COMET Volcanic and Magmatic Deformation Portal

ALOS	Advanced Land Observing Satellite (Japan).
ASAR	Advanced Synthetic-aperture Radar (Envisat, European Space Agency).
Azimuth	Distance measured from the early azimuth line to a particular point in the footprint of an imaging radar, in the direction parallel to the flight path of the radar.
Baseline	In GPS, a vector of coordinate differences between two points or an expression of the coordinates of one point with respect to the other. In common usage, the term often refers to the scalar distance between two points rather than to the vector. In InSAR, baseline refers to the perpendicular separation between satellite positions from two image pairs.
Bench mark	A relatively permanent, natural or artificial geodetic marker fixed to the Earth and bearing a marked point whose elevation above or below an adopted surface (datum) is known.
Caldera	A large, roughly circular volcanic depression caused by the collapse of surface rock over an underlying magma reservoir.
CGPS	Continuous GPS.
Coherence	Aspect of an InSAR interferogram related to the amount of change in the position or radar-reflective property of scatterers within corresponding pixels in the two SAR images used to create the interferogram. To measure surface displacement using InSAR the deforming area must be mostly coherent, otherwise phase-difference information is lost and the area is termed incoherent. Factors that lead to incoherence include snow cover, farming, and change in vegetation cover.
Constellation	In GPS context, the group of NAVSTAR satellites that form the heart of the US GPS system. Typically, there are around 21 active satellites and 3 orbiting spares ensuring that at least 4 satellites are above the horizon at all times.
Corner reflector	An optical reflector designed such that light incident on its surface is redirected back to its source on a path parallel to the incidence angle.
COSMO-SkyMed	An Italian constellation of four Earth observation SAR satellites for dual (civil and military) use.
CSA	Canadian Space Agency.
CVO	Cascades Volcano Observatory.
Decorrelation	In InSAR, a term used to describe the situation in which two radar images of the same area cannot be registered on a pixel by pixel basis.
Deformation	In geodesy, any change in the shape or dimension of the Earth’s surface.
DEM	Digital Elevation Model.
Dilation	Deformation by a change in volume but not shape.
DLR	Deutsches Zentrum fur Luft- und Raumfahrt (German Aeorospace Center).
Dry tilt	Equivalent to single-setup leveling. A term coined by the USGS at HVO to distinguish this differential leveling technique from the wet tilt method.
DTM	Digital Terrain Model.
EDM	Electro-optical distance meter, electronic distance meter, or electronic distance measurement.
Envisat	An inoperative ESA Earth-observing satellite still in orbit.
ERS	An ESA remote-sensing satellite (ERS-1 and ERS-2).
ESA	European Space Agency.
Fringe	In InSAR, the colour band from red-blue-green-yellow corresponding to centimeters of topographic relief change.
Galileo	Global Navigation Satellite System.
GEONET	GPS Earth Observation Network (continuous GPS network, Japan).
GIS	Acronym for geographic information system, a computer system capable of assembling, storing, manipulating and displaying geographically referenced information.
GLONASS	Global Navigation Satellite System (Russia).
GNSS	Global Navigation Satellite System.
GPS	Global Positioning System, specifically the US NAVSTAR GPS Global Navigation System.
GPS receiver	An electronic device that receives and processes signals from NAVSTAR or other GNSS satellites to calculate positioning information directly, or alternatively, to produce a dataset that can be further processed to obtain such information.
GVN	Global Volcanism Network.
IAVCEI	International Association of Volcanology and Chemistry of the Earth’s Interior.
IGS	International GPS Service.
InSAR	Interferometric Synthetic Aperture Radar. A satellite remote-sensing technique that measures ground deformation using the phase difference between two time-separated SAR images
Interferometry (radar)	A technique that uses the measured phase difference of a return radar signal between two satellite passes to detect ground surface movement. Two SAR images of the same target area on the Earth’s surface are taken at different times, and are flattened, co-registered, and differenced to produce an interferogram. Fringes represent the measured phase differences.
Interferogram (radar)	An image produced by differencing two radar-phase images of the same target area. An interferogram can be used to produce a DEM model or to measure surface deformation.
JAXA	Japan Aerospace Exploration Agency.
JERS	Japanese Earth Resources Satellite.
Kinematic (GPS)	A form of GPS surveying in which the position of a moving receiver is tracked by means of uninterrupted carrier-phase measurements following successful solution of the integer ambiguities.
Leveling	A surveying technique for determining relative height differences between fixed points on the Earth’s surface (bench marks) by sighting through a leveling instrument to one or more graduated rods.
Lidar	Light Detection and Ranging.
MSAS	MTSAT Satellite-based Augmentation System for GPS (Japan).
NASA	National Aeronautics and Space Administration.
NAVSTAR	Navigation Satellite Time and Ranging (US GPS satellite).
Network	In geodesy, a group of surveying stations that have been interconnected through measurements of their relative positions in such a way that the self-consistency of the measurements can be checked and adjusted.
PALSAR	Phased Array L-band Synthetic-aperture Radar (onboard JAXA’s ALOS satellite).
Phase unwrapping (InSAR)	A procedure for solving the 2-pi ambiguity inherent in radar interferograms, using one of several techniques to calculate the correct integer number of phase cycles to be added to each phase measurement. An unwrapped, flattened interferogram has a smooth range of pixel values corresponding to surface elevations.
Pixel	Short for ‘picture element’, the smallest part of a digital image. The number of pixels (width and height) in an image defines its size, and the number of pixels per inch defines its resolution.
RADAR	Radio detection and ranging.
RADARSAT	Commercial Canadian SAR satellites (Radarsat-1 and Radarsat-2).
SAR	Synthetic-Aperture Radar.
Sentinel	A new family of ESA satellites, called Sentinels, specifically for the operational needs of the Copernicus programme.
SRTM	Shuttle Radar Topography Mission.
Static GPS	A form of GPS surveying in which the position of a fixed point is determined by means of carrier-phase measurements. The integer ambiguities are resolved from an extended observation period through a change in satellite geometry.
Strainmeter	An instrument designed to measure changes in the shape or volume of a body as the result of stress.
Tiltmeter	An instrument that measures slight changes in the tilt of the ground surface, usually in relation to a liquid-level surface or the rest position of a pendulum.
TerraSAR-x	A commercial German synthetic aperture radar (SAR) Earth observation satellite.
Total station	A surveying instrument that combines the functions of an EDM and theodolite and thus is capable of measuring both distance and azimuth between fixed points.
USGS	United States Geological Survey.
VEI	Volcanic Explosivity Index.