Versatile Applications of Synthetic Aperture Radar

With the indigenous development of Radar Imaging Satellites – RISAT-1 and 2 – as well as the upcoming Indo-US joint venture of NASA-ISRO SAR (NISAR), Synthetic Aperture Radar (SAR) has become very popular in the Indian scientific community, as a powerful remote sensing technology to observe and understand the Earth’s surface.

The recent advancements in Synthetic Aperture Radar (SAR) technology have significantly enhanced its capabilities and expanded its applications. Advances in technology have led to the development of systems capable of capturing images with unprecedented detail.

High-resolution SAR images can now be used for various applications, including urban planning, infrastructure monitoring, and environmental assessment.

Advancements in SAR technology have led to the development of compact and lightweight systems that can be deployed on small unmanned aerial vehicles (UAVs) or satellites.

The technology is being increasingly integrated with other remote sensing technologies, such as optical and thermal imaging, to provide a more comprehensive understanding of the Earth’s surface.

This integration allows for the generation of multi-sensor data products that can be used for a wide range of applications. As technology continues to evolve, we can expect further enhancements in SAR capabilities, further expanding its potential applications and impact.

Charting the Way

The debut SAR mission was commenced by NASA in 1973 after launching SEASAT, along with a few other payloads. Thereafter, many airborne and space-borne SAR missions have been launched by various space agencies. One of the major game changers in the applications has been the open-access data products of Sentinel-1.

ISRO at Space Applications Centre developed its first SAR Radar Imaging satellite (RISAT-1) which was launched in 2012. As a follow-on mission to RISAT-1, RISAT-1A, also known as Earth Observation Satellite (EOS-04) launched in 2022, has an indigenously developed, C-band, active array SAR with several features and improvements over its predecessor to meet the user requirements.

With the new geospatial policy announced by Govt of India, Indian Remote Sensing Satellite data of spatial resolution 5m and above has also been made available openly without any cost by the National Remote Sensing Centre, ISRO through its Bhoonidhi portal.

Render of Radar Imaging Satellite RISAT-1 in deployed configuration

Aiding Agriculture

Ministry of Agriculture requires multiple pre-harvest acreage and production estimates of major kharif and rabi crops at various spatial scales, which has been done since the late 80s using remote sensing and ancillary data.

SAR technology has emerged as a powerful tool in agriculture, offering unique capabilities for monitoring crops and improving agricultural practices.

SAR, with its ability to penetrate clouds and operate day and night, provides valuable information that complements traditional optical remote sensing techniques. The reflection (backscatter in SAR) is highly sensitive to the moisture content as well as surface and canopy structure due to changes in dielectric constant and roughness.

One of the key applications of SAR in agriculture is crop monitoring and management. SAR can provide detailed information about crop growth, health, and stress levels. By analyzing that data, farmers can make informed decisions about irrigation, fertilization, and pest control, leading to improved crop yields and resource efficiency.

Another important application of SAR in agriculture is crop mapping and classification. Its data can be used to identify different crop types and monitor their spatial distribution. This information is invaluable for crop inventory, land use planning, and crop yield forecasting. The technology can also be used to monitor land degradation, such as soil erosion and desertification.

By analyzing this data over time, scientists and policymakers can assess the extent of land degradation and implement measures to mitigate its effects.

One of the major game changers in the applications has been the open-access data products of Sentinel-1. As SAR technology continues to evolve, its applications in agriculture are expected to grow, providing farmers with the tools they need to improve crop yields, reduce resource consumption, and adapt to a changing climate.

Forests and Climate

Forests play a very significant role in the global climate. Regular monitoring of forest areas and Above Ground Biomass is useful to manage forest degradation as well as in tracking total carbon storage.

SAR is revolutionizing forestry management by offering a wealth of information that is invaluable for monitoring forests, estimating biomass, detecting deforestation and illegal logging, monitoring forest fires, and assessing forest health.

Its data can provide detailed information about the extent and distribution of forests, allowing forestry agencies to monitor deforestation, degradation, and land use changes over time.

This information is crucial for sustainable forest management and conservation efforts. The technology is also used for estimating forest biomass, which is essential for assessing carbon stocks and monitoring forest health. It can also be used to detect deforestation and illegal logging activities in forests.

When it comes to managing wildlife disasters, SAR is instrumental in detecting and monitoring forest fires, allowing for the detection of heat sources associated with wildfires. This information is crucial for early detection and rapid response to forest fires, helping to minimize their impact on forest ecosystems and nearby communities.

By monitoring these changes over time, one can assess the health of forests and implement measures to mitigate the impact of forest pests and diseases.

Monitoring Earth’s Ice

The cryosphere, comprising of ice and snow, plays a crucial role in Earth’s climate system, affecting global sea levels, weather patterns, and ocean currents. Monitoring the cryosphere is essential for understanding and predicting climate change impacts.

SAR offers unique capabilities for studying ice and snow dynamics with unprecedented detail. The Himalayas, home to some of the world’s highest peaks and most extensive glaciers are a region of immense geological and ecological significance.

Monitoring this region is crucial for understanding climate change impacts, water resource management, and natural hazard mitigation. Glacier mass balance, used as a climate change indicator, requires regular monitoring.

However, due to the tough terrain, quality data collection through ground truth is limited, hence optical as well as SAR-based remote sensing is one of the best options for regular and long-term data collection for the analysis.

SAR has emerged as a vital tool for studying the Himalayas, offering unique capabilities for monitoring its glaciers, landforms, and environmental changes. The technology can provide detailed information about glacier flow rates, ice thickness changes, and glacier terminus positions.

This information is crucial for understanding glacier dynamics and predicting their response to climate change. The Himalayas are also prone to landslides and avalanches, which pose significant risks to human settlements and infrastructure.

It can detect and monitor landslides and avalanches in near real-time, providing early warning systems and valuable information for disaster management and risk assessment.

Disaster Management

SAR plays a crucial role in disaster management, providing valuable information for monitoring, assessing, and responding to various types of disasters worldwide like floods, earthquakes, subsidence, landslides, volcanic activity, oil spills, and cyclones.

Floods are one of the most frequent and vulnerable natural phenomena that occur due to both natural and anthropogenic changes.

Space-borne SAR can provide synoptic coverage and the capability to monitor flood-affected regions repeatedly, their progressions/recessions as well as damage assessment, making it an indispensable tool for the management and mitigation of such disasters.

During the floods, the technology provides detailed information about the extent of flooding, allowing authorities to assess the affected areas and plan evacuation and relief efforts accordingly. Its data is also used to monitor flood dynamics in real time, providing crucial information for flood forecasting and early warning systems.

Disasters like earthquakes are difficult to predict, however, SAR data is extremely useful in monitoring the seismic activities in the earthquake-prone zones by regular observations over those areas and computing the minute deformation through interferometric techniques. SAR technology is also used for earthquake damage assessment by comparing satellite images before and after an earthquake.

Oil Supervision

Oil spill is one of the major threats to the marine environment and the timely detection, monitoring, and mitigation of the threats using the easily scalable system, is one of the most important tasks for the ocean community.

With the increase in largescale developmental activities in the offshore and coastal regions, the chances of oil spills are also becoming very frequent.

Even though they originated in the deeper regions, they spread rapidly with the wind, waves, and currents. The development of sensors as well as techniques in recent times has increased the feasibility of detecting oil spills in real-time and computing their area as well as the direction of spread.

Due to its all-weather and day-night observation capability, SAR has become one of the extremely suitable instruments for oil spill detection tasks.

Oceanic Challenges

Ocean and coastal monitoring are of great scientific interest as well as economic interest. SAR applications for the ocean range from the ocean surface and internal waves, oil spills, currents, high-resolution surface winds, ships and pollutants detection, oil spill detection, underwater topography, coastline and inter-tidal zone classification, etc.

Many coastal and deep ocean activities such as coastal and offshore structure design, ship navigation, and the dissipation of marine pollution have substantial dependencies on ocean waves and wind information. SAR data can be used for all these problems.

Characteristics of the ocean surface and internal waves from SAR data are estimated using spectral analysis with various corrections during the estimation because of the modulation in the SAR image due to the ocean motion as well as imaging phenomena.

Similar to internal waves, other ocean phenomena revealed by SAR include the detection of ocean surface current patterns, eddies, and gyres, by their influence on surface waves.

Monitoring of underwater topography in coastal regions is vital for the exploration and exploitation of non-living and living resources such as operations on engineering structures, ship navigation, dredging, fishing, and laying pipelines/cables.

In addition to these activities, bathymetry plays a vital role in simulating storm surges, ocean currents, and waves in numerical models. The conventional techniques using ships and buoys over the vast stretches of coastal waters though provide data at higher spatial resolution and high accuracy, are found to be expensive and inadequate in terms of coverage.

NISAR’s Outreach

A dual-frequency NASA-ISRO Synthetic Aperture Radar (NISAR) is also planned to be in orbit in the very near future. NISAR, a dual-frequency (L and S band) SAR with an innovative SweepSAR technology with a high repeat cycle of 12 days, will be providing high resolution (2-30 m resolution) with a wide swath (>240 km) with the capability of acquiring full-polarimetric and repeat-pass interferometry.

NISAR has been conceptualized to provide a detailed view of the Earth to observe and measure some of the planet’s most complex processes, including ecosystem disturbances, glaciers, and ice-sheet dynamics, land deformations induced by tectonic and non-tectonic processes, coastal process dynamics and natural hazards.

This powerful and unique mission will produce a wealth of data over the global landmass, ice-covered regions, and oceans around India with enormous application potential.

The huge high-quality data of more than 40Tbits/day data will be transferred to ISRO and NASA ground stations for processing and will be provided to the research as well as user communities for science problems and numerous applications.

One of the major uniqueness of NISAR is the free and open data policy, in which all data will be provided freely to global users. From the Indian perspective, in addition to the science activities, NISAR data will be utilized for various SAR-based applications with free data for several operational project accomplishments.

For the initial as well as routine calibration, it has a comprehensive plan that involves setting up a network of corner reflectors, establishing corner reflectors around Indian research stations in Antarctica, and leveraging SAR calibration sites in the USA in addition to the homogeneously distributed targets.

Synthetic Aperture Radar is a versatile and powerful remote sensing technology with a wide range of applications. From environmental monitoring to disaster management, and urban planning to agriculture, SAR plays a crucial role in our understanding of the Earth’s surface and its changes over time. As SAR technology continues to advance, its applications are likely to expand, further enhancing its importance in modern remote sensing.

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