What does the APPLY orbit file for？

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The preprocessing steps are very tricky. See this post:

Radiometric & Geometric Correction Workflow

for SLC data I would suggest TOPS Split Apply Orbit file Thermal Noise Removal Calibration to Beta0* TOPSAR Deburst Radiometric terrain flattening (Speckle filtering) Range Doppler Terrain Correction If you don’t need the phase information you can also download it as a GRD product and only apply the following: Apply Orbit file Thermal noise removal Calibration to Beta0 Radiometric terrain flattening (Speckle filtering) Range Doppler Terrain Correction or (if you don’t have much topography)…

But in my point of view, do “apply orbit” before everything is totally OK.