REMOTE SENSING/Active Sensors 415 Radar Techniques Altimetry Radar altimeters are the simplest form of radar They are usually simple ranging devices recording the distance from the radar to the ground, pointing vertically downwards from a satellite Their footprint on the Earth’s surface is large (kilometre scale) and as a result their profiling measurements are only easily interpretable on surfaces that are relatively flat at this length scale (e.g., oceans and ice sheets) Their importance to geology has been that they have provided a measure of the shape of the geoid in the ocean basins that reflects the dynamic processes of plate tectonics (see Europe: Mediterranean Tectonics): subduction and spreading ridges Doppler Radar In an active remote sensing device, the engineer can control the rate of pulsing of signals transmitted to the surface and the range of frequencies of radiation transmitted and received This means that the measurements can remain ‘coherent’ from one measurement time to another, provided that the external conditions remain the same Such a coherent radar signal has three measurable quantities: timing, magnitude, and phase Any relative movement between the radar antenna and the reflecting surface from one pulse to another can be detected as a change in the frequency of return of these pulses – a Doppler shift The size of this shift allows the relative motion to be calculated Doppler radars of this type have been designed to measure the dynamics of volcanic ash columns Lower frequencies of radar signal are more sensitive to larger fragments Using multiple frequency bands in the same instrument, it is possible to measure the relative motions of different particle size fractions Because many ash eruption columns are usually obscured by fine turbulent cloud, Doppler radars offer the only practicable way of seeing through this and measuring the internal dynamics of the column Imaging Radar Figure Schematic of the typical operating geometries of a spaceborne SAR, an airborne lidar and a multi swath sonar The majority of Earth science applications of radar employ imaging techniques Table summarizes the characteristics of the main space-borne radar imaging systems to date and those planned in the next decade In addition there are a number of airborne systems and ground-based systems Future development of more compact systems and the exploitation Table Selected orbital SAR systems Seasat ERS 1/ JERS Radarsat Envisat Alos Palsar Terrasar X EVINSAR a Operation 1978 (3 months) 1991 2000 1995 1992 1998 1995 2002 2004 2007 2009 Country Frequency band Repeat period (days) Incidence angle Swath width (km) Resolution (m) USA L 17, Europe C 35 (1, 3, 176) Japan L 44 Canada C 24 Europe C 35 Japan L 44 Germany X 11 Europe L 11 23 23 26 41 20 60 20 50 60 20 55 30 46 100 100 85 10 500 100 485 70 350 10 100 250 25 28 18 10 100 25 10 100 16 45 a Proposed InSAR mission dedicated to ground motion measurement