SAR in the Space Sector Market - Serving Myriad Industries - 18/09/2018

The deployment of SAR in the space sector market has witnessed a tremendous increase lately, particularly due to the challenges in monitoring climate change and other environmental factors.

Environmental remote sensing can indeed be an asset in this regard, providing assistance in keeping track of problems in the atmosphere, land and ocean ecosystems. For this reason, Synthetic Aperture Radar (SAR) technology has acquired a powerful clientele base, as it allows government agencies, defence forces, and private organisations to use high resolution imagery in advanced monitoring systems.

It is therefore no surprise that the improbability of gathering reliable geospatial data in the most challenging of weathers has been countered by the application of SAR in numerous fields of research and remote surveying, providing a substantial impetus to the application of SAR in the space sector industry.

Looking beyond the obvious deployment of this technology in military surveillance satellites, SAR-enabled sensing equipment can provide credibility to geospatial surveys aimed at studying natural land deformations and subsurface characteristics. An uninterrupted flow of information is essential for precise analysis of geological changes that occur over a period of time. Theoretically, SAR captures several images of an area, which are formed by using time delay between the transmission of electromagnetic waves from the satellite and their reception after being reflected back from the target surface over a repeated path. Differential interferometry SAR (InSAR) is a technique gaining immense popularity in the investigation of surface deformities through effective remote sensing, by manipulating the round-trip signal components in SAR derived imaging. It outlines the temporal evolution of surface distortions and can display characteristics of the material that SAR signals interact with.

Consider a situation where a government agency and its scientists are required to continuously monitor the geological displacements occurring in or around an active volcano, since measuring the ground deformations in volcanic areas can give indications regarding the likelihood of an eruption. Using the differential InSAR technique, through sensors mounted on a satellite, eliminates the need for sending any personnel near the volcano to conduct a manual survey. The traditional methods of collecting data manually with the help of fixed point receivers and physical gauging have very limited measurement scope. However, deformation outlay on a colour coded map, delivered by an InSAR equipped satellite, will cover thousands of survey points and give more comprehensive figures. Analysis of minuscule changes in ground movement over a long period of time is also made possible by the differential InSAR technique, using historical data.

Having reviewed the benefit of deferential InSAR through the aforementioned example, one can surmise its potential in mapping surface deformations and producing average velocity charts related to earthquakes, landslides and other natural phenomena. InSAR images are quite definitive and can help to determine subsurface features for identifying potential hazards under man-made structures. The technology is able to detect small elevation changes in ground surface, to the magnitude of 1cm or less, and is proposed to be used in detecting the susceptibility of sinkhole subsidence. Engineering and mining industries can utilise geotechnical reports obtained from differential InSAR analysis to understand the soil quality or the amount of mineral deposits, supplementing their business decisions.

Elaborating on the significance for subsurface geology, a vital application of differential InSAR is the study of groundwater aquifers, necessary for knowing the water flow stability and how to sustain a uniform water supply. Unstable groundwater activities could result in subsidence, due to interaction with subsurface infrastructure in urban areas. By means of InSAR, numerous surface and underground measurements can be correlated for finding out different parameters affecting the aquifer system, while observing long-term and seasonal responses.

Citing an example, the northern Santa Clara Valley in California was the first region in the US where subsidence resulting from excess groundwater extraction was observed. Reports indicate that in the 1960s, around 246,700 cubic metres of groundwater was withdrawn every year to irrigate farms and meet the rapid urbanisation needs. This led to the unprecedented lowering of aquifer levels in the area and since then, surface water has been imported to fulfill the water demand and refresh the groundwater volumes. Using InSAR-derived information a few years ago, displacements showing recoverable seasonal subsidence and uplift of approx 30-40mm were observed.

Furthermore, multi-annual InSAR images helped in building a regional groundwater to surface water flow model of the Santa Clara subsurface basin, calibrated by using historical subsidence and hydro-geologic data of the area. It serves as an essential tool for resource administrators to minimize the effect of permanent subsidence and maximize water supply.

The Copernicus Earth observation programme, a European enterprise, features the Sentinel-1A and 2B satellites which are equipped with InSAR capabilities. In 2017, the outcome of a survey over Australia was published, which disclosed the changes related to groundwater in the Perth Basin. The Sentinel satellites repeat their path over Australia every 12 days, records stated. Applying differential InSAR to images taken at different times by the Sentinel-1 sensors, researchers in Perth were able to map ground displacements all over the greater Perth area and gain information about the regional aquifer withdrawal and other natural processes. The researchers aim to utilise the consistent and regular insights from the remote sensing abilities of the InSAR technology for improving water supply management, quickening emergency response time, and monitoring marine ecology.

In terms of the future prospects of SAR, global leaders are forming collaborations for working towards effective monitoring of climate and resources, so that no problem goes unnoticed. There are programmes in place for planning preventive actions or coping with the aftermath of calamities. As such, experts will need a vast amount of data, focused on identifying formation changes in the surface after an event, made possible by fresh SAR images being compared and analysed with images collected over the years from as many satellites as possible. Mapping the after-effect of an earthquake to find out the total elevation or subsidence in the affected area, and to study the faults for predicting the occurrence of another one, is also an application where governments have successfully made use of the differential InSAR technique.

With innovation being a critical success factor for most industries, numerous developments in image capturing and processing techniques have further bolstered the adoption of InSAR technology. Programmes are being designed for the launch of satellites with advanced SAR equipment to enable better understanding of our planet’s resources. NISAR is an upcoming co-operative venture between NASA and ISRO, scheduled to launch in 2021, and will be the first satellite with dual-frequency SAR sensors. The project is aimed at taking measurements of Earth’s most intricate processes, such as disturbances in the ecosystem and natural hazards. NISAR is also slated to be used for mapping resources, monitoring floods and coastal changes, oil spills and even estimating agricultural biomass, consequently representing the widespread applications of InSAR.

This article was published in Geomatics World September/October 2018

Last updated: 04/09/2020