Open-source investigators, human rights monitors, and conflict reporters rely heavily on earth observation satellites to verify troop build-ups, document wartime destruction, track industrial deforestation, and uncover clandestine infrastructure projects. While commercial satellite providers like Maxar Technologies and Planet Labs offer sub-30-centimeter imagery behind steep enterprise paywalls, public space agencies provide vast archives of high-resolution, calibrated multispectral imagery at zero cost.

This guide outlines where to find free satellite imagery, how to bypass cloud coverage using radar sensors (SAR), and how to calculate false-color spectral indices to identify burnt terrain, military fortifications, and water contamination.

┌────────────────────────────────────────────────────────────────────────┐
│             FREE PUBLIC SATELLITE CONSTELLATIONS COMPARISON            │
├────────────────────┬─────────────────┬──────────────┬──────────────────┤
│ CONSTELLATION      │ SPATIAL RES.    │ REVISIT RATE │ SENSOR TYPE      │
├────────────────────┼─────────────────┼──────────────┼──────────────────┤
│ Sentinel-2 (ESA)   │ 10m / 20m / 60m │ 5 Days       │ 13-Band Optical  │
├────────────────────┼─────────────────┼──────────────┼──────────────────┤
│ Landsat 8 & 9 (USGS│ 15m (Pan) / 30m │ 8 Days       │ 11-Band Thermal  │
├────────────────────┼─────────────────┼──────────────┼──────────────────┤
│ Sentinel-1 (ESA)   │ 10m (Interferom)│ 6 Days       │ C-Band Radar SAR │
├────────────────────┼─────────────────┼──────────────┼──────────────────┤
│ NAIP (USDA - USA)  │ 0.6m (Sub-meter)│ 2-3 Years    │ Aerial Orthophoto│
├────────────────────┼─────────────────┼──────────────┼──────────────────┤
│ NASA Worldview     │ 250m - 1km      │ Daily (2x)   │ MODIS / VIIRS    │
└────────────────────┴─────────────────┴──────────────┴──────────────────┘

1. Top Free Satellite Imagery Portals

A. Copernicus Browser (Sentinel Hub)

The European Space Agency’s Copernicus Browser (browser.dataspace.copernicus.eu) is the single most powerful free platform for investigative reporters: - Spatial Resolution: 10 meters per pixel for visible RGB channels (Band 4, Band 3, Band 2) and Near-Infrared (Band 8). - Temporal Cadence: Every 5 days globally using the twin Sentinel-2A and Sentinel-2B satellites. - Key Capability: Instant split-screen time-slider comparisons, cloud-cover filtering (< 10%), and client-side spectral calculations (NDVI, NDWI, Burn Ratios).

B. USGS EarthExplorer

Administered by the United States Geological Survey (earthexplorer.usgs.gov), EarthExplorer is the primary portal for historical imagery dating back to 1972: - Datasets: Full Landsat 1 through Landsat 9 archives, NASA ASTER Digital Elevation Models (DEM), and Declassified Military Satellite Photos (Corona missions, 1960–1972). - Format: Ingests raw GeoTIFF format files with complete uncompressed metadata for GIS software (QGIS, ArcGIS).

C. NAIP (National Agriculture Imagery Program)

For investigations located within the continental United States, the USDA’s NAIP dataset provides 60-centimeter (0.6m) aerial photography at zero cost. NAIP imagery reveals vehicle outlines, building roof alterations, and road trenches that are invisible on 10-meter Sentinel satellites.

D. NASA Worldview

When tracking active wildfire fronts, dust storms, industrial flaring, or massive artillery smoke plumes in near-real-time, NASA Worldview (worldview.earthdata.nasa.gov) updates twice daily using MODIS and VIIRS thermal sensors.

2. Unlocking Hidden Intelligence via Spectral Bands

Human eyes perceive light only in the visible spectrum ($\lambda \approx 400\text{–}700\text{ nm}$). Optical satellites capture wavelengths in Near-Infrared (NIR) and Shortwave Infrared (SWIR), which react distinctly to vegetation health, surface water, and artificial camouflage.

Essential Sentinel-2 Band Combinations

┌────────────────────────────────────────────────────────────────────────┐
│                   SENTINEL-2 BAND COMBINATION RECIPES                  │
├──────────────────────┬──────────────┬──────────────────────────────────┤
│ COMBINATION NAME     │ BAND RECIPE  │ INVESTIGATIVE APPLICATION        │
├──────────────────────┼──────────────┼──────────────────────────────────┤
│ Natural Color (RGB)  │ B04, B03, B02│ Standard baseline visual inspection│
├──────────────────────┼──────────────┼──────────────────────────────────┤
│ False Color Infrared │ B08, B04, B03│ Highlights healthy vegetation    │
│ (Vegetation / Camo)  │ (NIR, Red, G)│ (bright red); exposes dead turf  │
├──────────────────────┼──────────────┼──────────────────────────────────┤
│ Shortwave Infrared   │ B12, B8A, B04│ Penetrates light atmospheric haze;│
│ (Urban / Wildfires)  │ (SWIR, NIR, R)│ Active fires glow neon orange   │
├──────────────────────┼──────────────┼──────────────────────────────────┤
│ Moisture / Geology   │ B12, B11, B8A│ Highlights saturated marshland,   │
│ (Trench Water/Canals)│ (SWIR2, SWIR, │ dry riverbeds, concrete runways  │
└──────────────────────┴──────────────┴──────────────────────────────────┘

Computing the Normalized Difference Vegetation Index (NDVI)

Healthy living plants absorb red light for photosynthesis while strongly reflecting near-infrared radiation. By measuring the mathematical ratio, investigators can detect vehicle tracks across fields, bomb crater displacement of topsoil, and clandestine clearing:

$$\text{NDVI} = \frac{\text{B08 (NIR)} - \text{B04 (Red)}}{\text{B08 (NIR)} + \text{B04 (Red)}}$$

• Values between $0.6$ and $0.9$: Dense agricultural crops or lush forest canopy.
• Values near $0.0$: Bare rock, concrete structures, asphalt runways, or cleared dirt.
• Negative values ($-0.5$ to $-1.0$): Open water reservoirs or flooded trenches.

3. Piercing Clouds with Synthetic Aperture Radar (SAR)

Optical sensors cannot see through cloud decks or capture night operations. When weather obstructs an area of interest during breaking conflicts, pivot to Sentinel-1 C-Band SAR:

1. Active Microwave Pulses: Sentinel-1 does not measure reflected sunlight. It bounces microwave radio beams off the earth’s surface and records the return backscatter echo.
2. Rough vs. Smooth Reflectance: Calm open water reflects microwaves away from the sensor like a mirror (appearing pitch black). Metal hulls of naval vessels, reinforced concrete bunkers, and metal container roofs reflect radar beams directly back (appearing brilliant white).
3. Coherence Difference: By comparing the phase coherence between two radar passes taken 6 days apart, algorithms generate Damage Proxy Maps (DPM), revealing structural collapses across entire cities during siege warfare.

4. Step-by-Step Satellite Verification Workflow

[Target Verification Coordinates]
        │
        ├── 1. Check Copernicus Browser (Sentinel-2) for latest optical pass (<= 5 days)
        │
        ├── 2. High Cloud Cover? ──► YES ──► Pivot to Sentinel-1 Radar (SAR Backscatter)
        │                         │
        │                         └── NO ──► Apply False Color Infrared (B08-B04-B03)
        │
        ├── 3. Measure ground disturbance via NDVI change-detection calculation
        │
        └── 4. Cross-reference historical imagery using Google Earth Pro Historical Slider

Protocol for Open-Source Reporters:

1. Lock Lat/Long Coordinates: Never search by place names; input decimal coordinates (34.5338, 69.1372) to eliminate translation errors.
2. Set Cloud Ceiling to Zero: In Copernicus Browser, set the cloud coverage filter slider to under 15% to avoid downloading obscured tiles.
3. Document Sun Elevation & Azimuth: Check metadata headers for SUN_ELEVATION and SUN_AZIMUTH. These angles are critical when verifying shadow lengths in our Solar Chronolocator.
4. Preserve Raw Coordinate Metadata: When exporting imagery for publication, always download the calibrated GeoTIFF rather than taking a screenshot to preserve the cryptographic and coordinate bounding box headers.