ALOS              Advanced Land Observing Satellite (Japan).

Array             An ordered arrangement of monitoring instruments or measurement stations.

ASAR              Advanced Synthetic-aperture Radar (Envisat, European Space Agency).

AVO                Alaska Volcano Observatory.

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.

Backsight      In leveling, a reading on a leveling rod held in its unchanged position on a survey point of previously determined elevation, known as a turning point, after the leveling instrument has been moved to a new position.

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.

Bradyseism  An episode of gradual surface uplift or subsidence, usually several kilometers to a few tens of kilometers across, caused by inflation or deflation of a magma reservoir or by pressurisation or depressurisation of a hydrothermal system.

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.

Décollement A detachment structure above and below which the style of deformation in the rocks are independent.

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.

Deflation        In volcanology, a term used to describe shrinking of the ground surface, a volcanic edifice, or a sub-surface magma body. Deflation is a response to depressurisation, which can have several causes.

Deformation In geodesy, any change in the shape or dimension of the Earth’s surface.

DEM               Digital Elevation Model.

Dilatometer   Synonym for volumetric strainmeter.

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.

Footprint        For imaging radar systems, the area on the ground illuminated by the radar beam.

Foresight       A reading taken on a level rod to determine the elevation of the point on which the rod rests.

Fringe            In InSAR, the colour band from red-blue-green-yellow corresponding to centimeters of topographic relief change.

Galileo           Global Navigation Satellite System.

Geodetic moment   A measure of the energy associated with rock deformation, such as slip.

Geodimeter  Acronym for geodetic distance meter and a trade name for a specific brand.

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.

HVO                Hawaiian Volcano Observatory.

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.

Leveling, compensator      Leveling carried out with a leveling instrument having a compensator, or self-leveling device, for making line-of-sight horizontal.

Leveling, differential            Technique for determining the difference in elevation between two points by the incremental vertical displacements of a graduated rod. The two usual methods of differential are spirit leveling and compensator leveling, which differ in the way the instrument determines the horizontal line of sight.

Leveling, spirit                 Leveling with an instrument that depends on a spirit level for making its line-of-sight horizontal.

Lidar               Light Detection and Ranging.

LVO                Long Valley Observatory.

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.

SBAS              Satellite-Based Augmentation System (regional civilian GNSS augmentation system).

SBAS              Small baseline subset time-series of ground deformation.

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.

UNAVACO     Non-profit university-governed consortium that facilitates geoscience research and education using Geodesy.

USGS             United States Geological Survey.

VEI                  Volcanic Explosivity Index.

YVO                Yellowstone Volcano Observatory.