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Introduction to Synthetic Aperture Radar (SAR)
- 1. Center for Research and Application for Satellite Remote Sensing Yamaguchi University Introduction to Synthetic Aperture Radar (SAR)
- 2. • Remote Sensing and microwave remote sensing • Side looking radar • Real aperture radar • Synthetic aperture radar (SAR) • SAR geometry • SAR imaging • SAR parameters • Interpretation • Status and specification • Advantage and disadvantages 2 Contents
- 3. • Remote Sensing is defined as the science and technology by which characteristics of objects of interest can be identified without direct contact. • Microwave remote sensing uses electromagnetic radiation with a wavelength between 1 cm and 1 m (commonly referred to as microwaves) as a measurement tool https://upload.wikimedia.org/wikipe dia/commons/3/30/EM_spectrumre vised.png 3 Introduction to microwave remote sensing
- 5. • Real Aperture Radar (RAR) was the first active sensor to produce imagery of the terrain from backscattered microwave radiation. • It transmits a narrow angle beam of pulse radio wave in the range direction at right or left angles to fight direction (Azimuth direction) and receives the backscattering from the targets which will be transformed to a radar image from the received signals. 5 Side Looking Radar
- 6. The resolution in the range direction depends on the pulse width. Azimuth resolution is determined by the beam width. 6
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- 14. Radar Pulse Propagation Distance = Time/2 14
- 15. • Synthetic Aperture Radar (SAR) synthetically increase the antenna’s size or aperture to increase the azimuth resolution. It is complicated data processing of received signals and phase from moving targets with a small antenna. • Use radio wave : Microwave • Frequency : L, C, X-bands • Satellite emits microwave and observes reflection signal 15 Synthetic Aperture Radar
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- 20. SAR imaging azimuth ERS-1 © ESA SAR Raw Data azimuth range ERS-1 © ESA Focused SAR Data Range Compression 20
- 22. • System parameters • Frequency • Polarization • Look angle and look direction • Surface parameters • Surface roughness • Dielectric constant (moisture content, etc) • Topography (incidence angle) *** Parameters affect Radar Backscattering coefficient Characteristic of Synthetic Aperture Radars
- 23. Wavelength (λ) and frequency (f) are inter-related with light velocity (c = 3x108 m/s) and the conversion of frequency to wavelength is accomplished in equation below http://eijournal.com/print/articles/discover-the-benefits-of-radar-imaging 23 Frequency
- 24. Characteristics of backscatter corresponding to the frequency http://www.infoterra.es/asset/cms/ima ge/samples/radar02big.jpg http://www.jspacesystems.or.jp/ersdac/ Projects/JERS1/JSAR/images/fuji.jpg JERS-1 (L band) TerraSAR-X http://www.scielo.br/img/revistas/bcg/v18n4/a01fig2.jpg RADASAT (C band) and PALSAR (L band) 24
- 25. Transmitted and received radar signals propagate in a certain plane describes the orientation of the electric (E) vector in an electromagnetic wave, frequency “horizontal” (H) or “vertical” (V) in conventional imaging radar systems. There are three different levels of polarization complexity: • Single polarized: HH or VV or HV or VH • Dual polarized: HH and HV, VV and VH, or HH and VV • Four polarizations: HH, VV, HV and VH HH HV VH VV http://envisat.esa.int/handbooks/asar/CNTR1-1-5.html H H Transmit Receive 25 Polarizations
- 26. HH HV VH VV Sample data : ALOS PLR mode from http://www.alos-restec.jp/en/staticpages/index.php/service-sampledata-04 26 Polarizations
- 27. HH HV VH VV Sample data : ALOS PLR mode from http://www.alos-restec.jp/en/staticpages/index.php/service-sampledata-04 Sensitive Sensitive Water is dark Water is dark High backscattering High backscattering Low backscattering Low backscattering 27 Polarizations
- 30. Saturated soil →a high dielectric →strong reflector. http://www.geog.ucsb.edu/~jeff/115a/remote_sensing/radar/radarroughness.jpg Dry soil →low dielectric constant →little radar energy will be reflected. Dielectric constant depends on the type of material as well as its moisture state 30 Dielectric constant
- 37. http://step.esa.int/docs/tutorials/S1TBX%20SAR%20Basics%20Tutorial.pdf F F 37 Radar foreshortening
- 38. http://step.esa.int/docs/tutorials/S1TBX%20SAR%20Basics%20Tutorial.pdf L Low backscattering 38 Radar layovering
- 39. http://step.esa.int/docs/tutorials/S1TBX%20SAR%20Basics%20Tutorial.pdf 39 Ground range Slant range
- 40. http://envisat.esa.int/handbooks/asar/CNTR1-1-5.html 40 SAR images obtained with different incidence angles
- 41. ALOS sample image from http://www.alos-restec.jp/en/staticpages/index.php/service-sampledata-17 41 Slant range to ground range
- 42. Differences of multi-spectral and SAR Optical Sensor SAR Platform Radiation Spectrum Frequency Polarimetry Interferometry Acquisition time Weather Airborne/spaceborne Airborne/spaceborne Reflected sunlight Own radiation Visible/infrared Microwave Multi-frequency Single/Multi-frequency N.A. N.A. Day time Blocked by clouds See through clouds Day/night Polarimetric phase Polarimetric phase 42
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- 45. Status of SAR Satellites 45 SAR sensors 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 ERS-2 RADARSAT-1 ENVISAT ASAR ALOS PALSAR TerraSAR-X RADARSAT-2 COSMO-SkyMed RISAT-2 RISAT-1 HJ-1C Komsat-5 PAZ ALOS-2 SENTINEL 1a SENTINEL 1b SAOCOM 1A SAOCOM 1B Radarsat Constellation 1/2/3 X band (2.4-3.75 cm.) C band (3.75 -7.5 cm.) S band (7.5-15 cm.) L band (15-30 cm.) 11 24 15 25 25 4 28 11 14 12 12 16 16 4 Repeat day
- 46. ALOS-2 Modes Spotlight Strip Map Scan SAR Nominal Resolution Spotlight 1-3 m Strip Map 3-10 m ScanSAR: 100 m Swath width Spotlight: 25 km Strip Map: 50-70 km ScanSAR: 350 or 490 km Band L- Band Polarisation SP: HH or VV or HV DP: HH+HV or VV+VH FP: HH+HV+VH+VV Nominal Look Direction Left or Right Incidence angles between 8 and 70 degrees The PALSAR-2 aboard ALOS-2 is an L-band Synthetic Aperture Radar(SAR) sensor, a microwave sensor that emits L-band radio waves and receives their reflection from the ground to acquire information 46
- 47. ALOS-2 products Level Definition Format Level 1.1 This is complex number data on the slant range following compression of the range and azimuth. As one-look data, it includes phase information and will be the basis for later processing. In wide-area mode, image files are created for each scan. CEOS SAR/GeoTIFF Level 1.5 This is multi-look data on the slant range from map projection amplitude data, with range and azimuth compressed. CEOS SAR/GeoTIFF Level 2.1 Geometrically corrected (orthorectified) data using the digital elevation data from Level 1.1. CEOS SAR/GeoTIFF Level 3.1 Image quality-corrected (noise removed, dynamic range compressed) data from Level 1.5. CEOS SAR/GeoTIFF 47
- 48. Sentinel-1 Sentinel-1 is a two-satellite constellation with the prime objectives of Land and Ocean monitoring by the European Space Agency. The goal of the mission is to provide C-Band SAR data • Orbit Type: Sun-synchronous, near-polar, circular •Inclination: 98.18° • Repeat Cycle: 175 orbits in 12 days 48
- 50. • ALOS-2 • https://auig2.jaxa.jp • Sentinel-1 • https://scihub.copernicus.eu/dhus/#/home 50 Data Hub
- 51. Advantages: • Observe both night and day time • Small dependence on atmospheric conditions such as solar radiation, cloud cover • Controllability of the emitted electromagnetic radiation such as power, frequency, polarization, radiation direction and etc. Disadvantages: • Black and white image • Difficult to understand without http://www.eorc.jaxa.jp/en/hatoyama/etc/images/use/image/rn_12_1.jpg http://www.eorc.jaxa.jp/en/hatoyama/etc/images/use/image/rn_12_2.jpg Advantages and Disadvantages of SAR