Satellite Imagery Glossary

An active remote sensing technology that uses microwave radar pulses to image the Earth's surface.

SAR works by transmitting microwave pulses and measuring the reflected signal. Unlike optical sensors, SAR can operate day or night and penetrate clouds, smoke, and light rain. The "synthetic aperture" technique combines multiple radar pulses to achieve high spatial resolution. Sentinel-1 uses C-band SAR at 5.405 GHz (5.6 cm wavelength).

A radar technique that combines two or more SAR images to measure surface deformation or generate digital elevation models.

InSAR uses the phase difference between SAR acquisitions to detect millimeter-scale ground movements. Applications include monitoring earthquakes, volcanic activity, landslides, subsidence, and glacier flow. Differential InSAR (DInSAR) removes topographic effects to isolate surface displacement. Sentinel-1's 6-day repeat cycle enables regular InSAR monitoring.

The European Union's Earth observation programme, coordinated by the European Space Agency (ESA), providing free and open satellite data.

Copernicus is the world's largest single Earth observation programme. It includes the Sentinel satellite family: Sentinel-1 (SAR), Sentinel-2 (optical), Sentinel-3 (ocean/land), Sentinel-5P (atmosphere), and upcoming Sentinel-6 (altimetry). All Copernicus data is free and open under the Copernicus Open Access policy, enabling commercial and non-commercial use with attribution.

Satellite imagery captured in multiple discrete spectral bands, typically 3-10 bands spanning visible to near-infrared wavelengths.

Multispectral sensors capture light in specific wavelength ranges (bands). Sentinel-2 has 13 spectral bands from visible (blue, green, red) through near-infrared (NIR) to shortwave infrared (SWIR). Different bands reveal different surface properties: red/NIR for vegetation, SWIR for moisture, coastal aerosol for water quality. Combining bands enables vegetation indices and land cover classification.

A Sentinel-1 SAR product that has been multi-looked and projected to ground range using an Earth ellipsoid model.

GRD products have reduced speckle noise due to multi-looking (averaging multiple looks). The data is projected from slant range to ground range, making it easier to use for geographic applications. Available in VV and VH polarizations. Resolution is approximately 10m for Sentinel-1 IW mode.

A processing technique that corrects SAR imagery for terrain-induced radiometric distortions using a Digital Elevation Model (DEM).

RTC products provide more accurate backscatter values in mountainous or hilly terrain by accounting for local incidence angle variations. This correction is essential for quantitative analysis and time-series comparisons. Microsoft Planetary Computer provides Sentinel-1 RTC products processed with the Copernicus DEM.

A microwave frequency band (4-8 GHz) used by Sentinel-1 SAR. Sentinel-1 operates at 5.405 GHz (5.6 cm wavelength).

C-band radar provides good balance between penetration capability and resolution. It can penetrate vegetation canopy partially and is sensitive to surface roughness and soil moisture. Compared to L-band (longer wavelength), C-band has less vegetation penetration but higher resolution.

The primary acquisition mode for Sentinel-1 over land, providing 250 km swath width with 5×20m resolution.

IW mode uses the TOPSAR (Terrain Observation with Progressive Scans SAR) technique, capturing data in three sub-swaths with burst synchronization for interferometric applications. This is the standard mode used over most land surfaces.

The orientation of the electromagnetic wave transmitted and received by SAR. Sentinel-1 IW mode provides VV and VH polarizations.

VV (vertical-vertical): transmits vertically polarized waves and receives vertically polarized returns. Sensitive to surface roughness and soil moisture. VH (vertical-horizontal): transmits vertically and receives horizontally polarized returns (cross-polarization). Useful for vegetation analysis and flood mapping as water has low cross-polarization return.

A specific range of wavelengths captured by a satellite sensor, representing a portion of the electromagnetic spectrum.

Each band captures different surface properties. Common bands include: Blue (coastal/aerosol), Green (vegetation vigor), Red (chlorophyll absorption), NIR (vegetation structure), SWIR (moisture content). Sentinel-2 bands: B2 (Blue, 490nm), B3 (Green, 560nm), B4 (Red, 665nm), B8 (NIR, 842nm), B11/B12 (SWIR). Band combinations create different visualizations.

A sensor channel on VIIRS designed to detect low-light signals including city lights and moonlit clouds.

The DNB has extremely high sensitivity, detecting radiances from full sunlight down to quarter moon illumination. NASA Black Marble products use DNB data to create nighttime lights composites, useful for monitoring urbanization, economic activity, and power outages.

Visible Infrared Imaging Radiometer Suite - a sensor on Suomi NPP and NOAA-20 satellites providing global daily coverage.

VIIRS provides 22 spectral bands from visible to thermal infrared. For Off-Nadir Delta, we use the Day-Night Band (DNB) products from NASA Black Marble (VNP46A2) for nighttime lights analysis. Resolution is approximately 500m.

A Sentinel-2 product level indicating atmospherically corrected surface reflectance data.

L2A products are processed from Level-1C (top-of-atmosphere reflectance) using the Sen2Cor algorithm. They provide orthorectified, atmospherically corrected surface reflectance values at 10m, 20m, and 60m resolution depending on the band. Off-Nadir Delta uses L2A products for all Sentinel-2 imagery.

A GeoTIFF file format optimized for cloud-based access, enabling efficient partial reads over HTTP.

COGs use internal tiling and overviews (pyramids) organized to allow HTTP range requests. This means you can request just the portion of the image you need without downloading the entire file. Essential for web-based satellite imagery viewing and analysis. Off-Nadir Delta streams COG data directly from cloud storage.

An open standard format for encoding geographic data structures using JavaScript Object Notation (JSON).

GeoJSON supports geometry types including Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon, and GeometryCollection. It is widely used for web mapping applications due to its simplicity and compatibility with JavaScript. Off-Nadir Delta uses GeoJSON for vector data export and footprint display.

A standard protocol for serving pre-rendered georeferenced map tiles over the internet.

WMTS serves map images as tiles (typically 256×256 pixels) organized by zoom level, row, and column. This tiled approach enables efficient caching and fast map display. Related protocols include WMS (Web Map Service) for on-demand rendering and XYZ tiles (used by OpenStreetMap, Google Maps). Off-Nadir Delta uses tile-based streaming for satellite imagery.

An open specification for describing geospatial data assets, enabling standardized search and discovery.

STAC provides a common language for describing satellite imagery metadata including spatial extent, temporal coverage, and available assets (bands, formats). Major providers like AWS Earth Search and Microsoft Planetary Computer use STAC APIs. Off-Nadir Delta queries STAC APIs to search and access satellite imagery.

A public domain metadata standard for embedding georeferencing information in TIFF image files.

GeoTIFF stores coordinate reference system (CRS), map projection, and georeferencing information within the TIFF file structure. This allows GIS software to correctly position the image on the Earth. Most satellite imagery is distributed in GeoTIFF or Cloud Optimized GeoTIFF format.

A standardized index measuring vegetation greenness, calculated from red and near-infrared reflectance.

NDVI = (NIR - Red) / (NIR + Red). Values range from -1 to +1. Healthy vegetation typically shows values of 0.3-0.8 due to high NIR reflectance and low red absorption. Water bodies and bare soil typically show values near 0 or negative. NDVI is widely used for agriculture monitoring, drought assessment, and land cover classification.

An optimized vegetation index that corrects for atmospheric and soil background effects.

EVI uses blue band information to correct for atmospheric aerosols and is less sensitive to soil brightness variations. It performs better than NDVI in high-biomass regions where NDVI tends to saturate. Formula: EVI = 2.5 × (NIR - Red) / (NIR + 6×Red - 7.5×Blue + 1).

An index for detecting and monitoring water content in vegetation or water bodies.

Multiple NDWI formulations exist. McFeeters NDWI uses (Green - NIR) / (Green + NIR) for water body detection. Gao NDWI uses (NIR - SWIR) / (NIR + SWIR) for vegetation water content. Both help distinguish water from land and monitor drought conditions.

The angle between the satellite sensor's viewing direction and the vertical (nadir) direction pointing straight down to Earth.

Nadir means "directly below" - a nadir view looks straight down. Off-nadir imaging tilts the sensor to view targets at an angle, enabling more frequent revisits, stereo imaging for 3D models, and better views of building facades. However, off-nadir images have more geometric distortion and atmospheric path length. The name "Off-Nadir Delta" reflects the value of viewing Earth from different perspectives.

The portion of radar energy that is reflected back toward the SAR sensor from the target surface.

Backscatter intensity depends on surface roughness, dielectric properties (moisture content), and local geometry (slope facing toward/away from sensor). Smooth surfaces like calm water produce low backscatter (specular reflection away from sensor), while rough surfaces and urban structures produce high backscatter.

The process of removing geometric distortions from satellite imagery to create a planimetrically correct image.

Raw satellite images have distortions from sensor tilt, terrain relief, and Earth curvature. Orthorectification uses a Digital Elevation Model (DEM) and sensor geometry to correct these distortions, producing images where features are in their true geographic positions. Essential for accurate measurements and GIS overlay. Sentinel-2 L2A products are orthorectified.

A 3D representation of terrain elevation, used for orthorectification, terrain correction, and topographic analysis.

DEMs represent the bare earth surface (without vegetation or buildings). Related products include DSM (Digital Surface Model, including buildings/trees) and DTM (Digital Terrain Model). Common sources include SRTM (30m global), Copernicus DEM (30m global), and ALOS World 3D (30m). DEMs are essential for RTC processing and orthorectification.

A satellite image displayed using red, green, and blue bands mapped to the corresponding RGB display channels, showing natural-looking colors.

True color (natural color) composites use visible bands: Red→R, Green→G, Blue→B. For Sentinel-2: B4→R, B3→G, B2→B. The result looks similar to what human eyes would see. However, some features are better visualized using false color composites that include non-visible bands like near-infrared.

A satellite image displayed by mapping non-visible spectral bands to RGB display channels, revealing features not visible to the human eye.

Common false color combinations: NIR-Red-Green (vegetation appears red, useful for agriculture), SWIR-NIR-Red (distinguishes vegetation types and moisture), Urban (SWIR-SWIR-Red for built-up areas). For Sentinel-2 vegetation: B8→R, B4→G, B3→B shows healthy vegetation as bright red.

The fraction of incident electromagnetic radiation reflected by a surface, expressed as a value between 0 and 1.

Surface reflectance (Bottom of Atmosphere, BOA) represents the true reflectivity of the Earth's surface after atmospheric correction. Top of Atmosphere (TOA) reflectance includes atmospheric effects. Sentinel-2 L2A products provide surface reflectance values. Reflectance values are used to calculate vegetation indices and perform quantitative analysis.

The size of the smallest feature that can be detected in satellite imagery, typically expressed as the ground distance represented by one pixel.

Higher resolution (smaller pixel size) reveals more detail but covers less area. Sentinel-2: 10m (visible/NIR), 20m (red-edge/SWIR), 60m (atmospheric). Sentinel-1: 10m (IW mode). VIIRS: ~500m. Resolution trade-offs affect revisit time, data volume, and processing requirements. "Resolution" also refers to spectral (number of bands), temporal (revisit frequency), and radiometric (bit depth) properties.

A data layer that identifies cloud-covered pixels in optical satellite imagery.

Clouds obstruct the view of Earth's surface in optical imagery. Cloud masks classify each pixel as clear, cloudy, cloud shadow, or other categories. Sentinel-2 includes the Scene Classification Layer (SCL) with cloud/shadow detection. Cloud masks are essential for time-series analysis and composite generation. SAR imagery is not affected by clouds.

The geographic boundary of a satellite image scene, typically represented as a polygon.

Footprints show the spatial extent of available imagery. Off-Nadir Delta displays search result footprints on the map, allowing you to see coverage and select specific scenes. Footprints may overlap, and the same location may have multiple images from different dates or orbit passes.

A single satellite image acquisition covering a specific geographic area at a specific time.

Also called a "granule" or "tile" depending on the data provider. Sentinel-2 scenes are organized into 100×100 km tiles in the Military Grid Reference System (MGRS). Each scene has associated metadata including acquisition time, cloud cover percentage, and processing level. Multiple scenes are often mosaicked for large-area analysis.

A granular noise pattern inherent to SAR imagery caused by coherent interference of radar waves.

Speckle appears as a salt-and-pepper pattern and can obscure fine details. Multi-looking (averaging multiple observations) reduces speckle but decreases resolution. GRD products are multi-looked for reduced speckle. Speckle filters can be applied for further noise reduction.

The time interval between successive observations of the same location by a satellite.

Sentinel-1 constellation has a 6-day exact repeat cycle. Sentinel-2 has a 5-day revisit at the equator (with both satellites). VIIRS provides daily coverage. Shorter revisit times enable more frequent monitoring but may reduce spatial resolution.

The width of the area imaged by a satellite sensor in a single pass.

Sentinel-1 IW mode has a 250 km swath width. Sentinel-2 has a 290 km swath. Wider swaths provide more coverage per orbit but may have lower resolution at swath edges. Trade-off exists between swath width and spatial resolution.

Processing to remove the effects of atmospheric scattering and absorption from satellite imagery.

Sentinel-2 Level-2A products are atmospherically corrected, providing surface reflectance values instead of top-of-atmosphere radiance. This correction is essential for quantitative analysis and time-series comparisons. SAR is less affected by atmosphere due to microwave wavelength.