
- geoid height
- the height difference between the geoid and ellipsoid at any given point on the earth's surface (also referred to as geoid undulation)
- geoid undulation
- the height difference between the geoid and ellipsoid at any given point on the earth's surface (also referred to as geoid height )
- geomagnetic storms
- occur when solar flares cause irregular ionization of the ionosphere which in turn causes irregular refraction delays of a radio wave passing through the non-homogeneous medium
- gross errors
- errors which result from some equipment malfunction or observer's mistake (also referred to as blunders)
- ground control segment
- the ground based system used to operate the satellites on a continual basis
- ionosphere
- the layer of free electrons ranging from about 50 to 1000 km above the earth
- ionospheric errors
- the delay resulting from GPS signals being transmitted through the ionosphere
- kinematic
- refers to the type of GPS positioning where a receiver is moving while data is being collected
- mask angle
- the elevation angle below which GPS signals are not recorded due to an option set in the GPS receiver or GPS processing software (also referred to as cutoff angle)
- measurement method
- method of differential positioning whereby the measurements at the monitor site are used to correct the measurements at the remote site (see position method)
- monitor
- refers to a stationary GPS receiver set up on a point of known coordinates, providing a basis from which measurements to the rover receiver may be referenced
- multipath
- refers to a reflected signal that combines with a true signal resulting in a weaker position determination
- observing session
- the time period during which simultaneous GPS observations are taken
- obstructions
- obstacles which block the line of sight between a GPS receiver's antenna and a satellite
- orbital error
- the difference between the satellite position as calculated using its ephemerides and its "true" position in space
- orthometric heights
- heights above the geoid which have traditionally been determined through levelling
- P code
- the precise GPS code which through anti-spoofing will be unavailable to civilian users; it is modulated on both the L1 and L2 carriers at a frequency of 10.23 MHz
- point positioning
- the determination of the coordinates of a receiver with respect to the earth's reference frame by intersection of the signals from four or more satellites (also referred to as absolute positioning or single point positioning)
- position method
- method of differential positioning whereby the measurements at the monitor site are used to compute a position, which is in turn used to correct the position computed at the remote site (see measurement method)
- post-mission processing
- data processing to compute positions which is carried out after the observing session is completed
- precise ephemeris
- a set of parameters which precisely describe the location of satellites with respect to time, which are based on computations of data collected from GPS tracking stations around the world
- precision
- the closeness of an estimate to its mean estimate
- probable error
- a magnitude of error which represents 50% uncertainty
- pure kinematic
- type of GPS positioning where at least one GPS receiver continuously tracks GPS satellites while moving, with the objective of providing the position of the trajectory of the receiver (also referred to as continuous kinematic)
- random errors
- the errors which remain if all gross and systematic errors are removed; random errors tend to be distributed about the mean following the normal probability distribution function
- rapid static
- a form of static GPS positioning which requires minutes instead of hours of observations due to special ambiguity resolution techniques which use extra information such as P code measurements or redundant satellites
- real-time processing
- positions computed as soon as data is collected
- receiver clock errors
- errors due to the inaccuracy of the receiver clock in measuring the signal reception time
- receiver noise
- a quantification of how well a GPS receiver can measure code or carrier observations
- relative accuracy
- the accuracy of a measurement between two points (i.e. the accuracy of one point measured relative to another)
- relative positioning
- the determination of the position of one point with respect to another point with known coordinates
- remote
- refers to a GPS receiver which either moves along a trajectory to be positioned (e.g. for kinematic positioning) or from point to point to be positioned (e.g. semi-kinematic or rapid static), and whose measurements are combined with those from a monitor receiver for relative positioning (also referred to as a rover)
- root mean square (rms)
- measure of the dispersion of observations about the mean
- rover
- refers to a GPS receiver which either moves along a trajectory to be positioned (e.g. for kinematic positioning) or from point to point to be positioned (e.g. semi-kinematic or rapid static), and whose measurements are combined with those from a monitor receiver for relative positioning (also referred to as a remote)
- satellite constellation
- the complete set of satellites and their configuration in space
- satellite message
- the package of information modulated on both the L1 and L2 carrier frequencies which includes among other information, the broadcast ephemerides and satellite health status
- selective availability (SA)
- a technique which is used to limit real-time accuracy achievable by civilian users and which consists of degradation of the broadcast orbit (i.e. the satellites' "known" position in space) and dithering of the satellite clocks
- semi-kinematic
- a relative positioning method in which the rover receiver remains stationary on each point for several seconds, and maintains lock on satellites while moving between points (also referred to as stop and go)
- site
- a term often used to refer to a point on the earth's surface at which GPS observations are collected (also referred to as station)
- single point positioning
- the determination of the coordinates of a receiver with respect to the earth's reference frame by intersection of the signals from four or more satellites (also referred to as absolute positioning or simply point positioning)
- standard deviation
- a measure of the dispersion of observations about the mean, sometimes limited to apply only to normal distributions, but more commonly used to refer to any distribution, in which case it is the same as root mean square
- static
- refers to the type of positioning where a GPS receiver is stationary while data is collected
- station
- a term often used to refer to a point on the earth's surface at which GPS observations are collected (also referred to as site)
- stop and go
- a relative positioning method in which the rover receiver remains stationary on each point for several seconds and maintains lock on satellites while moving between points (also referred to as semi-kinematic)
- systematic errors
- those errors which have some known pattern or behavior which biases the observations
- troposphere
- the layer of the atmosphere up to 80 km above the earth
- tropospheric errors
- the delay of GPS signals due to transmission through the troposphere
- user equivalent range error (UERE)
- the contribution of all errors to single point positioning accuracy
- user segment
- includes all those who use GPS tracking equipment to receive GPS signals to satisfy specific positioning requirements
- window
- the periods during the 24 hour day when there are a sufficient number of satellites in view to meet the positioning requirement
- Y code
- the encrypted GPS code which replaces the P code when anti-spoofing is implemented
