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Feature of each frequency;  Advantage of L-band

The common wavelengths used in the InSAR technique are X-band (~3 cm), C-band (~6 cm), or L-band (~24 cm).

The shorter the wavelength is, the more precise the capability of detecting the ground movement is. However, the difference of the wavelengths greatly influences the interference property of the ground surface. This is because the radio wave with shorter wavelength than C-band hardly penetrates vegetations, e.g., leaves and branches of trees.

Therefore observing a forest area by C- or X-band radar, the radio wave is reflected by leaves at the upper part of trees and cannot reach to the ground. Coherent signals cannot be observed at vegetated areas because leaves swing in the wind or grow up, whereas the signals in urban areas are coherent.

On the other hand, a L-band radar, which has a long wavelength, can penetrate the vegetation. Most of trees are transmissive for L-band. This means that the L-band radar can receive a reflected wave from the ground and be coherent even in a forest area.

A Baseline, a distance of a SAR antenna at two observations, affects the coherence; the long baseline leads to low coherence. Moreover, in a rugged mountainous area the coherence tends to be low. L-band, however, can achieve sufficient coherence even if the baseline is not very short.

Thus the L-band radar have a big advantage to keep a coherence in a vegetated or rugged mountainous area, with which Japan is widely covered, whereas the C- or X-band radar can obtain useful InSAR results only in urban areas. L-band is the optimal wavelength for the land of Japan.

JERS-1, which was launched by NASDA, carried L-band SAR (wavelength of 23.5cm). Since JERS-1 was terminated in October 1998, however, there had been only C-band satellites in operation until ALOS was launched. After ALOS stopped its operation in May 2011, again we could use only C- and X-band SAR data. Now ALOS-2, launched in May, 2014, carries L-band SAR and we can get L-band SAR data.

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