1.1 Theoretical background

IBIS-L , a ground based interferometric radar with synthetic aperture (InSAR), is a remote sensing tool, used to detect surface displacements (Bernadini et al., 2007 [4]). It is all-weather suitable and works to a distance up to 4 km. The main advantage compared to spaceborne InSAR systems is its short repeat cycle (several minutes) resulting in a much higher coherence.

The radar sensor emits microwaves (Ku-band: 17.2 GHz, bandwith 200 MHz) and records amplitude and phase of the backscattered signal. The resolution in range (line-of-sight) is 0.75 m. The synthetic aperture is generated by moving the sensor on a 2 m long rail. In this way a resolution in cross-range of 4.5 mrad is obtained (i.e. 4.5 m resolution in 1 km distance to the sensor).

The difference of two phase images yields an interferogram. The so obtained phase difference
    
of one pixel is always wrapped in the range of -π bis +π.The observed interferometric phase is a function of line-of-sight surface displacement d_los, atmospheric disturbance, noise and the phase ambiguity n
    
with
    
whereas λ is the wavelength.

To obtain the surface displacement from the observed phase all other effects have to be removed. The atmospheric phase can either be removed by observing atmospheric parameters (i.e. humidity, temperature, pressure) or by estimating it using stable targets (control points) within the observed scene (Roedelsperger et al., 2009 [2]). The estimation of atmospheric phase can be done at the wrapped interferogram. Thus, if it is assumed that the displacement between two consecutive images is within the unambiguous range of -λ to +λ, the displacement can be directly determined from the corrected interferogram.

The accuracy of the obtained displacement depends highly on the material of the surface, e.g. rock is a good reflector while vegetation is a poor reflector and also constantly moving due to wind. To estimate the phase accuracy, the coherence can be computed. The coherence ranges between 0 and 1, whereas 1 is perfect coherence (i.e. high accuracy).

The obtained displacements are still given in a local coordinate system and have to be georeferenced (i.e. transformed into a global reference system). To do this, the displacement maps have to be projected onto a Digital Elevation Model (DEM). IBIS-L is also capable of generating a DEM by introducing a spatial baseline between to acquisitions. Then the observed interferometric phase is also dependent on topography (Roedelsperger et al., 2010 [3]).