| 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 |
