have been successfully measured using satellite interferometry. The technique allows remote, high-resolution measurement of changes in glacial structure, ice flow, and shifts in ice dynamics, all of which agree closely with ground observations. Infrastructure and building monitoring InSAR can also be used to monitor the stability of built structures. Very high resolution SAR data (such as derived from the TerraSAR-X StripMap mode or COSMO-Skymed HIMAGE mode) are especially suitable for this task. InSAR is used for monitoring highway and railway settlements, dike stability, forensic engineering and many other uses. DEM generation Interferograms can be used to produce digital elevation maps (DEMs) using the stereoscopic effect caused by slight differences in observation position between the two images. When using two images produced by the same sensor with a separation in time, it must be assumed other phase contributions (for example from deformation or atmospheric effects) are minimal. In 1995 the two ERS satellites flew in tandem with a one-day separation for this purpose. A second approach is to use two antennas mounted some distance apart on the same platform, and acquire the images at the same time, which ensures no atmospheric or deformation signals are present. This approach was followed by NASA's SRTM mission aboard the Space Shuttle in 2000. InSAR-derived DEMs can be used for later two-pass deformation studies, or for use in other geophysical applications. See also Coherence (physics) Optical heterodyne detection Remote sensing ROI PAC Further reading B. Kampes, Radar Interferometry – Persistent Scatterer Technique, Kluwer Academic Publishers, Dordrecht, The Netherlands, 2006. References External links InSAR, a tool for measuring Earth's surface deformation Matthew E. Pritchard USGS InSAR factsheet InSAR Principles, ESA publication, TM19, February 2007. Geophysical survey Geodesy Synthetic aperture radar Interferometry