How can I process the multi temporal Sentinel-1 image?

Hi!

I want to compare the multi temporal Sentinel-1 image. I am using the Level-1 GRD sentinel-1.
At the same thing (a house, unchange by time), there are difference dB value in the multi temporal image. The standard deviation is very high in the difference time, for example 3dB.

How can I process to calibrate the multi temporal image?

You will need to calibrate each image first before coregistering. The calibration vectors in the products refer to specific locations in the that image. After coregistration the images have been mapped to the master images coordinate space thereby loosing the ability to calibrate it.