Understanding GSD
How GSD is calculated
GSD follows directly from optics and orbit height: GSD = pixel size × altitude ÷ focal length. Flying lower shrinks the GSD, which is why the sharpest Earth-observation satellites sit in low orbit (roughly 450–700 km); a longer focal length — a physically larger telescope — does the same job without dropping altitude. As an illustration, an 8-micrometre detector pixel viewed through a 16 m focal length from 600 km projects to about 0.30 m on the ground. GSD is always quoted 'at nadir', looking straight down. When a satellite tilts to image a target off to one side, the pixel footprint stretches and the effective GSD grows — WorldView-3, for example, is 0.31 m at nadir but about 0.34 m at 20° off-nadir.
Why GSD is not the same as resolution
A 30 cm GSD does not mean a 30 cm object can be identified. GSD measures how finely the scene is sampled, not the smallest feature the optics can resolve — that is limited by lens sharpness (the modulation transfer function) and atmospheric blur. Reliably recognising an object takes several pixels across it, so the smallest identifiable feature is typically a few times the GSD. This is why 30 cm imagery shows that a vehicle is a car, and perhaps its rough class, but never a face or a number plate — the 'read a licence plate from space' idea is a myth. Beware, too, of '15 cm HD' marketing: those products are interpolated (super-resolved) from ~30 cm native pixels, not genuinely finer sampling. Commercial sharpness was long capped by regulators — the US limit was 0.5 m until 2014, when it was relaxed to 0.25 m panchromatic.
GSD across the imaging fleet
GSD spans nearly three orders of magnitude across today's Earth-observation satellites — from sub-metre commercial 'tasking' platforms that must be pointed at a target, to free, wide-swath public missions used for climate, agriculture and land-cover monitoring:
| Satellite / sensor | GSD at nadir | Access |
|---|---|---|
| WorldView-3, WorldView Legion (Maxar) | ~0.30 m panchromatic | Commercial |
| Pléiades Neo (Airbus) | 0.30 m panchromatic | Commercial |
| Planet SkySat | ~0.50 m | Commercial |
| Planet PlanetScope (Dove) | ~3 m | Commercial |
| Sentinel-2 (ESA / Copernicus) | 10 m multispectral | Free / open |
| Landsat 8/9 (USGS / NASA) | 15 m pan, 30 m multispectral | Free / open |
| MODIS (Terra / Aqua) | 250–1000 m | Free / open |
The resolution–coverage trade-off
Sharper pixels come at a price. Halving the GSD quarters the ground area each pixel represents, so a sensor must either accept a much narrower swath width or generate far more data per scene. Narrower swaths mean each satellite images less of Earth per pass, which lengthens revisit time — the wait before the same spot is photographed again. This is the central trade-off in Earth observation: Maxar and Airbus fly a handful of large, very sharp satellites that are tasked to point at specific targets, while Planet flies a big flock of smaller satellites at coarser GSD to image the whole landmass daily. Synthetic-aperture radar satellites play by different rules, seeing through cloud and darkness, and most optical imagers use a sun-synchronous orbit so surface lighting stays consistent between passes.