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Synthetic aperture radar (SAR) is an active imaging radar that sends microwave pulses from a satellite or aircraft and records the echoes to form an image of the ground. Microwaves pass through cloud and smoke and need no sunlight, so SAR images the ground at any hour. Each pixel stores backscatter, the share of energy the surface returns to the antenna. Complex SAR data also keeps the phase of the echo, the point in its wave cycle at which it returns. The European Union's Sentinel-1 mission publishes SAR data free, and analysts use it to map floods, detect ships, and measure subsidence. This article argues that water, buildings, and vegetation return distinct backscatter and coherence, a measure of how similar a pixel's echoes are on two passes. The harbor walkthrough below measures both values on surfaces labeled with Overture map data.
A SAR antenna transmits a microwave pulse to one side of the flight track and records the fraction the surface scatters back. Echo delay fixes each target's position in range, the direction across the track.
Resolution along the track, called azimuth, depends on antenna length. A real antenna fine enough for 10 m pixels from orbit would be kilometers long (NASA Earthdata SAR primer). SAR replaces that antenna with motion. Each target stays inside the beam while the satellite travels a stretch of its orbit. The echo frequency shifts as the satellite passes the target (the Doppler shift). A processor uses that shift to combine every echo from the stretch into one pixel. That stretch of orbit is the synthetic aperture. Moreira and colleagues' tutorial on synthetic aperture radar (2013) derives the math.
Side-looking geometry distorts terrain through foreshortening, layover, and shadow:
Brightness depends on roughness, geometry, and moisture. Calm water reflects the pulse away from the antenna and appears dark. Fields and forests scatter part of it back and appear gray. Walls and the ground beside them form corner reflectors that return a double bounce, so cities, ships, and flooded forests appear bright. Interference between scatterers adds a grainy texture called speckle to every SAR image. Goodman (1976) showed that fully developed speckle has a contrast of one. Averaging M independent looks, separate estimates of the same pixel, reduces it by 1/√M. Lee (1980) introduced a filter that uses the local mean and variance to smooth uniform areas and keep edges.
Longer wavelengths pass through leaves to branches, trunks, and the ground, and shorter ones scatter from the first surface they meet. The band ranges follow NASA Earthdata.
L-band phase stays stable over vegetated ground for longer than C-band phase, so L-band InSAR works in forests where C-band coherence is lost.
Polarization is the orientation of the radar wave's electric field, horizontal (H) or vertical (V). Each channel is named by two letters: the transmit orientation, then the receive orientation. HH and VV are co-polarized: the radar receives in the same orientation it sent. HV and VH are cross-polarized. Rough bare surfaces return most energy in VV and HH. A tree canopy scatters the wave many times and rotates part of it, so cross-pol returns rise with vegetation volume. Double bounce from buildings and flooded trunks is strongest in HH. Sentinel-1 acquires VV and VH over most land, and HH and HV over polar ice.
Polarimetric decompositions split a pixel with all four channels into these scattering mechanisms. Cloude and Pottier (1997) classify pixels by two values computed from the channels' coherency matrix: entropy, how mixed the scattering is, and the mean scattering angle. Freeman and Durden (1998) fit surface, double-bounce, and volume components.
Processing levels, named as for Sentinel-1:
Sentinel-1 measurement layers ship as GeoTIFF, and cloud archives repackage them as Cloud Optimized GeoTIFF. The Alaska Satellite Facility runs HyP3, an on-demand service that produces RTC and InSAR products from Sentinel-1.
SAR and optical imagery measure different physical properties, so Earth observation programs such as Copernicus use both.
Optical sensors such as Landsat, Sentinel-2, and hyperspectral instruments identify materials by spectrum. SAR supplies images on cloudy dates, and its backscatter and phase measure structure and ground motion. Wherobots' Strait of Hormuz analysis counted hulls in Sentinel-2 and reported that "Traffic through the surveyed corridor fell 95%". The post cites a separate study that combined Sentinel-1 radar with AIS, the positions ships broadcast by radio.
Ship detection, threshold flood mapping, and sea ice charting need one scene. Other uses compare two dates by phase (deformation and elevation), coherence (damage), or backscatter (change detection).
Public missions supply SAR data free:
Commercial operators such as ICEYE, Capella Space, and Umbra fly constellations of small X-band SAR satellites and sell tasked spotlight images at sub-meter resolution.
The header image above shows the ports in Sentinel-2 and Sentinel-1. Kellndorfer and colleagues' Global seasonal Sentinel-1 interferometric coherence and backscatter data set (Scientific Data, 2022) processed about 205,000 Sentinel-1 Interferometric Wide scenes from December 2019 to November 2020. The output is seasonal layers in 1 by 1 degree tiles at 3 arcsecond pixel spacing. Each season has mean RTC backscatter in VV and VH and median VV coherence for repeat intervals of 6, 12, 18, 24, 36, and 48 days. It also has the parameters of an exponential model of coherence decay. WherobotsDB reads the tiles in place from a public AWS bucket with RS_FromPath as out-db rasters, rasters left in their files in storage.
RS_FromPath
The analysis joined these sources:
buildings_building
base_water
base_land_cover
base_land_use
The test area, 118.36°W to 118.08°W and 33.68°N to 33.84°N in tile N34W119, holds 64,512 pixels of the summer 2020 layers (June to August). One WherobotsDB query reads both raster files and joins their pixels to two Overture tables. Raster and vector in one engine; most stacks pick one. In the query, RS_BandAsArray and posexplode turn each raster into one row per pixel. ST_PolygonFromEnvelope builds each pixel's footprint. Spatial joins to Overture data then assign a surface type. A pixel's building share is the area of Overture building footprints inside it (ST_Intersection), divided by the pixel's area. A pixel counts as open water when its center falls in an Overture water polygon of class ocean, bay, water, or dock and no building touches it.
RS_BandAsArray
posexplode
ST_PolygonFromEnvelope
ST_Intersection
WITH r AS ( SELECT RS_BandAsArray(RS_FromPath('s3://sentinel-1-global-coherence-earthbigdata/data/tiles/N34W119/N34W119_summer_vv_AMP.tif'), 1) AS vv, RS_BandAsArray(RS_FromPath('s3://sentinel-1-global-coherence-earthbigdata/data/tiles/N34W119/N34W119_summer_vv_COH12.tif'), 1) AS coh ), pix AS ( -- one row per pixel, with its footprint as a polygon SELECT pos, z.vv, z.coh, ST_PolygonFromEnvelope(-119 + (pos % 1200) / 1200.0, 34 - (pos DIV 1200 + 1) / 1200.0, -119 + (pos % 1200 + 1) / 1200.0, 34 - (pos DIV 1200) / 1200.0) AS cell FROM r LATERAL VIEW posexplode(arrays_zip(vv, coh)) t AS pos, z WHERE pos DIV 1200 BETWEEN 192 AND 383 AND pos % 1200 BETWEEN 768 AND 1103 ), bld AS ( -- share of each pixel covered by Overture building footprints SELECT p.pos, SUM(ST_Area(ST_Intersection(p.cell, b.geometry))) / MAX(ST_Area(p.cell)) AS frac FROM pix p JOIN wherobots_open_data.overture_maps_foundation.buildings_building b ON ST_Intersects(p.cell, b.geometry) WHERE b.bbox.xmax > -118.36 AND b.bbox.xmin < -118.08 AND b.bbox.ymax > 33.68 AND b.bbox.ymin < 33.84 GROUP BY p.pos ), water AS ( -- Overture water, clipped to the test area SELECT ST_Intersection(geometry, ST_PolygonFromEnvelope(-118.36, 33.68, -118.08, 33.84)) AS geom FROM wherobots_open_data.overture_maps_foundation.base_water WHERE class IN ('ocean', 'bay', 'water', 'dock') AND bbox.xmax > -118.36 AND bbox.xmin < -118.08 AND bbox.ymax > 33.68 AND bbox.ymin < 33.84 ), wat AS ( SELECT DISTINCT p.pos FROM pix p JOIN water w ON ST_Contains(w.geom, ST_Centroid(p.cell)) ) SELECT CASE WHEN w.pos IS NOT NULL AND COALESCE(b.frac, 0) = 0 THEN 'open water' WHEN b.frac >= 0.25 THEN 'dense buildings' ELSE 'other land' END AS class, COUNT(*) AS pixels, ROUND(20 * LOG10(percentile_approx(p.vv, 0.5)) - 83, 1) AS vv_db_median, ROUND(percentile_approx(p.coh, 0.5) / 100, 2) AS coh12_median, ROUND(AVG(CASE WHEN 20 * LOG10(p.vv) - 83 < -15 THEN 1 ELSE 0 END), 3) AS share_below_minus15db FROM pix p LEFT JOIN bld b ON p.pos = b.pos LEFT JOIN wat w ON p.pos = w.pos GROUP BY 1 ORDER BY pixels DESC
The data set stores both layers as scaled integers, called digital numbers (DN). The query converts them: 20 log10(DN) minus 83 gives γ⁰ in dB, and DN / 100 gives coherence. The query returned:
Joins to Overture land cover and land use split the other land class. The 6, 24, and 48 day layers show coherence decay over time:
Medians per class:
Each class matches scattering physics:
A second query bins VV backscatter in 1 dB steps for water and land. Of 23,637 open-water pixels, 20,291 fall below -15 dB. Of 40,875 land pixels, 507 (1.2%) do. The 448 water pixels at -5 dB or brighter are consistent with the ships at anchor in San Pedro Bay and the harbor structures.
That split is the starting point of SAR flood mapping. On a post-event scene, RS_MapAlgebra applies the same threshold to mark new water. RS_ZonalStats summarizes that water mask under each Overture transportation segment. The SpatioTemporal Asset Catalog (STAC) is a standard for imagery catalogs. The STAC reader loads scenes from an imagery catalog, one row per scene. The AWS Glue Data Catalog announcement applies this to supply chain disruption. It joins Sentinel-1 imagery and National Oceanic and Atmospheric Administration (NOAA) HRRR (High-Resolution Rapid Refresh) weather forecasts to the Overture road network to score each corridor. RasterFlow (Public Preview) builds mosaics from imagery, runs computer vision inference on them, and writes the results as vectorized outputs on the same platform.
Language models were trained on text, documents, databases, and the internet, which hold no record of last week's flood. The Model Context Protocol (MCP) is a protocol that lets AI agents call data tools. Through the Wherobots MCP Server, an AI agent can run a query like the one above and return the closed road segments.
Carl Wiley of Goodyear proposed in 1951 that the Doppler shifts of radar echoes could sharpen a side-looking radar's resolution along its track. He later described the principle in "Synthetic aperture radars: a paradigm for technology evolution" (IEEE Transactions on Aerospace and Electronic Systems, 1985). The first civilian SAR in orbit flew on NASA's Seasat in 1978. Its L-band radar imaged the oceans for about three months before a power failure.
Graham proposed SAR interferometry for topographic mapping in 1974 (Proceedings of the IEEE). Goldstein, Zebker, and Werner solved two-dimensional phase unwrapping (Radio Science, 1988), the step that turns wrapped phase, which repeats every wave cycle, into height. Gabriel, Goldstein, and Zebker (1989) introduced differential interferometry, which removes topography and leaves surface motion of a centimeter or less. Rosen and colleagues (Proceedings of the IEEE, 2000) wrote the standard review.
Every InSAR method needs coherence. Zebker and Villasenor (1992) separated the sources of decorrelation: baseline geometry, thermal noise, and change in the surface between passes. The port table measures the last of those directly.
Single interferograms carry atmospheric noise. Ferretti, Prati, and Rocca (2001) picked out permanent scatterers, points such as building corners that stay coherent for years, and estimated millimeter-scale motion at each one. Berardino, Fornaro, Lanari, and Sansosti (2002) built the small baseline subset (SBAS) method, which combines many interferograms with short orbital separations to recover deformation over distributed targets. Both depend on stable urban phase like the 0.51 coherence the port buildings keep after 48 days.
Backscatter change detection compares two dates with a log ratio. Speckle multiplies the signal, and the logarithm turns it into addition. Bazi, Bruzzone, and Melgani (2005) set the threshold on the log-ratio image automatically from a generalized Gaussian model. For floods, Wagner and colleagues (Remote Sensing of Environment, 2026) review the scientific challenges of the fully automatic Copernicus Global Flood Monitoring service. Hard cases for any threshold include floods under vegetation and in cities, where double bounce brightens floodwater, and dry sand, which can be as dark as water.
Zhu and colleagues (2021) review deep learning for SAR and the pitfalls that speckle, side-looking geometry, and complex values create for methods designed for optical images. Labeled data sets set the benchmarks: Sen1Floods11 (Bonafilia and colleagues, 2020) for floods, and xView3-SAR (Paolo and colleagues, 2022) for dark vessels. SSL4EO-S12 (self-supervised learning for Earth observation with Sentinel-1 and Sentinel-2; Wang and colleagues, 2023) pairs Sentinel-1 and Sentinel-2 patches from 251,079 locations for self-supervised pretraining. It is a basis for geospatial AI foundation models that read radar and optical data together.
A SAR scene is a raster, and the decisions it supports are about vectors: which buildings sit in the flood, which road segments closed, which terminals slowed down. Answering them takes a distributed spatial join of millions of radar pixels to millions of features. Jia Yu, Jinxuan Wu, and Mohamed Sarwat introduced GeoSpark (ACM SIGSPATIAL 2015) for spatial data processing on Apache Spark. Yu, Zongsi Zhang, and Sarwat described its partitioning, indexing, and join design in Spatial data management in Apache Spark (GeoInformatica, 2019). GeoSpark became Apache Sedona, which runs the pixel-to-polygon joins in the port analysis. WherobotsDB is 100% code compatible with Sedona across all spatial functions.
Analysis-ready formats make those joins repeatable. CEOS Normalised Radar Backscatter fixes the calibration. The STAC SAR extension records polarizations, frequency band, and instrument mode, so a query can filter scenes on catalog metadata before reading any pixel.
At the ports, SAR backscatter and coherence separated water, buildings, industrial yards, and vegetation.
Join SAR flood extents and ship detections to roads and buildings with a Wherobots free trial at cloud.wherobots.com.
In remote sensing, SAR stands for synthetic aperture radar. It is an imaging radar on a satellite or aircraft. It combines the echoes recorded along its path as if one much longer antenna had recorded them, which gives sharp microwave images of the ground.
SAR data is the imagery a synthetic aperture radar produces. Each pixel holds backscatter, the strength of the microwave echo from that patch of ground. Complex products also keep the phase of the echo, the point in its wave cycle at which it returns. Common SAR data products are Single Look Complex (SLC), Ground Range Detected (GRD), terrain-corrected backscatter, and interferograms.
A SAR antenna sends microwave pulses to one side of its flight path and records the returning echoes. Echo timing places each target across the track. As the platform moves, the same target returns echoes from many positions. Processing combines them into one sharp pixel, as if a single antenna as long as that stretch of the path had recorded them.
SAR is used for flood mapping, ship and oil spill detection, and measuring ground deformation from earthquakes, volcanoes, and subsidence. Other uses are sea ice charting, crop and forest monitoring, and damage mapping after disasters. Its main advantage is that it collects images at night and through cloud.
Synthetic aperture radar supplies its own illumination, so it images day and night, and its microwaves pass through cloud, smoke, and haze. Its echoes change with surface roughness, moisture, and structure. Comparing the phase of two passes, called interferometry, measures ground movement of centimeters or less from orbit.
Both are active sensors that measure echoes of their own signal. SAR uses microwaves with wavelengths of centimeters to a meter, images wide swaths from orbit, and works through cloud. LiDAR uses laser light, measures precise heights as a point cloud, and needs a clear line of sight, so clouds block it.
Bright pixels mean strong backscatter, and dark pixels mean weak backscatter. Calm water and smooth pavement appear dark because they reflect energy away from the sensor. Rough ground scatters energy back toward the sensor and appears brighter. Vegetation scatters energy inside its canopy and appears mid-gray. Buildings, ships, and flooded forest appear brightest, because the pulse bounces off a wall or trunk and the ground beside it straight back to the sensor. Comparing images from two dates shows change.
Open SAR data is free. Copernicus distributes Sentinel-1 data through the Copernicus Data Space Ecosystem. NASA’s Alaska Satellite Facility DAAC distributes Sentinel-1 data and NISAR L-band data, and ISRO releases NISAR S-band data through its Bhoonidhi portal. Commercial SAR imagery from ICEYE, Capella Space, and Umbra is licensed.
Carl Wiley of Goodyear proposed the principle in 1951: using the Doppler shifts of radar echoes to sharpen resolution along a side-looking radar’s flight path. The first civilian SAR satellite was NASA’s Seasat, launched in 1978, which carried an L-band radar.
Coherence is a value from 0 to 1 that measures how similar the radar echoes from the same pixel are on two passes. Stable surfaces such as buildings stay coherent, and water, moving vegetation, and changed ground lose coherence. At the Ports of Los Angeles and Long Beach, summer 2020 Sentinel-1 data gives a median 12-day coherence of 0.57 over dense buildings. It is 0.35 over vegetation and 0.03 over open water.
VV means the radar transmits and receives vertically polarized waves, and VH means it transmits vertical and receives horizontal. VV is stronger over rough surfaces and water waves. VH comes mostly from scattering inside vegetation canopies, which rotates part of the wave. The VH to VV ratio therefore tracks vegetation. Sentinel-1 collects both over most land.
Calm water reflects the radar pulse away from the sensor, so it appears dark. A backscatter threshold separates it from land. At the Ports of Los Angeles and Long Beach, 85.8% of open-water pixels fall below -15 dB in VV, against 1.2% of land pixels. Flood mapping applies the same threshold, or a change test against a dry reference scene, to a post-event image.
Synthetic aperture radars are imaging radars carried on satellites, aircraft, and drones. Each one sends microwave pulses to the side of its path. It combines the echoes recorded along the path as if a much longer antenna had recorded them, which gives fine resolution from far away. Sentinel-1, NISAR, RADARSAT, TerraSAR-X, and the ICEYE, Capella Space, and Umbra constellations are synthetic aperture radars in orbit today.
Synthetic aperture radar is a radar camera. It sends its own microwave signal and records the echo, so it takes pictures at night and through cloud. Moving the radar along its path makes a small antenna act like a much larger one, which sharpens the image. NISAR, the joint NASA and ISRO satellite launched from India in July 2025, is one example.
Well-known examples include the InSAR map of ground movement from the 1992 Landers earthquake in California and damage maps made within days of the 2015 Gorkha earthquake in Nepal. Others are Sentinel-1 flood maps from the Copernicus Emergency Management Service, dark vessel detection in the xView3-SAR benchmark, and the TanDEM-X global elevation model behind the Copernicus DEM.
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