In this article, we are going to focus exclusively on Sentinel data. First though, we will describe some of the basics. There are many types of imaging satellites: some like Sentinel-1 capture radar, others like Sentinel-2, capture light. As you may be aware, the light spectrum is significantly larger than what our human eyes can see, and the Sentinel-2 satellites can see far more than we can.
Sentinel-2 satellites carry the Multi-Spectral Instrument (MSI) that images Earth in 13 spectral bands across different wavelengths. Here are the specific wavelengths for both Sentinel-2A and Sentinel-2B satellites.
| Band | Sentinel-2A Central Wavelength (nm) | Sentinel-2B Central Wavelength (nm) | Spatial Resolution (m) |
|---|---|---|---|
| 1 | 442.7 | 442.2 | 60 |
| 2 | 492.4 | 492.1 | 10 |
| 3 | 559.8 | 559.0 | 10 |
| 4 | 664.6 | 664.9 | 10 |
| 5 | 704.1 | 703.8 | 20 |
| 6 | 740.5 | 739.1 | 20 |
| 7 | 782.8 | 779.7 | 20 |
| 8 | 832.8 | 832.9 | 10 |
| 8A | 864.7 | 864.0 | 20 |
| 9 | 945.1 | 943.2 | 60 |
| 10 | 1373.5 | 1376.9 | 60 |
| 11 | 1613.7 | 1610.4 | 20 |
| 12 | 2202.4 | 2185.7 | 20 |
These bands range from visible light (bands 2-4), through near-infrared (bands 5-9), to short-wave infrared (bands 10-12), with specialized bands for coastal aerosol detection (band 1) and cirrus cloud detection (band 10).
The spatial resolution and spectral nature of this data is quite intuitive and easy to understand with the right explanation.
The 5-Day Revisit & The Swath
Sentinel-2 satellites revisit every location on Earth approximately every 5 days, creating frequent opportunities to monitor change.
Between the pair of Sentinel-2 sats (Sentinel-2A and Sentinel-2B) we have a revisit time of around 5 days, with a swath width of ~290km. This means that the sats are taking a constant image that is 290km wide; the resultant image data files are square-ish chunks from this neverendingly long swath. In short, we get a full, new picture of the landmass of the planet every 5-ish days.
Luckily for our calculations, Gaza falls directly in the path of one of these swaths, so there is no need to stitch raster files together. We always get a clean single raster, weather permitting.
This frequent, open-source revisit cycle is what makes platforms like Sentinel Bird possible. By continuously ingesting these 5-day chunks, we can build high-fidelity timelapses and interactive change-detection sliders that document physical transformation on the ground in near real-time.
What’s Next?
Now that we know where the data comes from and how often we get it, we need to understand what the data actually looks like under the hood. In the next section, we will break down exactly what a “raster” is, and what “10 meters per pixel” actually means when you are looking at a warzone.
(Next: The Pixels: Spatial Resolution)