Swath · subswath · TOPSAR
A Swath is the entire observation strip Sentinel-1 sees in a single pass (about 250km in IW mode), and a Subswath (IW1·IW2·IW3) is a subregion that splits that strip into three pieces of about 80km each, slightly overlapping. Sentinel-1 IW uses TOPSAR (Terrain Observation by Progressive Scans), rapidly alternating the beam across IW1→IW2→IW3, so it does not image 250km at once but in pieces to avoid limits on data volume, antenna, and resolution. The most confusing point is that IW1→IW3 are arrayed along Range (the side direction), not Azimuth (the satellite's flight direction); the IW chosen in SNAP TOPSAR Split is exactly the subswath, and burst, deburst, and coregistration are all processed per subswath.
Swath vs Subswath
- A Swath is the entire width the satellite sees in one pass, about 250km in Sentinel-1 IW mode.
- A Subswath is a subregion of that width — IW1·IW2·IW3 each about 80km, summing to roughly 250km, with slight overlap between them.
- In other words a single swath contains three subswaths, and IW1/2/3 are the units into which the swath is divided.
TOPSAR — why image it in pieces
- Imaging 250km at high resolution all at once would explode data volume, strain the antenna, and degrade resolution, so the beam is steered electronically to scan the subswaths alternately.
- This is TOPSAR (Terrain Observation by Progressive Scans), sweeping the beam along the flight direction in bursts to fill each subswath.
- The beam rapidly alternates in the order IW1→IW2→IW3→IW1→IW2→…, filling the 250km wide area piece by piece rather than in a single shot.
Range vs Azimuth — the most confusing point
- IW1→IW3 are arrayed along Range (the side direction), not along the satellite's flight direction (Azimuth).
- The satellite flies upward (Azimuth) while IW1·IW2·IW3 lie side by side next to its track (Range).
- Memorizing this picture prevents coordinate-system confusion — remember that the IW numbers are strips lined up sideways, not stages along the flight direction.
| Azimuth | Range | |
|---|---|---|
| Direction | Satellite flight direction (upward) | Sideways from the track |
| IW1·IW2·IW3 | Not this direction | Arrayed side by side here |
Connecting to SNAP in practice
- In the TOPSAR Split step you select which subswath (IW1/IW2/IW3) to process, and if the AOI of interest lies only in IW2 you cut out and process IW2 alone.
- Burst and Deburst decompose and merge by the burst units inside a subswath, while Coregistration aligns per subswath.
- This structure is exactly why separate IW1·IW2·IW3 folders appear in SNAP, and to reduce processing cost the key is to pick only the subswath containing the AOI of interest.
Mistaking IW1→IW3 for the Azimuth direction instead of Range (the side direction) tangles the entire coordinate system — it is safest to memorize the picture. Also, a burst count of at least 2 is required: with a single burst (FIRST=LAST=1) the ESD overlap is 0, so the pipeline reports success with exit 0 yet produces empty output. Conversely, when the AOI of interest falls within one subswath you only process that one — the Songdo track materialized just the IW2 single swath (VV only) of the rel127 north slice, shrinking it to about 121GB; knowing swath/subswath greatly cuts materialization cost.