Synthetic Aperture Radar, or SAR, is an incredible technology used to create detailed images of the Earth
It works day or night and doesn’t care about the weather – rain, clouds or sun, it can “see” everything the same way.
What is SAR?
SAR is a type of radar that sends energy waves towards the Earth and then captures what “returns” after those waves hit things like mountains, forests, rivers or even ice.
Unlike regular cameras, which rely on sunlight to take pictures, SAR creates its own images using those energy waves.
That’s why it’s great for studying places like Antarctica (to see icebergs), tracking oil spills in swamps or mapping wetlands like those in Alaska.
Why “synthetic”?
The name “synthetic” comes from a pretty clever idea.
To get really sharp images with a radar, you would need a huge antenna – imagine an antenna that was over 4,000 meters long, bigger than dozens of football fields put together! That’s impossible to put on a satellite.
So scientists came up with a way to use a small antenna, but combine several measurements from it as the satellite moves.
It’s like they “pretend” to have a huge antenna, which gives super-detailed images.
How does SAR measure things?
SAR calculates the distance between the satellite and the ground by measuring how long it takes the waves to go back and forth.
This distance is called the “slant range”.
When projected onto the ground, it becomes the “ground range”.
The path the satellite follows is called the “flight direction” or “azimuth”, and the direction perpendicular to this path is the “range direction”.
Two important angles are
– Appearance angle: the angle between the radar and the lowest point below it (called the nadir).
– Angle of incidence: the angle between the radar signal and the ground surface.
– These angles help to understand how the waves “hit” the ground and bounce back, which can create shadows or dark areas in the images, depending on the type of terrain.
Frequency and wavelength: what does this mean?
Unlike regular cameras, which capture visible or infrared light, SAR uses longer waves, measured in centimeters or meters.
This allows it to “see” through clouds or even treetops.
These waves are divided into “bands”, such as X, C, L and P, each with a different size:
– Band X (3 cm): only “sees” the tops of trees.
– Band L (23 cm): can “go deeper” into forests and see branches and trunks.
The size of the wave changes what the radar can capture.
For example, larger waves penetrate deeper into forests, soil or even ice.
This is so useful that scientists have used SAR to find ancient cities hidden under sand or vegetation!
Polarization – how the waves “dance”
SAR waves can be sent and received in different directions, such as horizontal (H) or vertical (V).
This is called polarization.
For example:
– HH: wave sent and received horizontally.
– VH: sent vertically and received horizontally.
These combinations show different things
– Ground or water reflects more in VV.
– Leaves and branches show up better in VH or HV.
– Buildings or logs reflect well in HH.
The type of wave (short or long) also changes what you see, because larger waves “go” deeper into the ground.
Resolution and “snow”
Resolution is how well the radar can separate small things on the ground.
The better the resolution, the more detail you can see.
But sometimes the images have “speckle” in them, which is like noise caused by very small objects that the radar can’t separate well.
This noise can be reduced with special techniques.
Interferometry – measuring changes
SAR can also measure changes in the terrain with a technique called InSAR.
By comparing two images of the same place at different times, it detects whether the ground has risen or fallen – sometimes to within a centimetre! This is used to study earthquakes or volcanoes.
Where is the data?
There are several SAR satellites, such as Sentinel-1 and RADARSAT, that provide data to scientists and researchers.
Some places, such as the Alaska Satellite Facility, even offer ready-to-use images without much extra work.
In short, SAR is like a super eye in the sky: it sees what normal cameras can’t, creates detailed images and helps us better understand our planet, from forests to hidden ruins!
The table below notes the SAR bands with their associated frequency and wavelength, along with typical applications for that band.
Data Availability
| Software | Developer | Analysis Type | Applicable Platforms |
|---|---|---|---|
| Sentinel Application Platform (SNAP) Sentinel 1 Toolbox (S1TBX) |
ESA (European Space Agency) | A graphical user interface (GUI) used for both polarimetric and interferometric processing of SAR data. Start to finish processing includes algorithms for calibration, speckle filtering, coregistration, orthorectification, mosaicking, and data conversion. |
|
| pyroSAR | John Truckenbrodt, Friedrich-Schiller-University Jena / Deutsches Zentrum German Aerospace Center |
A Python framework for large-scale SAR satellite data processing that can access GAMMA and SNAP processing capabilities. Specializes in the handling of acquisition metadata, formatting of preprocessed data for further analysis, and options for exporting data to Data Cube. | Sentinel and various past and present satellite platforms |
| Generic Mapping Tools Synthetic Aperture Radar (GMTSAR) |
ConocoPhillips, Scripps Institution of Oceanography, and San Diego State University | GMTSAR adds interferometric processing capabilities to Generic Mapping Tools (GMT), command line tools used to manipulate geographic data and create maps. GMTSAR includes two main processors: 1. an InSAR processor that can focus and align stacks of images, maps topography into phase, conducts phase unwrapping, and forms complex interferograms, and 2. a postprocessor to filter the interferogram and create coherence, phase gradient, and line-of-sight displacement products. |
|
| Delft object-oriented radar interferometric software (DORIS) |
Delft University of Technology | Interferometric processing from single look complex (SLC) to complex interferogram and coherence map. Includes geocoding capability, but does not include phase unwrapping. | Single Look Complex data from ERS, ENVISAT, JERS, RADARSAT |
| Statistical-Cost, Network-Flow Algorithm for Phase Unwrapping (SNAPHU) | Stanford Radar Interferometry Research Group | Software written in C that runs on most Unix/Linux platforms. Used for phase unwrapping (an interferometric process). The SNAPHU algorithm has been incorporated into other SAR processing software, including ISCE. | Input data is interferogram formatted as a raster, with single-precision (float, real*4, or complex*8) floating-point data types |
| Hybrid Pluggable Processing Pipeline (HyP3) |
Alaska Satellite Facility | Online interface for InSAR processing, including steps such as phase unwrapping (using the Minimum Cost Flow algorithm). Includes access to some GAMMA and ISCE processing capabilities for interferometry. Also includes Radiometric Terrain Correction (RTC) and change detection tools. | Dependent on process |
| InSAR Scientific Computing Environment (ISCE) |
NASA’s Jet Propulsion Laboratory and Stanford University | Interferometric processing packaged as Python modules. Interferometric processing from raw or SLC to complex interferogram and coherence map. Includes geocoding, phase unwrapping, filtering, and more. |
|
| MapReady | Alaska Satellite Facility | A GUI used to terrain-correct, geocode, and apply polarimetric decompositions to multi-polarimetric SAR (PolSAR) data. | ALOS Palsar and other older datasets in ASF’s catalog (SNAP S1TBX recommended for Sentinel-1 datasets) |
| Python Radar Analysis Tools (PyRat) |
Andreas Reigber | A GUI implemented in Python for post-processing of both airborne and space-based SAR imagery. Includes various filters, geometrical transformations and capabilities for both interferometric and polarimetric processing. | Airborne and space-based SAR data |
| Polarimetric SAR data Processing and Education Toolbox (PolSARpro) |
ESA | A GUI for high-level polarimetric processing. Includes analysis capabilities for PolSAR, PolinSAR, PolTomoSAR, and PolTimeSAR data, including functionalities such as elliptical polarimetric basis transformations, speckle filters, decompositions, parameter estimation, and classification/segmentation. Includes a fully polarimetric coherent SAR scattering and imaging simulator for forest and ground surfaces. |
Supports upcoming missions:
|
The table below lists the SAR instruments that have or are currently producing data, as well as the data parameters.
| Band | Frequency | Wavelength | Typical Application |
|---|---|---|---|
| Ka | 27?40 GHz | 1.1?0.8 cm | Rarely used |
| K | 18?27 GHz | 1.7?1.1 cm | Rarely used |
| Ku | 12?18 GHz | 2.4?1.7 cm | Rarely used |
| X | 8?12 GHz | 3.8?2.4 cm | High resolution SAR (urban monitoring,; ice and snow, little penetration into vegetation cover; fast coherence decay in vegetated areas) |
| C | 4?8 GHz | 7.5?3.8 cm | SAR Workhorse (global mapping, change detection, monitoring of areas with low to moderate penetration, higher coherence); ice, ocean, maritime navigation |
| S | 2?4 GHz | 15?7.5 cm | Increasing use for SAR-based Earth observation and agriculture monitoring (NISAR will carry an S-band channel; expends C-band applications to higher vegetation density) |
| L | 1?2 GHz | 30?15 cm | Medium resolution SAR (geophysical monitoring, biomass and vegetation mapping, high penetration, interferometric SAR [InSAR]) |
| P | 0.3?1 GHz | 100?30 cm | Biomass, vegetation mapping, and assessment. Experimental SAR band. |
Published in 04/02/2025 04h29
Text adapted by AI (ChatGPT / Gemini) and translated via Google API in the English version. Images from public image libraries or credits in the caption. Information about DOI, author and institution can be found in the body of the article.
Reference article:
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