{"id":2228,"date":"2025-04-02T04:29:07","date_gmt":"2025-04-02T04:29:07","guid":{"rendered":"http:\/\/brazilianinstituteoftechnology.com\/?page_id=2228"},"modified":"2025-04-02T04:29:07","modified_gmt":"2025-04-02T04:29:07","slug":"radar-de-abertura-sintetica-sar-explicado-de-forma-simples","status":"publish","type":"page","link":"https:\/\/terrarara.com.br\/en\/?page_id=2228","title":{"rendered":"Synthetic aperture radar (SAR) ? simply explained"},"content":{"rendered":"
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Image by Wikipedia<\/a><\/figcaption><\/figure><\/div>
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\r\n\t#SAR<\/a>\u00a0<\/p>

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\r\n\tSynthetic Aperture Radar, or SAR, is an incredible technology used to create detailed images of the Earth<\/strong><\/p>

It works day or night and doesn’t care about the weather – rain, clouds or sun, it can “see” everything the same way.\r\n<\/p>

\r\nWhat is SAR?\r\n<\/p>

\r\nSAR 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.<\/p>

\r\nUnlike regular cameras, which rely on sunlight to take pictures, SAR creates its own images using those energy waves.<\/p>

\r\nThat’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.\r\n<\/p>

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Geometria das observa\u00e7\u00f5es utilizadas para formar a abertura sint\u00e9tica para o alvo P na posi\u00e7\u00e3o ao longo da via x = 0. Cr\u00e9dito: NASA SAR Handbook.<\/figcaption><\/figure><\/div>
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Why “synthetic”?\r\n<\/p>

\r\nThe name “synthetic” comes from a pretty clever idea.<\/p>

\r\nTo 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.<\/p>

\r\nSo scientists came up with a way to use a small antenna, but combine several measurements from it as the satellite moves.<\/p>

\r\nIt’s like they “pretend” to have a huge antenna, which gives super-detailed images.\r\n<\/p>

\r\nHow does SAR measure things?\r\n<\/p>

\r\nSAR calculates the distance between the satellite and the ground by measuring how long it takes the waves to go back and forth.<\/p>

\r\nThis distance is called the “slant range”.<\/p>

\r\nWhen projected onto the ground, it becomes the “ground range”.<\/p>

\r\nThe path the satellite follows is called the “flight direction” or “azimuth”, and the direction perpendicular to this path is the “range direction”.\r\n<\/p>

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Configura\u00e7\u00e3o de digitaliza\u00e7\u00e3o para um SAR de apar\u00eancia esquerda com uma antena retangular. Cr\u00e9dito: Coert Olmsted, Scientific SAR Guia do Usu\u00e1rio. Instala\u00e7\u00e3o do Alasca SAR, 1993.<\/figcaption><\/figure><\/div>
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Two important angles are<\/b>\r\n<\/p>

\r\n– Appearance angle: the angle between the radar and the lowest point below it (called the nadir).\r\n<\/p>

\r\n– Angle of incidence: the angle between the radar signal and the ground surface.<\/p>

\r\n– 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.\r\n<\/p>

\r\nFrequency and wavelength: what does this mean?\r\n<\/p>

\r\nUnlike regular cameras, which capture visible or infrared light, SAR uses longer waves, measured in centimeters or meters.<\/p>

\r\nThis allows it to “see” through clouds or even treetops.<\/p>

\r\nThese waves are divided into “bands”, such as X, C, L and P, each with a different size:\r\n<\/p>

\r\n– Band X (3 cm): only “sees” the tops of trees.<\/p>

\r\n– Band L (23 cm): can “go deeper” into forests and see branches and trunks.\r\n<\/p>

\r\nThe size of the wave changes what the radar can capture.<\/p>

\r\nFor example, larger waves penetrate deeper into forests, soil or even ice.<\/p>

\r\nThis is so useful that scientists have used SAR to find ancient cities hidden under sand or vegetation!\r\n<\/p>

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O espectro eletromagn\u00e9tico com bandas SAR destacadas.<\/figcaption><\/figure><\/div>
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Polarization – how the waves “dance”<\/b>\r\n<\/p>

\r\nSAR waves can be sent and received in different directions, such as horizontal (H) or vertical (V).<\/p>

\r\nThis is called polarization.<\/p>

\r\nFor example:\r\n<\/p>

\r\n– HH: wave sent and received horizontally.<\/p>

\r\n– VH: sent vertically and received horizontally.\r\n<\/p>

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O espalhamento forte em HH indica uma predomin\u00e2ncia do espalhamento do dobro-salto (por exemplo, vegeta\u00e7\u00e3o stemmy, estruturas humano-criadas), quando o VV forte se relacionar \u00e0 dispers\u00e3o de superf\u00edcie \u00e1spera (por exemplo, terra desencapada, \u00e1gua); as varia\u00e7\u00f5es espaciais na polariza\u00e7\u00e3o dupla indicam a distribui\u00e7\u00e3o de dispersores do volume (por exemplo, vegeta\u00e7\u00e3o e tipos de solo da alto-penetra\u00e7\u00e3o tais como a areia ou outros solos porosos secos). Cr\u00e9dito: NASA SAR Handbook.<\/figcaption><\/figure><\/div>
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These combinations show different things<\/b>\r\n<\/p>

\r\n– Ground or water reflects more in VV.<\/p>

\r\n– Leaves and branches show up better in VH or HV.<\/p>

\r\n– Buildings or logs reflect well in HH.\r\n<\/p>

\r\nThe type of wave (short or long) also changes what you see, because larger waves “go” deeper into the ground.\r\n<\/p>

\r\nResolution and “snow”\r\n<\/p>

\r\nResolution is how well the radar can separate small things on the ground.<\/p>

\r\nThe better the resolution, the more detail you can see.<\/p>

\r\nBut sometimes the images have “speckle” in them, which is like noise caused by very small objects that the radar can’t separate well.<\/p>

\r\nThis noise can be reduced with special techniques.\r\n<\/p>

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Sensibilidade das medi\u00e7\u00f5es de SAR \u00e0 estrutura da floresta e penetra\u00e7\u00e3o no dossel em diferentes comprimentos de onda usados para observa\u00e7\u00f5es de sensoriamento remoto no ar ou no espa\u00e7o da superf\u00edcie terrestre. Cr\u00e9dito: NASA SAR Handbook.<\/figcaption><\/figure><\/div>
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Interferometry – measuring changes<\/b>\r\n<\/p>

\r\nSAR can also measure changes in the terrain with a technique called InSAR.<\/p>

\r\nBy 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.\r\n<\/p>

\r\nWhere is the data?\r\n<\/p>

\r\nThere are several SAR satellites, such as Sentinel-1 and RADARSAT, that provide data to scientists and researchers.<\/p>

\r\nSome places, such as the Alaska Satellite Facility, even offer ready-to-use images without much extra work.\r\n<\/p>

\r\nIn 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!<\/p>

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A deforma\u00e7\u00e3o da superf\u00edcie em Okmok, um vulc\u00e3o nas Ilhas Aleutas, \u00e9 revelada nesta imagem interferom\u00e9trica SAR (InSAR) ERS-2. Cada ciclo de cor representa 2,8 cm de movimento da superf\u00edcie do solo. Cr\u00e9dito: NASAilitis Alaska Satellite Facility Distributed Active Archive Center (DAAC ASF)<\/figcaption><\/figure><\/div>
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\r\nThe table below notes the SAR bands with their associated frequency and wavelength, along with typical applications for that band.\r\n<\/p>

\r\nData Availability<\/b>\r\n<\/p>

\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Software<\/th>\r\nDeveloper<\/th>\r\nAnalysis Type<\/th>\r\nApplicable Platforms<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
Sentinel Application Platform (SNAP) Sentinel 1 Toolbox\u00a0
(S1TBX<\/a>)<\/td>\r\n		ESA (European Space Agency)<\/td>\r\nA 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.<\/td>\r\n\r\n
  • Sentinel-1<\/li>\r\n
  • ERS-1 and 2<\/li>\r\n
  • ENVISAT<\/li>\r\n
  • ALOS PALSAR<\/li>\r\n
  • TerraSAR-X<\/li>\r\n
  • COSMO-SkyMed<\/li>\r\n
  • RADARSAT-2<\/li>\r\n<\/ul><\/td>\r\n<\/tr>\r\n
pyroSAR<\/a><\/td>\r\nJohn Truckenbrodt, Friedrich-Schiller-University Jena \/ Deutsches Zentrum

German Aerospace Center<\/td>\r\n		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.<\/td>\r\nSentinel and various past and present satellite platforms<\/td>\r\n<\/tr>\r\n
Generic Mapping Tools Synthetic Aperture Radar\u00a0
(GMTSAR<\/a>)<\/td>\r\n		ConocoPhillips, Scripps Institution of Oceanography, and San Diego State University<\/td>\r\nGMTSAR 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.<\/td>\r\n\r\n
    \r\n
  • ERS-1\/2<\/li>\r\n
  • Envisat<\/li>\r\n
  • ALOS-1<\/li>\r\n
  • TerraSAR-X<\/li>\r\n
  • COSMOS-SkyMed<\/li>\r\n
  • Radarsat-2<\/li>\r\n
  • Sentinel-1<\/li>
  • ALOS-2<\/li>\r\n<\/ul>\r\n<\/td>\r\n<\/tr>\r\n
Delft object-oriented radar interferometric software\u00a0
(DORIS<\/a>)
\u00a0<\/td>\r\n		Delft University of Technology<\/td>Interferometric processing from single look complex (SLC) to complex interferogram and coherence map. Includes geocoding capability, but does not include phase unwrapping.<\/td>\r\nSingle Look Complex data from ERS, ENVISAT, JERS, RADARSAT<\/td>\r\n<\/tr>\r\n
Statistical-Cost, Network-Flow Algorithm for Phase Unwrapping\u00a0
(SNAPHU<\/a>)<\/td>		Stanford Radar Interferometry Research Group<\/td>\r\nSoftware 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.<\/td>\r\nInput data is interferogram formatted as a raster, with single-precision (float, real*4, or complex*8) floating-point data types<\/td>\r\n<\/tr>\r\n
Hybrid Pluggable Processing Pipeline\u00a0
(HyP3<\/a>)<\/td>\r\n		Alaska Satellite Facility<\/td>\r\nOnline 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.<\/td>\r\nDependent on process<\/td>\r\n<\/tr>\r\n
InSAR Scientific Computing Environment\u00a0
(ISCE<\/a>)<\/td>\r\n		NASA’s Jet Propulsion Laboratory and Stanford University<\/td>\r\nInterferometric processing packaged as Python modules. Interferometric processing from raw or SLC to complex interferogram and coherence map. Includes geocoding, phase unwrapping, filtering, and more.<\/td>\r\n\r\n
    \r\n
  • ALOS<\/li>\r\n
  • ALOS2<\/li>\r\n
  • COSMO_SKYMED<\/li>\r\n
  • ENVISAT<\/li>\r\n
  • ERS<\/li>\r\n
  • KOMPSAT5<\/li>\r\n
  • RADARSAT1<\/li>\r\n
  • RADARSAT2<\/li>\r\n
  • RISAT1<\/li>\r\n
  • Sentinel-1<\/li>\r\n
  • TERRASARX<\/li>\r\n
  • UAVSAR<\/li>\r\n<\/ul>\r\n<\/td>\r\n<\/tr>\r\n
MapReady<\/a><\/td>\r\nAlaska Satellite Facility<\/td>\r\nA GUI used to terrain-correct, geocode, and apply polarimetric decompositions to multi-polarimetric SAR (PolSAR) data.<\/td>\r\nALOS Palsar and other older datasets in ASF’s catalog (SNAP S1TBX recommended for Sentinel-1 datasets)<\/td>\r\n<\/tr>\r\n
Python Radar Analysis Tools\u00a0
(PyRat<\/a>)<\/td>\r\n		Andreas Reigber<\/td>\r\nA 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.<\/td>\r\nAirborne and space-based SAR data<\/td>\r\n<\/tr>\r\n
Polarimetric SAR data Processing and Education Toolbox
(PolSARpro<\/a>)<\/td>\r\n		ESA<\/td>\r\nA 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.<\/td>\r\n\r\n