Terrain correction and coregistration

Hello everyone,
Which order is better for pre-processing two sentinel-1 images in the same area at different times: applying Terrain correction first and then apply Coregistration, or vice versa?

depends on if your data is from the same track or from different tracks

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Thanks. It is from the same track, so which order should I apply? and why?

then you calibrate them to Sigma0, then apply Coregistration and terrain correct the stack in the end.

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Thank you so much. But can you please tell me why we don’t apply Terrain correction before coregistration in this situation?

because coregistration provides a more accurate alignment of the single images (at sub-pixel level) and you potentially lose some of this information when the data is resampled during the terrain correction.

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Pardon me for a further question: why do we have to do the opposite in the situation of the images from different track?

images of different lootracks cannot be coregistrated because the matching of points often fails due to the different looking directions. In such cases, a solution is to terrain correct first and select “Product Geolocation” as offset method, so that SNAP already “knows” which pixels roughly belong together.

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Thank you so much for your clear explanation!