Range & azimuth resolution
The two directions of a SAR pixel are governed by completely different principles. Range (front-back separation) is set by pulse bandwidth, expressed as ΔR=c/2B, while Azimuth (side separation) is set by the synthetic aperture and is effectively tied to the antenna length. On Sentinel-1 IW, Range (~5m) is far better than Azimuth (~20m), giving a rectangular 5m×20m pixel; this is because TOPS mode trades away Azimuth resolution to image a 250km-wide swath (SM mode is ~5m×5m). Because of this asymmetry, InSAR multilook for speckle reduction is usually applied as 3×1, averaging mainly in the Range direction.
Defining the two directions
- Range resolution measures how close two objects can be along the radar line-of-sight while still being separated (front-back separation).
- Azimuth resolution measures how well two objects are separated along the satellite's flight direction, that is, the sideways direction.
Range resolution — set by bandwidth
- Like a bat's ultrasound, if two echoes arrive almost simultaneously they sound as one, and if there is a time gap they sound as two. SAR separates objects by this time difference, where near objects return early and far objects return late.
- How finely this time gap can be resolved depends on bandwidth, so wider bandwidth yields shorter pulses and lets closer objects be separated.
- Sentinel-1's roughly 56MHz bandwidth corresponds to a theoretical ~2.7m resolution, while the actual method of producing short pulses is handled by Chirp and range compression.
- Range-direction resolution
- Speed of light
- Pulse bandwidth
Azimuth resolution — set by the synthetic aperture
- A small antenna spreads its beam too widely to separate the side direction, so Sentinel-1's mere 12.3m antenna observes the same target hundreds to thousands of times while moving and phase-coherently sums them, building a virtual antenna several km long via the synthetic aperture.
- As a result, Azimuth resolution is effectively tied to the real antenna length, approximated as Δ_az ≈ L/2 (L being the real antenna length).
- L/2 ≈ 6m is only the theoretical approximation for Stripmap (SM) mode; IW uses TOPS mode, which electronically swings the beam in azimuth to image a 250km-wide swath and thereby sacrifices Azimuth resolution down to ~20m.
- In other words, the actual resolution is not set by antenna length alone but by antenna, TOPS scanning, processing, and operating mode combined, yielding ~20m for IW.
Comparison across modes and multilook implications
- On Sentinel-1 IW, Range is far better than Azimuth. Range is good thanks to chirp and bandwidth (56MHz allows an actual 2~3m, becoming ~5m in products after multilook, resampling, and GRD), while Azimuth is poor because the TOPS design prioritized a 250km-wide swath over high resolution.
- Seeing a rectangular SLC pixel in SNAP is the normal result of Range 5m / Azimuth 20m, so there is no need to be alarmed.
- Since Azimuth is already poor at 20m, smearing it further would drop resolution excessively, so InSAR multilook for speckle reduction is usually applied as 3×1, averaging mainly in Range.
| Range | Azimuth | |
|---|---|---|
| What it determines | Front-back separation | Side separation |
| Governing factor | Pulse bandwidth (ΔR=c/2B) | Synthetic aperture (Δ_az≈L/2) |
| S1 IW value | ~5m | ~20m |
| S1 SM value | ~5m | ~5m |
Do not naively apply the L/2 ≈ 6m formula to the 12.3m antenna and expect IW Azimuth to be 6m. L/2 is only the theoretical approximation for Stripmap (SM) mode; IW uses TOPS mode, which swings the beam in azimuth to gain a 250km-wide swath and thereby sacrifices resolution to ~20m.