A Guide to SAR Data Download and Digital Processing: Algorithms and Implementation

Digital Processing of Synthetic Aperture Radar Data Download

Synthetic aperture radar (SAR) is a powerful remote sensing technique that can provide high-resolution images of the Earth's surface in all weather conditions and day or night. SAR data can be used for various applications, such as land cover mapping, disaster monitoring, urban planning, environmental assessment, and more. However, SAR data are complex and require digital processing to extract useful information from them. In this article, we will explain what SAR is, why digital processing of SAR data is important, how to download SAR data, and what are the basic steps and software tools for digital processing of SAR data.

digital processing of synthetic aperture radar data download

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Introduction

What is synthetic aperture radar (SAR)?

SAR is a type of radar that uses the motion of the sensor (usually mounted on a satellite or an airplane) to create a large synthetic antenna aperture that can produce high-resolution images of the target area. SAR emits microwave pulses that bounce off the ground and return to the sensor, where they are recorded as echoes. The echoes contain information about the distance, angle, and reflectivity of the objects on the ground. By combining the echoes from different positions of the sensor along its orbit, SAR can form a two-dimensional image of the scene.

Why is digital processing of SAR data important?

SAR data are not easy to interpret by human eyes because they are affected by various factors, such as noise, speckle, distortion, interference, and polarization. Therefore, digital processing of SAR data is necessary to enhance the quality, correct the errors, and extract meaningful information from the raw data. Digital processing of SAR data can also enable advanced techniques, such as interferometry and polarimetry, that can reveal more details about the physical properties and changes of the surface.

How to download SAR data?

There are many sources of SAR data available online, depending on the sensor, resolution, coverage, and access policy. Some of the most popular sources are:

• Copernicus Open Access Hub: This is the official portal for accessing the data from the Sentinel-1 satellites operated by the European Space Agency (ESA). Sentinel-1 provides global coverage with C-band SAR at various modes and resolutions. The data are free and open to anyone.

• Alaska Satellite Facility (ASF): This is a NASA facility that distributes data from various SAR sensors, such as ALOS PALSAR, RADARSAT-1, RADARSAT-2, UAVSAR, and NISAR (upcoming). The data are mostly free and open to anyone.

• European Space Imaging (EUSI): This is a commercial provider of very high-resolution SAR data from the TerraSAR-X and TanDEM-X satellites operated by the German Aerospace Center (DLR). The data are available for purchase or through a license agreement.

To download SAR data from these sources, you need to register an account, search for your area of interest, select the data products, and download them to your local computer. The data are usually in the form of compressed files that contain the image and metadata files.

Digital Processing of SAR Data: Basic Steps

Pre-processing

Pre-processing is the first step of digital processing of SAR data, which aims to improve the quality and usability of the data. Pre-processing includes the following sub-steps:

Calibration

Calibration is the process of converting the raw SAR data into physical units, such as radar backscatter or brightness temperature. Calibration also removes the system-induced biases and variations from the data, such as antenna pattern, range spreading loss, and thermal noise. Calibration is essential for ensuring the accuracy and consistency of the data.

Geocoding

Geocoding is the process of assigning geographic coordinates to each pixel of the SAR image, based on the sensor's position, orientation, and imaging geometry. Geocoding also resamples the SAR image from the slant-range coordinate system (where the pixels are aligned along the sensor's line of sight) to the ground-range or map coordinate system (where the pixels are aligned along the north-south and east-west directions). Geocoding is important for facilitating the spatial analysis and comparison of the data.

Speckle filtering

Speckle filtering is the process of reducing the speckle noise from the SAR image, which is caused by the coherent interference of the microwave pulses reflected by multiple scatterers within a resolution cell. Speckle noise reduces the contrast and visibility of the features in the image and affects the accuracy of the information extraction. Speckle filtering applies various algorithms to smooth or average the pixels in the image while preserving the edges and details.

Processing

Processing is the second step of digital processing of SAR data, which aims to extract information and features from the data. Processing includes the following sub-steps:

Interferometry

Interferometry is a technique that uses two or more SAR images acquired from slightly different positions to measure the phase difference between them. The phase difference can be related to the topography or deformation of the surface, depending on the temporal and spatial baseline between the images. Interferometry can generate digital elevation models (DEMs) or displacement maps from SAR data.

Polarimetry

Polarimetry is a technique that uses SAR images acquired with different polarizations to analyze the scattering mechanisms and properties of the surface. Polarization refers to the orientation of the electric field vector of the microwave pulse, which can be horizontal (H) or vertical (V). Polarimetry can use different combinations of transmit and receive polarizations, such as HH, VV, HV, VH, or circular polarizations. Polarimetry can provide more information about the structure, orientation, and dielectric constant of the surface.

Classification

Classification is a technique that uses SAR images to identify and label different types of land cover or objects in the scene. Classification can be supervised or unsupervised, depending on whether prior knowledge or training data are available. Classification can use various features or parameters derived from SAR data, such as backscatter, texture, coherence, polarimetric decomposition, or machine learning algorithms.

Post-processing

Post-processing is the third step of digital processing of SAR data, which aims to visualize, analyze, and export the results of processing. Post-processing includes:

Visualization

Visualization is a technique that uses color coding, contrast enhancement, histogram equalization, or other methods to display the SAR images or products in a more intuitive and appealing way. Visualization can help to highlight the features or patterns in the data and facilitate the interpretation and communication of the results.

Analysis

Analysis is a technique that uses statistical methods, geographic information systems (GIS), or other tools to quantify, measure, or compare the SAR images or products. Analysis can help to evaluate the accuracy, reliability, or significance of the results and provide insights or answers to specific questions or problems.

Export

Export is a technique that uses file formats, compression methods, or metadata standards to save or share the SAR images or products with other users or applications. Export can help to ensure the compatibility, interoperability, or reproducibility of the results.

Digital Processing of SAR Data: Software and Tools

SNAP (Sentinel Application Platform)

SNAP (Sentinel Application Platform)

SNAP is a free and open source software tool developed by ESA for processing, analysis, and visualization of various types of remote sensing data, including SAR data from Sentinel-1 and other sensors. SNAP provides a user-friendly graphical interface that allows users to perform various operations on SAR data, such as calibration, geocoding, speckle filtering, interferometry, polarimetry, classification, and more. SNAP also supports scripting and batch processing for automation and customization of workflows. SNAP can be downloaded from https://step.esa.int/main/download/.

PolSARpro (Polarimetric SAR Data Processing and Educational Tool)

PolSARpro is a free and open source software tool developed by ESA for processing and analysis of polarimetric SAR data. PolSARpro provides a comprehensive set of algorithms and tools for polarimetric decomposition, classification, segmentation, speckle filtering, and more. PolSARpro also includes educational modules and tutorials for learning the basics and applications of polarimetry. PolSARpro can be downloaded from https://earth.esa.int/web/polsarpro/home.

ENVI SARscape (SAR Image Analysis and Interpretation)

ENVI SARscape is a commercial software tool developed by L3Harris Geospatial for processing and analysis of SAR data. ENVI SARscape integrates with ENVI, a popular software for remote sensing image analysis, and provides a seamless workflow for processing SAR data from various sensors, such as Sentinel-1, TerraSAR-X, RADARSAT-2, ALOS PALSAR, and more. ENVI SARscape offers advanced capabilities for interferometry, polarimetry, classification, change detection, and more. ENVI SARscape can be purchased or licensed from https://www.l3harrisgeospatial.com/Software-Technology/ENVI/ENVI-SARscape.

Other software and tools

There are many other software and tools available for digital processing of SAR data, such as GAMMA Remote Sensing Software (https://www.gamma-rs.ch/), ASF MapReady (https://asf.alaska.edu/software-tools/mapready/), Orfeo Toolbox (https://www.orfeo-toolbox.org/), Google Earth Engine (https://earthengine.google.com/), and more. Each software or tool has its own advantages and disadvantages in terms of functionality, usability, compatibility, cost, and support. Users should choose the software or tool that best suits their needs and preferences.

Conclusion

Digital processing of synthetic aperture radar data download is a complex but rewarding task that can provide valuable information and insights about the Earth's surface. In this article, we have explained what SAR is, why digital processing of SAR data is important, how to download SAR data, and what are the basic steps and software tools for digital processing of SAR data. We hope that this article has helped you to understand the basics and benefits of digital processing of SAR data download.

FAQs

• What are the advantages of SAR over optical remote sensing?

SAR has several advantages over optical remote sensing, such as:

• SAR can operate in all weather conditions and day or night, while optical remote sensing is limited by clouds and sunlight.

• SAR can penetrate through vegetation and soil to some extent, while optical remote sensing can only detect the surface reflectance.

• SAR can measure the phase difference between two images to derive elevation or deformation information, while optical remote sensing can only measure the intensity difference.

• SAR can use different polarizations to analyze the scattering mechanisms and properties of the surface, while optical remote sensing can only use different wavelengths.

• What are the challenges of digital processing of SAR data?

Digital processing of SAR data faces several challenges, such as:

• SAR data are large and complex, requiring high computational power and storage capacity.

• SAR data are affected by various sources of noise, distortion, and interference, requiring sophisticated algorithms and methods to correct and enhance them.

• SAR data are sensitive to the sensor's position, orientation, and imaging geometry, requiring accurate metadata and models to geocode and register them.

• SAR data are not intuitive and easy to interpret by human eyes, requiring color coding, contrast enhancement, or other methods to visualize them.

• What are the applications of digital processing of SAR data?

Digital processing of SAR data can enable various applications, such as:

• Land cover mapping: SAR data can be used to classify different types of land cover or objects in the scene, such as forests, crops, buildings, roads, water bodies, etc.

• Disaster monitoring: SAR data can be used to detect and monitor natural or human-induced disasters, such as floods, landslides, earthquakes, volcanoes, oil spills, etc.

• Urban planning: SAR data can be used to map and analyze the urban environment, such as building density, height, structure, layout, etc.

• Environmental assessment: SAR data can be used to assess and monitor the environmental conditions and changes, such as soil moisture, vegetation biomass, deforestation, wetlands, etc.

• How to learn more about digital processing of SAR data?

There are many resources available online for learning more about digital processing of SAR data, such as:

• Courses: There are several online courses that teach the basics and applications of SAR and digital processing of SAR data, such as Synthetic Aperture Radar: Imaging with Radar, Optical Earth Observation: The Big Picture, SAR-EDU, etc.

• Tutorials: There are several online tutorials that demonstrate how to use various software and tools for digital processing of SAR data, such as SNAP Tutorials, PolSARpro Tutorials, ENVI SARscape Tutorials, etc.

• Books: There are several books that provide comprehensive and in-depth knowledge and examples of SAR and digital processing of SAR data, such as Synthetic Aperture Radar Signal Processing with MATLAB Algorithms, Polarimetric Radar Imaging: From Basics to Applications, Synthetic Aperture Radar Polarimetry, etc.

• How to get help or feedback on digital processing of SAR data?

There are many online communities and forums where you can get help or feedback on digital processing of SAR data, such as:

• STEP Forum: This is the official forum for SNAP users and developers, where you can ask questions, report issues, share ideas, or find solutions related to SNAP and other ESA tools.

• GIS Stack Exchange: This is a Q&A site for cartographers, geographers, and GIS professionals, where you can ask questions or answer questions related to GIS and remote sensing topics.

• ResearchGate: This is a social network for researchers and scientists, where you can follow topics, join groups, post questions or answers, or find publications related to SAR and digital processing of SAR data.

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