Dear Reda,

U have to first process the data and check of any sinks or garbage values or no data pixels. then re-sample your data and then check the elevation of same point  on all the DEMS . you error will be reduced

also their is the difference in the elevation if you compare different DEMS a particular DEM can give you best results at some place but there can be large errors at some other places it depends upon the surface roughness in case of SAR data and also the view angle and other sensor properties.

try to validate the results with GPS i.e filed data and check which DEM is showing close relation to your field data. 

Best of Luck 

Dear Mohammd ,

Thanks for your explanation

The elevations of SAR DEM are relative!

How could they be absolute? What softwares to be used?

Best Regards

I think the best option for you is drone survey then check for the best drone and the best thing is that you can use it at other places also