over the same area two times while through the differences 
between these observations, height can be extracted. In three-pass 
interferometry (or DInSAR) the obtained interferogram of a 
double-pass InSAR for the commonly tandem image pairs is 
subtracted from the third image with wider temporal baseline 
respective to the two other images (Figure 1). 
master image 
slave image 
     
Ln réccnsences 
i coregistered 
slave image 
  
interferogram coherence 
  
  
euhanced 
interferogram 
  
   
unwrapped phase 
digital 
I' displacement 
model 
Figure 1: DInSAR Method flowchart (image source: Author) 
In single-pass InSAR, space-craft has two SAR instrument aboard 
which acquire data for the same area from different view angles at 
the same time. With single-pass, the third dimension can be 
extracted and the phase difference between the first and second 
radar imaging instruments give the height value of the point of 
interest using mathematical method. (Henderson, et al. 1998) 
SRTM (Shuttle Radar Topography Mission) used the single-pass 
interferometry technique in C- and X-band. Earth’s height model 
generated by InSAR-SRTM with 90-m horizontal resolution is 
available while the DEM with 4-to-4.5-m relative accuracy is also 
available for restricted areas around the globe. (Tarikhi, 2009) 
When using InSAR technique the resulted interferogram expresses 
surface deformation in the form of color changes which represents 
the whole range of the phase from 0 to 27 in a full color cycle. The 
slight deformation of the surface causes the change of color 
accordingly; therefore, the steep deformation of the surface 
corresponds to the steep change of color. In an interferogram 
moving from red to violet in the pattern indicates that the surface 
  
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
has moved closer to the satellite (uplift). Change of color bands in 
the reverse order, indicates that the surface has moved farther to 
the satellite. Each cycle of colors represents a change in the ground 
height in the direction of platform that depends on satellite 
geometry. For instance one cycle in the interferogram generated by 
the SAR sensors of the Envisat, ERS and Radarsat which use the 
C-band wavelength of 5.66cm corresponds to the half of the 
wavelength that is 2.83cm. 
      
   
    
  
descending orbit kA 
west looking 
    
common crea of 
ihe successive 
images’ coverage 
image seuscce: Parvia Terikhi 
  
+. Da 
ascending orbit 
east looking 
Figure 2: Concept of ascending and descending orbits 
In practice SAR satellite observes obliquely downward (off nadir) 
and not directly below (nadir). Therefore, when it 1s in ascending 
orbit (northward) the satellite observes from west and in 
descending orbit (southward) the satellite observes from east 
(Figure 2). In case of the westward movement of the surface, if 
satellite observes in ascending orbit (northward), the surface 
moves near to the satellite. Contrarily, if satellite observes in 
descending orbit (southward), the ground moves far from the 
satellite. That is why the ascending and descending SAR images of 
the same area looks different. 
3. COMPARISON; WHY INSAR IS PREFERRED 
InSAR ability to generate topographic and displacement maps has 
been proven in wide range of applications such as earthquakes, 
mining, landslide, volcanoes. Although other facilities like GPS, 
total stations, laser altimeters are also used, comparison between 
InSAR and these tools reveals its reliability. Laser altimeters can 
generate high resolution DEM and low resolution displacement 
maps in contrary to InSAR with the spatial resolution of 25m. 
However, most laser altimeters record narrow swaths. Therefore, 
for constructing a DEM by laser altimetry, more overlapping 
images are required. Displacement map precision obtained by 
terrestrial surveying using GPS and total stations is similar or 
better than InSAR. GPS generally provides better estimation of 
horizontal displacement and with permanent benchmarks slow 
deformations is monitored for years without being concerned about 
surface de-correlation. The most important advantage of InSAR 
over GPS and total stations are wide continuous coverage with no 
or less need for fieldwork. Therefore, wide and continuous 
coverage, high precision, cost effectiveness and feasibility of 
recording data in all weather conditions are its main privileges. 
However, it is important to note that the displacement estimated by 
InSAR is in the line-of-the-sight direction and to decompose this 
vector to parallel and normal components the terrestrial data or 
extra interferograms with different imaging geometry are required. 
Data availability and atmospheric effects limit using InSAR, 
however processing of its data is challenging. For each selected 
    
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