RAdio Detection and Ranging (RADAR) according to the Satellite Navigation and Position Group (2004) is the measuring of microwave signals’ strength and round-trip time, which are released from a radar antenna and echoed off a far-off object or surface. Ibid (2004) comments that microwave wavelengths of 1cm to 1m in range (frequency corresponding to 300MHz to 30GHz) and polarizations (which are waves that are polarized singularly in a horizontal or vertical plane) are alternately transmitted and received in pulses by the radar antenna.
Ibid (2004) maintains that satellite operating radar sensors measure the quantity of energy that arrives back to the satellite following its contact with the earth's surface, (represented as an intensity image), as well as also containing ‘the round-trip time between the sensor and the surface’ (Satellite Navigation and Position Group, 2004) ( represented as the phase image). Ibid (2004) states that as opposed to optical sensors, the microwave energy of radar can penetrate cloud cover, rain, dust and haze, acquiring images independent of the sun's presence, allowing radar to obtain data through a broad range of atmospheric conditions.
Synthetic Aperture Radar (SAR)
The Canada Centre for Remote Sensing (2002) states that Synthetic Aperture Radar (SAR) produces high resolution remote sensing imagery through a coherent radar system.
An image is created through processing the signal with reference to the magnitude and phase of the returned ‘signals over successive pulses from elements of a ‘synthetic aperture’ ’ (Satellite Navigation and Position Group, 2004).
The effect of increasing the length of the antenna occurs through a synthetic aperture being created by signal processing, such that ‘the line of sight direction changes along the radar platform trajectory’ (European Space Agency, 2002). Ibid (2002) remarks that the attainable azimuth resolution of a SAR does not rely on platform altitude (distance), and is roughly equivalent to one-half the length of the actual (real) antenna. Ibid (2002) comments that p ulse compression techniques are made possible through high range resolution. Ibid (2002) states that the ground surface is mapped as the radar beam is positioned to the trajectory of the platform’s side, while in addition possessing an adequately wide antenna beam width in the along-track direction. In this way, an area or identical object/target can be observed multiple times without altering the antenna look angle.Interferometric SAR (InSAR)
According to Chang H.C. et al. (2003)The principle of InSAR is that using a repeat-pass, the satellite obtains two SAR images of the same region and registers them together. Ge L. (2001) states an interferogram can then be created from the phase difference between the two SAR images of the same area. Qin L. (2003) remarks the images are analysed to construct Digital Elevation Models (DEMs) and topographic information
InSAR provides 25m spatial resolution. InSAR has been claimed to be able to monitor any point on the earth’s surface without the assistance of any ground based receivers or targets. While this is true, InSAR easily becomes inaccurate due to a number of errors that cannot be eliminated using only results from SAR. These can be minimised or eliminated through coupling InSAR with ground survey methods, in particular GPS.Differential InSAR (DInSAR)
Differential Interferometric Synthetic Aperture Radar (DInSAR) is currently being tested for a number of applications, including deformation due to underground mining.
DInSAR takes two InSAR images and processes them so that ground deformations can be identified.