SLC vs GRD product levels
SLC stores each pixel's complex value (I + jQ) as-is, preserving phase, so it serves as the input for InSAR; GRD keeps only amplitude (|I + jQ|), discarding phase, as a general-purpose product. The reason GRD makes up roughly 80–90% of the ESA archive while SLC is only ~10–20% is not that 'only some SLC gets converted to GRD,' but a combination of demand, storage volume, and acquisition mode — most users do not need phase, and SLC is 3–5 times larger than GRD. Looking only at the InSAR pipeline makes SLC feel like the 'default product,' but that is an illusion of the expert's viewpoint; from an operational standpoint the default is GRD.
SLC vs GRD — what is the difference
- SLC (Single Look Complex) stores both I and Q to preserve phase φ, while GRD (Ground Range Detected) stores only amplitude and discards phase.
- Phase is the core ingredient of InSAR, which measures the difference between two acquisitions in millimeters, so GRD — having discarded phase — fundamentally cannot be used for InSAR.
- Conversely, questions like 'where is flooded, where has changed' are solvable with amplitude alone, so GRD suffices for change detection, flood mapping, and classification.
| SLC | GRD | |
|---|---|---|
| Phase | Preserves phase as complex I+jQ | Amplitude only, phase discarded |
| Use | InSAR/DInSAR/SBAS/PSI only | Change detection, flood mapping, classification |
| Archive share | ~10–20% | ~80–90% |
Why GRD is 80–90%
- Most uses — flood, agriculture, ocean, forestry, disaster, GIS — do not need phase and are served by GRD, while InSAR users running DInSAR/SBAS/PSInSAR are a niche even among SAR users.
- In volume, IW SLC is roughly 3–8GB versus GRDH at about 0.8–1.5GB — SLC is 3–5 times larger — and since ESA receives tens of TB daily it cannot archive every mode as SLC forever, so it prioritizes the higher-demand GRD.
- Some acquisition modes such as Wave (WV, ocean) lean more toward OCN/GRD than SLC, adding a per-mode bias on top.
Position in the processing chain and the illusion
- GRD is a downstream product made by further processing SLC (multilook, detection, ground projection), so SLC comes first in processing order yet GRD dominates in distribution volume.
- The paradox that 'the more-processed product is distributed more' is entirely due to demand.
- Working only through RAW → SLC → Coregistration → Interferogram → SBAS/PSI in SNAP/MintPy/MiaplPy makes SLC feel like the starting point and 'default,' but by global distribution volume the default product is GRD.
Always specify the product type when searching the catalog — default results in CDSE/ASF mix in a lot of GRD, so for InSAR you must explicitly filter for 'IW_SLC' to avoid wasted effort. That said, abundant GRD does not mean 'SLC coverage is lacking' — what is scarce is global SLC, not local SLC, and your area of interest may already have plenty of SLC.