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What is the difference between SAR and InSAR?

In geodesy, earth observation and geospatial analysis, synthetic aperture radar (SAR) and interferometric synthetic aperture radar (InSAR) are both powerful remote sensing technologies. Both use RADAR signals and are based either from an aircraft or satellite, but there are key differences between them in terms of objective, data processing and application.


This article outlines the differences between SAR and InSAR, how the technologies work and when you should use one over the other. It will therefore serve as a practical reference for GIS Professionals, researchers and Earth Scientists.


Difference between SAR and InSAR
Difference between SAR and InSAR

What Is SAR (Synthetic Aperture Radar)?


SAR (Synthetic Aperture Radar) is/has a remote sensing system that uses active microwave radar signals to visualize the earth’s surface. 


Conversely to optical sensors, SAR Systems transmit their own energy and record backscattered signals; therefore, SAR operates regardless of time of day, as well as different weather conditions such as cloudy skies, rain, and snow! 


How SAR Works


SAR Sensors (on satellite or aircraft):


  1. Emit microwave pulses towards the earth

  2. Record the reflected signals from the earth’s surface

  3. Use the platform’s motion to create an artificial antenna (synthetic aperture)

  4. Create high quality radar images.


Key Features of SAR


  1. Has an active sensing technology

  2. Can work in all weather, at any time of day or night

  3. Is sensitive to surface conditions such as; roughness, moisture content, and whether there are structures on the surface

  4. Produce images of the varying levels of radar intensity (grayscale).


Common Uses of SAR


  1. Land use & land cover mapping

  2. Flood & disaster monitoring

  3. Forest areas – biomass estimates and the structure of trees

  4. Ocean surfaces – oil spill detection

  5. Military reconnaissance/surveillance


What Is InSAR (Interferometric Synthetic Aperture Radar)?


Interferometric Synthetic Aperture Radar (InSAR) is an innovative method of measuring ground movement and elevation changes by combining information from at least two different Synthetic Aperture Radar (SAR) images of the same area taken at different times or angles.


In contrast to SAR imaging that relies on the radar intensity to determine ground properties, InSAR is able to derive ground properties from comparing the change in phase between two SAR radars that are measuring the same piece of ground.


How InSAR Is Performed


  1. Two SAR images of the same ground are acquired at two different times

  2. The change in the two radar image is the difference in the phase from the two SAR radars, called the interferogram.

  3. By calculating the phase difference of the two SAR radars, the change in either the displacement of the ground or change in the topographic height of the ground is determined with millimeter accuracy/ precision.


InSAR Characteristics


  1. Requires at least two SAR image acquisitions

  2. Can measure both ground deformation and ground elevation

  3. Can achieve high vertical accuracy (millimetric to centimetric)

  4. Can be influenced by atmospheric effects or temporal decorrelation


InSAR Applications


  1. Earthquake deformation studies

  2. Volcanic monitoring

  3. Land subsidence or uplift detection

  4. Glacier or ice sheet movement

  5. Infrastructure stability (bridges, dams, buildings)


Key Differences Between SAR and InSAR

Feature

SAR

InSAR

Full Name

Synthetic Aperture Radar

Interferometric Synthetic Aperture Radar

Data Requirement

Single radar image

Two or more SAR images

Primary Output

Radar intensity image

Interferogram and deformation map

Main Purpose

Imaging and surface characterization

Measuring elevation and surface displacement

Accuracy

Meter-level spatial resolution

Millimeter-level vertical accuracy

Processing Complexity

Moderate

High

Sensitivity

Surface roughness, moisture

Phase stability, atmospheric effects

SAR vs InSAR: When to Use Which?


Use SAR for:


  • Obtaining maps of large regions

  • Tracking flooding and vegetation

  • Operating in cloudy or dark situations

  • Examining surface texture and backscatter characteristics


Use InSAR for:


  • Calculating changes in land or construction sizes

  • Tracking seismic or volcanic movement

  • Discovering dry land sinking slowly

  • Determining if land structures remain stable over time


In the real world, InSAR is created from SAR data, indicating that SAR provides the basis for InSAR's special analysis methods.


Advantages and Limitations


SAR Benefits


  • Imaging That Works Regardless of Weather

  • Resolution is Very Fine

  • Coverage of a Large Area

  • Can Be Used for Ongoing Monitoring with Assurance


SAR Limitations


  • Images Have Speckle Noise Problems

  • Image Interpretation by Specialist Individuals Will Be Required

  • Limited Elevation/ Displacement Info Provided Directly


InSAR Advantages


  • Deformation Measurement Has Very High Accuracy

  • Geohazards Can Be Monitored Effectively Using InSAR

  • Larger Areas Can Be Monitored with Greater Accuracy than Ground-Based Sensors


InSAR Limitations


  • Stable Surface Conditions Required to Produce Accurate Results

  • Atmospheric Disturbances Can Affect Output Reliability

  • Complex Processing Required to Generate Therequired Outputs from InSAR Data

  • In Shrub/Trees Cover Areas With Vegetation Overhead, Results May Become Unreliable Due to Decorrelated Conditions o Between the Sensor and Ground.


Role of SAR and InSAR in Modern Geodesy


The use of Synthetic Aperture Radar (SAR) and Interferometric Synthetic Aperture Radar (InSAR) is critical in the field of geodesy & geospatial science and supports the following functions:


  • Monitoring changes on the earth's surface or studying the workings of the planet and environment

  • Assisting Global Navigational Satellite Systems (GNSS) and leveling data

  • Creating new Digital Elevation Models (DEMs)

  • Aiding in improving Disaster Risk Management


These systems allow for the accurate, global, and consistent measurement of data, thus fulfilling the mission of GeoWGS84.com when used with coordinate reference systems such as the World Geodetic System 1984 (WGS84).


While there are some similarities between the two technologies, there are fundamental differences that distinguish the two methods in their application and processing methods.


Specific to application, the primary objective of SAR is to capture images of the Earth's surface while the primary objective of InSAR is to extend the use and capabilities of SAR for measuring ground deformation and elevation change with a high level of accuracy.


Geospatial professionals can use this knowledge to select the most appropriate technologies for mapping, monitoring, and conducting research in Earth sciences.


For more information or any questions regarding the SAR and InSAR, please don't hesitate to contact us at


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