INSAR
Radar is an extremely useful tool for monitoring changes in the Earth’s surface. Interferometric synthetic aperture radar, or INSAR, can accurately measure tiny changes in the Earth’s surface.
Radar waves pass through Earth’s atmosphere and will bounce off land surfaces. Radar is a form of light - electromagnetic radiation - so like light it is a type of wave with a frequency/wavelength. When radar bounces from the surface to a spacecraft, it arrives back at a specific point in each wave. It's difficult to turn small changes in wavelength from pixel to pixel into any useful information since they’re a function of many properties of the surface, but if 2 images taken close together in time are subtracted from each other, tiny changes from one frame to another stand out. Those tiny changes in the wavelength of the returning energy are reflected in changes in the colors on an INSAR interferogram like this one. As the wavelength of the returning energy changes, it either builds constructively on the waves in the previous pass or interferes destructively.
INSAR is a tool that can show changes in the Earth’s surface from one scene to the next – changes like deformation in a volcano or motion from an earthquake. If 2 closely timed radar images are available, earthquake motions can be understood and volcanic eruption warnings can be given, but doing so requires a satellite built to do the measurement.
Previous satellites could do these measurements, but they weren’t built to do rapid overpasses. A few years ago, the European Space Agency launched its Sentinel-1A satellite (http://on.fb.me/1B1f6Jj), the first in its next-generation series of Earth-observing spacecraft. That satellite carries radar specifically designed for INSAR – it will be able to image every point on Earth’s surface every 12 days.
This INSAR image shows ground deformation in an earthquake in Oaxaca , Mexico last year. The ground moved by a maximum of about 40 centimeters in this earthquake, so each of the contours of color represents about 3 centimeters of ground motion. The points marked show estimates of the earthquake epicenter based on seismic measurements made by the USGS and Mexican seismic networks; clearly based on this image, the nearby seismic instruments from the Mexican seismic network produced a more precise estimate of the epicenter location.
-JBB
Image credit: EOS/ESA/NASA https://disasters.nasa.gov/oaxaca-mexico-earthquake-2018/insar-map-deformation-due-m72-oaxaca-earthquake https://eos.org/project-updates/earthquake-monitoring-gets-boost-new-satellite
