—- '2> 2 
e 
  
— M NM RP o0 SS MD Ar A M DD MA LU 
"—- -—- 
  
  
fig. 18: Partially unwrapped phase interferogram 
image 
both informations. Actually we decided to resample 
the SPOT DEM into the slant range geometry. 
Using the coarse orbitographic informations for the 
reference scene, we resampled the 40 by 40 
meters SPOT DEM grid into the equivalent of a 50 
by 50 meters grid in slant range geometry as 
shown figure 20. 
  
fig. 19: SPOT DEM with interferometric data 
coverage framed. 
  
fig. 20: SPOT DEM resampled into the radar 
geometry 
The resampled SPOT DEM can then be compared 
with the unwrapped interferogram. Because of the 
crossed orbit the relative altimetry of the 
unwrapped phases at the bottom of the scene is 
very poor. Figure 21 shows a small portion of the 
area for which the interferometric process works 
normally (reasonable baseline). Visually, on this 
small area, interferometric altimetry information 
gives nice details that are smoothed on SPOT 
DEM. Although SPOT DEM generation is not a 
stable process and can still be improved, the 
interferometric result compares quite favorably with 
it. It is reassuring to notice that interferometry 
gives altimetric information under the clouds! 
Further quantitative evaluation such as local noise 
and mean quadratic error over the scene can be 
achieved once we compute the absolute altimetry 
informations from the interferometric process. 
  
fig. 21: SPOT DEM on top (14 meters (XS) mean 
quadratic error), and unwrapped phases at the 
bottom for a small area 
4. INTERACTION 
After simulation and evaluation let us present some 
experiments to demonstrate that SPOT DEM or 
coarse elevation model can help in the unwrapping 
process. The method followed is simple 
interferogram subtraction. The resulting 
interferogram should be easier to unwrap and is 
related to the altimetry differences between the two 
initial products. 
4.1 Approximated DEMs 
A first experiment shows how approximated DEMs 
can be used in this unwrapping process. 
Coarse altimetric informations are generated by 
simply smoothing the initial SPOT DEM. Then 
interferograms are simulated with the same 
geometry and parameters (ERS-1 parameters) for 
each DEM. Figure 22 shows the simulated 
interferogram with a 100 meters interferometric 
baseline for the initial SPOT DEM. With a very low