A Field Guide to Free Satellite Data Sources: Sentinel-1, Sentinel-2, VIIRS, and NASA FIRMS

Why Source Selection Matters More Than Resolution

When people start with satellite imagery, the first question is almost always "how sharp is it?" But resolution is rarely the thing that makes or breaks an analysis. The thing that matters is what the sensor actually measures — reflected sunlight, radar backscatter, emitted light at night, or thermal anomalies. Each answers a different question.

The good news: the most useful sources are free and open. ESA's Copernicus programme and NASA distribute global, multi-year archives at no cost, with attribution. This guide walks through the four sources that cover the vast majority of monitoring work, plus one analysis-ready radar product, and explains when to reach for each.

For a structured reference with resolution, revisit, and attribution for every source, see the Data Sources page.

Sentinel-2: The Optical Workhorse

Sentinel-2 is what most people picture when they think "satellite image" — natural-looking optical imagery, captured in 13 spectral bands from visible light through shortwave infrared.

• Resolution: 10 m for visible and near-infrared bands
• Bands: 13 spectral bands
• Revisit: ~5 days with both satellites
• Best for: vegetation health, crop and forest monitoring, land cover, water quality

Because it captures multiple bands, Sentinel-2 supports spectral indices like NDVI, EVI, and NDWI — simple band-math formulas that turn raw reflectance into measures of vegetation vigor or water content. Its one weakness is fundamental to all optical sensors: it cannot see through cloud. Over the wet tropics, weeks can pass without a clear scene.

Sentinel-1: Radar That Ignores the Weather

Sentinel-1 solves the cloud problem. It is a C-band Synthetic Aperture Radar (SAR) — an active sensor that transmits its own microwave pulses and measures what bounces back. Radar penetrates cloud, smoke, and light rain, and works just as well at night.

• Resolution: ~10 m (Interferometric Wide swath mode)
• Polarization: VV and VH
• Revisit: 6-day repeat cycle
• Best for: flood mapping, surface-change detection, vessel detection, all-weather monitoring

SAR imagery looks unfamiliar at first — it measures surface roughness and moisture, not color — but that is exactly why it is powerful. Calm water appears dark (it reflects radar away from the sensor), while cities and rough terrain appear bright. For disaster response, where clouds almost always accompany the event, SAR is often the only sensor that can deliver an image on the day it matters.

OPERA RTC-S1: Analysis-Ready Radar

Raw SAR backscatter varies with terrain: the same forest looks brighter on a slope facing the sensor than on one facing away. OPERA RTC-S1 removes that distortion. It is a Radiometrically Terrain Corrected product derived from Sentinel-1 (so it is still C-band radar, not L-band NISAR), distributed as Cloud-Optimized GeoTIFFs by the Alaska Satellite Facility.

• Source: Sentinel-1 (C-band)
• Processing: terrain correction using a digital elevation model
• Best for: time-series backscatter comparison and terrain-aware change detection

If you are comparing radar values across dates in hilly terrain, RTC makes those comparisons meaningful in a way raw GRD products cannot.

VIIRS Nighttime Lights: A Proxy for Human Activity

VIIRS nighttime lights measure something none of the above can: visible light emitted at night. NASA's Black Marble products calibrate and atmospherically correct the VIIRS Day/Night Band so that city lights become a usable signal.

• Resolution: ~500 m
• Product: NASA Black Marble (VNP46A2)
• Revisit: daily
• Best for: urbanization and electrification trends, economic-activity proxies, power-outage detection after disasters

At ~500 m, VIIRS is coarse, but its daily cadence and a clean signal make it ideal for tracking change over time rather than mapping fine detail. A sudden drop in nighttime brightness after a storm is one of the fastest indicators of a grid-scale power outage.

NASA FIRMS: Active Fire, Updated Daily

NASA FIRMS provides near real-time thermal fire detections from VIIRS and MODIS. Hotspots are published daily and require no account or API key to browse.

• Resolution: 375 m (VIIRS), 1 km (MODIS)
• Latency: near real-time, daily
• Best for: wildfire situational awareness, tracking fire spread

FIRMS is not imagery in the usual sense — it is a point layer of thermal anomalies — but overlaid on a basemap or satellite scene it turns "is something burning here?" into an answerable question within hours.

Putting It Together

The single biggest mistake is treating these as competitors. They are complementary:

Source	Measures	Resolution	Revisit	See through cloud?
Sentinel-2	Reflected light (13 bands)	10 m	~5 days	No
Sentinel-1	Radar backscatter	~10 m	6 days	Yes
OPERA RTC-S1	Terrain-corrected backscatter	~30 m	~12 days	Yes
VIIRS	Light emitted at night	~500 m	Daily	Partly
NASA FIRMS	Thermal fire anomalies	375 m / 1 km	Daily	Yes (thermal)

A typical workflow uses more than one. Map a flood with Sentinel-1 while cloud blocks the optical view, then confirm the recovery weeks later with a clear Sentinel-2 scene. Spot a thermal anomaly in FIRMS, then pull the Sentinel-2 burn scar once skies clear. Watch a region's nighttime lights for an outage, then task radar to see what changed on the ground.

How to Choose

• Need color, vegetation, or land cover? Start with Sentinel-2.
• Clouds in the way, or need night coverage? Use Sentinel-1.
• Comparing radar across dates in rough terrain? Use OPERA RTC-S1.
• Tracking human activity or outages? Use VIIRS nighttime lights.
• Is something on fire right now? Use NASA FIRMS.

Every source above is free to browse in Off-Nadir Delta — no GIS software, no code, no API keys. For definitions of the technical terms used here, see the glossary.

Attribution reminder: When you export or publish imagery, credit the source — "Contains modified Copernicus Sentinel data" for Sentinel-1/2, "NASA Black Marble VNP46A2" for VIIRS, "OPERA RTC courtesy NASA/JPL-Caltech," and "NASA FIRMS" for active fire data.