Satellite imagery used to be expensive and restricted, but today some of the world's best Earth observation data is free to anyone with an internet connection. Programs run by NASA, the European Space Agency, and the US Geological Survey distribute imagery covering the whole planet, updated regularly, under open licences. Whether you are a student, a farmer checking crop health, or a GIS analyst building a project, these sources are where most real work begins.

The catch is choice: each source differs in resolution, revisit time, spectral bands, and ease of download, and picking the wrong one wastes hours. A 10-metre image is superb for regional land cover but useless for counting individual trees, while a daily coarse image suits drought monitoring better than a sharp image updated once a month. This guide matches sources to uses so you download the right data the first time.

The European Space Agency's Sentinel-2 satellites provide 10-metre optical imagery in visible and near-infrared bands, with a revisit time of about five days when both satellites operate. Data is free through the Copernicus Data Space Ecosystem, and it is the default choice for vegetation monitoring, land cover mapping, and change detection at regional scales. Its 13 spectral bands support indices like NDVI, and its archive stretches back to 2015.

For daily global coverage, NASA's MODIS instruments on the Terra and Aqua satellites, and the newer VIIRS sensor, provide imagery at 250 metres to 1 kilometre, updated once or twice daily. That resolution cannot map fields, but it is ideal for tracking fires, floods, dust storms, and vegetation trends across continents, and the data is available through NASA Worldview for quick browsing before you download anything.

Start by defining your area and dates, then use a browser-based portal, Copernicus Browser for Sentinel, EarthExplorer for Landsat, Worldview for MODIS, to preview scenes before downloading, since cloud cover ruins many otherwise perfect images. Most portals let you filter by cloud percentage; under 10 to 20 percent is a good target for optical work. Downloads arrive as GeoTIFF files that open directly in QGIS, where you can combine bands into true-colour or false-colour composites.

One final habit separates casual downloaders from reliable analysts: keep a simple data log for every scene you use, noting the sensor, acquisition date, cloud cover, and processing level. That log makes your work reproducible, lets you defend your results when someone questions them, and saves hours when you return to a project months later and need the exact same inputs again.