RADAR is the most commonly used active remote sensing system used (although the use of LiDAR is certainly increasing). As mentioned in the history of remote sensing lecture, RADAR became an operational technology in the late 1930's, early 1940's, when it proved to be an important sensing system during the Battle of Britain. Since then its uses have expanded considerably, and RADAR is now widely used in weather forecasting systems, for sea ice mapping, for monitoring changes related to earthquakes, and for mapping areas that cannot be imaged using passive systems.

Processing RADAR requires specialized software and specialized knowledge (that is, to properly interpret RADAR requires knowledge of physics). None-the-less, every remote sensing student should have some basic knowledge of RADAR since you are likely to encounter images made from RADAR data.

• NRCan's RADAR tutorial (and here as well); related to The Canadian Space Agency's RADARSAT.
• Links to another RADAR tutorial, and another one (European Space Agency) [ESA's glossary of RADAR terms].
• A detailed discussion on Europe's (ESA / ENVISAT) Advanced Synthetic Aperture RADAR (ASAR); the FAQ presented in Chapter 4 is particularly relevant.
• A page create by the Japanese RADAR group analysing the earth movements associated with earthquake in 2011.
• A discussion of how RADAR can be used to detect the movements associated with earthquakes. (The Wiki page on SAR describes the interferometry process; here is a definition of the term.)
• The Shuttle RADAR Topography Mission (SRTM) -- a DEM for the whole earth. A poster that provides an overview of the process [note: a very large TIF].
• The Distance Early Warning (DEW) line.
• RADAR 'discovered' ancient river beds in the Sahara.
• A tutorial on RADAR meteorology.
• A discussion of ground-penetrating RADAR.
• A recent article on the use of RADAR to map forests around the world.

Text: Chapter 10.2: RADAR

I was asked a very interesting question--if radar is touted as an all-weather remote sensing technology, how is it that weather forecasters use radar to highlight clouds and precipitation levels? (That is, if the radar used by, for example, RADARSAT, is able to 'see' through clouds, how is it that weather radars can discern clouds, etc.?)

The answer can be found on these two pages:

A technical description of RADARSAT, and a technical description of CloudSat.

In particular, note that for RADARSAT the frequency is listed as 5.3GHz/C-band 5.6 cm, while for CloudSat it is listed as 94 GHz (1 GHz = 109 Hz). From the figure below we can see that 94 GHz would produce a shorter wavelength, one that would be reflected by clouds, while the longer RADARSAR wavelengths would pass through them.