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IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India,2002 
GENERATION AND VALIDATION OF DEM USING 
SAR INTERFEROMETRIC TECHNIQUES AND DIFF ERENTIAL GPS 
P. Jayaprasad*, B. Narender, S.K. Pathan and Ajai 
Forestry, Land use and Photogrammetry Group, 
Space Applications Centre, ISRO, Ahmedabad. 380 015, India 
Jp_pallipad @ yahoo.com 
KEYWORDS: SAR Interferometry, Coherence, DEM, Differential GPS 
ABSTRACT: 
Synthetic Aperture Radar (SAR) interferometry or InSAR is a useful technique for topographic mapping from Space. It utilizes phase 
content of the radar signal derived from the complex radar data as additional information for extracting three-dimensional information of 
the surface. The objective of the present study was to generate DEM using InSAR techniques in a moderately undulating terrain and 
validate it using differential GPS measurements. DEM was generated using InSAR software developed at SAC. Differential GPS survey 
was carried out to establish GCP's for validating DEM. 
1. INTRODUCTION 
Synthetic Aperture Radar (SAR) interferometry or InSAR is a 
useful technique for topographic mapping from space. It utilizes 
phase content of the radar signal as an additional information 
source derived from the complex radar data for extracting three- 
dimensional information of the surface. SAR images record both 
intensity and the phase values of the backscattered signal. The 
intensity gives the nature of the surface and the phase values give 
the terrain elevation or topography of the surface. Complex 
images recorded by different antennas or at different times are 
combined to form interferograms. If the two antennas are within 
the scattered beam of a ground resolution cell, then due to 
coherent nature of the radar signals, the signals reflected from the 
same scatterer on the ground will interfere each other. The phase 
components of a pair of images interfere to produce a fringe 
pattern. This resulting phase difference image is known as 
interferogram. Since phase is a function of distance, the fringes 
represent the elevation pattern in an encoded form due to 
wrapping of phase from — zm to +r. Using one known scene 
elevation, phase unwrapping can be carried out and these phase 
values can be related to the terrain elevation using the SAR 
geometry. 
SAR interferometry offers advantages with respect to other 
techniques in having all weather capability and high accuracy 
especially for differential height measurements. Differential 
interferometry allows one to map elevation changes at centimeter 
level by means of three SAR images of the same area separated in 
time with adequate baseline. 
This study attempts to further explore the use of this technique for 
DEM estimation in a moderately undulating terrain. The 
validation of DEM was done using differential GPS 
measurements. 
2. InSAR THEORY 
A radar interferometer is formed by relating the signals from two 
spatially separated radar antennas,The separation of the two 
antennas is called the baseline. The two antennas may be mounted 
on a single platform or a synthetic interferometer may be realized 
by utilizing a single antenna on a satellite in a nearly exact 
repeating orbit. However, for repeat orbit case, temporal 
decorrelation constitutes an important source of error in 
topographic mapping. 
The geometry of interferometer is as shown in fig.1. Al and A2 
are two satellite positions separated by baseline B. a is the base 
line angle i.e. the angle of the baseline with respect to the 
horizontal and 0 the look angle. R and R+3 are the slant ranges, z 
the surface topography and H the satellite height. Radar signals 
transmitted and received from two antennae will, when properly 
resampled and cross multiplied, form an interferogram. For repeat 
pass imaging geometries, on each pass the radar acts as both a 
transmitter and receiver, therefore the total path difference for 
each radar observation to a given point on the surface is twice 
what would be expected if a single spacecraft or aircraft with two 
physical antennas were used. The phase difference at each 
location is proportional to the path difference 2 8 with a 
component of proportionality 2 z/ A. From the geometry the 
following relations can be derived. 
ô = 2/42 Q where 97 2z/4*26 (1) 
(R8) 2 R^ « B? - 2RB Sin (a - 0) (2) 
Sin (a — 0) = ((R - 8) - R? -B?y 2RB (3) 
z= h-RCos 0 (4) 
Using this equation topographic heights can be estimated using 
values of 0 and B. Knowing the height differences in X and Y 
directions we can estimate slope of the terrain. The phase errors 
and attitude errors are the two sources of error in height 
estimation. 
   
   
   
    
   
  
  
  
  
   
  
    
     
     
    
   
  
    
    
   
    
    
   
  
     
   
   
    
    
    
    
   
     
    
     
  
   
 
	        
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