Interferometric Synthetic Aperture Radar (InSAR)

Radar (RAdio Detection and Ranging) measures the strength and round-trip time of the microwave signals that are emitted by a radar antenna and reflected off a distant surface or object. The radar antenna alternately transmits and receives pulses at particular microwave wavelengths (typically in the range 1cm to 1m, which corresponds to a frequency range of about 300MHz to 30GHz) and polarisations (waves polarised in a single vertical or horizontal plane).

In the case of satelliteborne radar sensors, they measure the amount of energy which returns to the satellite after it interacts with the earth's surface, which is presented in the form of an intensity image, but also includes the round-trip time between the sensor and the surface, which is represented as the phase image. Unlike optical sensors, the radar's microwave energy penetrates clouds, rain, dust, or haze, and acquires images regardless of the sun's illumination, enabling radar to collect data under almost all atmospheric conditions.

A synthetic aperture radar, or SAR, is a coherent radar system that generates high resolution remote sensing imagery. Signal processing uses magnitude and phase of the received signals over successive pulses from elements of a 'synthetic aperture' to create an image.

Interferometric SAR (InSAR or IfSAR) systems exploit the phase difference from two SAR images acquired over the identical scene, thereby useful geodetic information such as the digital elevation model (DEM) can be derived. Two sets of SAR image values are registered by using correlation techniques in the InSAR processing software to an accuracy of 1/8 of a pixel. The phase differences between the two images are then calculated to generate the interferogram. These phase differences in the interferogram wrap around in cycles of 2pi, and need to be 'unwrapped' in order to obtain absolute phase values. Finally, the absolute phase can be converted into height information.

Based on InSAR, a new technique called differential InSAR (DInSAR) allows for the measurement of small deformations of the terrain (down to the millimetre level) that have occurred between the two different image acquisitions. In DInSAR analysis, the topographic contribution to the phase difference is carefully removed using a DEM. Atmospheric disturbances can be either accounted for using independent observations such as GPS, or neglected if the tropospheric delay is considered to be homogeneous at the time of the radar image acquisition.