Geoscience Reference
In-Depth Information
Galvanic Contact Resistivity Methods Resistivity geophysical methods that employ electrodes to
directly apply electric current into the subsurface.
Geographic Coordinate System A spatial reference system using latitude and longitude to describe
locations on the Earth's surface.
Geographic Information System (GIS) A computer-based information management system capable
of storing, editing, integrating, analyzing, and displaying spatially referenced data.
GIS: Layer A group of geographic features with thematic similarities having the same geographic
extent, coordinate system, and attributes.
GIS: Raster Data A data format in which attribute values are referenced to individual cells within
a spatially defined grid.
GIS: Vector Data A data format in which features and associated attributes are spatially refer-
enced to points, lines, or areas enclosed by polygons.
Geophone A device for recording artificially generated seismic waves, accomplished by convert-
ing ground vibrations into voltage measurements.
Geophysics There are several deinitions for geophysics, but in the context of this topic, geophysics
is defined as the application of physical quantity measurement techniques to provide in situ
information on conditions or features beneath the Earth's surface.
Global Positioning System (GPS) A global satellite navigation system employing twenty-four sat-
ellites in medium Earth orbit, which transmit precise microwave signals, thereby enabling
signal receivers to be accurately located.
GPS: 2D Operating Mode A two-dimensional GPS position fix that includes only horizontal
coordinates (no GPS elevation). It requires a minimum of three visible satellites.
GPS: 3D Operating Mode A three-dimensional GPS position fix that includes horizontal coordi-
nates, plus elevation. It requires a minimum of four visible satellites.
GPS: Accuracy A measure of how close an estimate of a GPS position is to the true location (how
close a fix comes to the actual position).
GPS: Almanac A set of parameters included in the GPS satellite navigation message that a receiver
uses to predict the approximate location of a satellite. The almanac contains information
about all of the satellites in the constellation.
GPS: Ambiguity The initial bias in a carrier-phase observation of an arbitrary number of cycles.
The initial phase measurement made when a GPS receiver first locks onto a GPS signal is
ambiguous by an integer number of cycles because the receiver has no way of knowing the
exact number of carrier wave cycles between the satellite and the receiver. This ambiguity,
which remains constant as long as the receiver remains locked on the signal, is established
when the carrier-phase data are processed.
GPS: Anti-Spoofing Encryption of the P-code to protect the P-signals from being “spoofed”
through the transmission of false GPS signals by an adversary.
GPS: Carrier Frequency A radio wave that conveys or carries some kind of modulation. The
frequency of an unmodulated output of a radio transmitter. The GPS L1 carrier frequency is
1575.42 MHz and L2 carrier frequency is 1227.6 MHz.
GPS: Carrier Phase The accumulated phase of either the L1 or L2 carrier of a GPS signal, mea-
sured by a GPS receiver since locking onto the signal (also called integrated Doppler).
GPS: Coarse/Acquisition Code (C/A Code) The standard positioning signal the GPS satellite
transmits to the civilian user. It contains the information the GPS receiver uses to fix its
position and time, and is accurate to 100 meters or better.
GPS: Coordinates A set of numbers that describes your location on or above the Earth. Coordi-
nates are typically based on latitude and longitude lines of reference or a global or regional
grid projection.
GPS: Cycle Slip A discontinuity in GPS carrier-phase observations, usually of an integer number
of cycles, caused by temporary signal loss. If a GPS receiver loses a signal temporarily, due
to obstructions for example, when the signal is reacquired there may be a jump in the integer
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