ROCK MECHANICS/Passive Sensors 439 Figure Conical scanning mechanism Passive Microwave Sensors Thermal radiation from natural surfaces, such as the land surface, extends from its peak in the thermal infrared region into the microwave region An Earth observation microwave imagining radiometer operates in this spectral region to receive the microwave radiation from the Earth As a passive sensor system, it is important to understand that a microwave radiometer is fundamentally different from a radar sensor, that is a ranging system The only resemblance between the two is that they both operate in the microwave spectral range A passive microwave sensor system, a microwave imaging radiometer, works more like a thermal sensor system It collects emitted energy radiated from the Earth in the microwave spectral range that provides useful information relating to surface temperature, topography, and material dielectric properties This type of sensor has been used for global temperature mapping, polar ice mapping, and regional soil moisture monitoring A space-borne microwave imaging radiometer is often a multichannel scanner, such as the SMMR (Scanning Multichannel Microwave Radiometer), which was on board of Seasat and Nimbus in 1978 and the Microwave Imager on board the Tropical Rainfall Measuring Mission (TRMM) satellite in 1997 It is composed of an antenna together with its scanning mechanism, a receiver, and a data handling system The received emitted microwave signals are highly related to the observation angle and the RIFT VALLEYS See AFRICA: Rift Valley path length in the atmosphere Ensuring that these scanning parameters are constant, thus they can significantly increase the accuracy of the derivation of surface parameters from the microwave brightness temperature A conical scan configuration is, therefore, popular for passive microwave scanners As shown in Figure 6, the antenna observation direction is offset at a fixed angle from a nadir rotating scan around the vertical (nadir) axis and thus sweeps the surface of a cone If the scan is configured for a full 360 , double coverage fore and aft of the spacecraft is obtained With the moving forward of a satellite along its orbit, a belt of land surface is imaged Obviously, in a conical scanning geometry, the observation angle and distance to any scanned position are constants Space-borne passive microwave scanners are usually of low spatial resolution from several kilometres to several tens of kilometres, because of weak signals in the microwave spectral range See Also Geological Field Mapping Remote Sensing: Active Sensors Further Reading Drury SA (2001) Image Interpretation in Geology, 3rd edn Chelternam: Blackwell Science Elachi C (1987) Introduction to the Physics and Techniques of Remote Sensing New York: John Wiley & Sons Lilesand TM and Kiefer RW (2000) Remote Sensing and Image Processing, 4th edn New York: John Wiley & Sons Colwell RN (ed.) (1983) Manual of remote sensing, Vol.1, 2nd edn Theory, instruments and techniques Falls Church, Va: American Society of Photogrammetry Robinson N (1966) Solar Radiation Amsterdam, London and New York: Elsevier Sabins FF (1996) Remote Sensing: Principles and Interpret ation, 3rd edn Basingstoke: WH Freeman ASTER user Handbook v.2, on line at http://asterweb.jpl nasa.gov/ Landsat handbook, on line at http://landsat.gsfc.nasa gov/ Remote sensing learning sources of Canada Centre for Remote Sensing, on line at http://www.ccrs.nrcan.gc.ca/ ccrs/learn/learn e.html The SPOT Payload, on line at http://www.spotimage fr/automne modules files/standard/public/p229 file LINKEDFILE Spot payload.pdf