What the Sentinel-1 archive actually delivered over An Giang
Retrieved 7 September 2026. This is the one part of the LuaZero prototype backed by data we pulled rather than by arithmetic or a synthetic sample. It is a measurement of data availability, not a detection of anything happening in a field.
What we did
We queried the public Sentinel-1 archive for a 17 km by 17 km box over An Giang province in the Mekong Delta, took every scene on a single relative orbit across two crop seasons, and read a small window of real backscatter pixels from each one. No account, licence or payment was involved at any step.
| Setting | Value |
|---|---|
| Catalogue | Microsoft Planetary Computer STAC, collection `sentinel-1-rtc` |
| Product | Sentinel-1 GRD, radiometrically terrain corrected, VV polarisation, gamma0 |
| Area | An Giang, Mekong Delta. Bounding box 105.10–105.25 E, 10.35–10.50 N |
| Track | Relative orbit 18, descending, on its own |
| Window | 1 November 2025 to 31 May 2026 |
| Sample | Five points, 70 m by 70 m each, mean of the enclosed pixels |
| Access | Anonymous SAS token, then HTTP range reads against the cloud-optimised GeoTIFF |
Only a few kilobytes moved per scene, because a windowed read fetches the tiles it needs rather than the whole image.
What we found
30 distinct acquisition dates in seven months on one track. The gaps between consecutive passes:
| Gap between passes | Times observed |
|---|---|
| 1 day | 2 |
| 6 days | 19 |
| 11 days | 1 |
| 12 days | 7 |
Median gap 6 days; mean 7.28 days.
Three satellites appear in the record — Sentinel-1A on 16 dates, Sentinel-1C on 12 and Sentinel-1D on 2 — which is the constellation reconfiguration of 2026 visible in the data itself.
The search returned 41 granules on this track but only 30 acquisition dates. The 11 extra granules are neighbouring slices of the same pass whose footprint does not cover our sample points. No acquisition date was lost.
VV backscatter at the five sample points spanned 7.8 to 16.2 dB between its lowest and highest value over the period. Four points sat in the −20 to −5 dB range with large seasonal swings; the fifth stayed near −2 dB throughout, which is the signature of a permanently bright scatterer such as built-up ground rather than a flooded and replanted paddy.
What this proves, and what it does not
It proves the revisit assumption on the LuaZero site is not theoretical. A median gap of 6 days is exactly the two-satellite repeat. But a quarter of the intervals were 12 days, so the 6-day figure is a best case, not a planning assumption. Any monitoring design that promises a pass every six days over every field is overstating what the archive delivers.
It does not prove anything about drainage. We have no ground truth, no field boundaries and no land-cover check, so none of these pixels is confirmed to be a rice paddy — the fifth point almost certainly is not one. Backscatter variation is not a drainage event, a classification accuracy, or an emissions result. Establishing that link is the entire purpose of the proposed pilot, and it needs the water-level loggers and the agronomist-defined labels described in the validation protocol.
We also used a single orbit direction on purpose. Ascending and descending passes are flown at different incidence angles and times of day; pooling them without per-track calibration would inflate the apparent revisit and mix incomparable measurements.
Reproducing it
The retrieval script is `tools/fetch-observed-revisit.py` in the project repository, and its condensed output is published at `/observed-revisit.json` on the site. It needs `rasterio` and a network connection, nothing else. Re-running it against a different bounding box, track or date range is a one-line change, and we expect anyone assessing this application to be able to check the numbers rather than take them on trust.