Exploring Synthetic Aperture Radar (SAR) with Open Data Cube

A peek into history

The invention of Radio Detection and Ranging or Radar, as a concept for detecting and localizing objects in a three-dimensional space, dates back to the turn of the 20th century. Once invented, radar technology developed rapidly during the period of World War II. Later, new and better radar systems emerged in the 1950s, expanding the application realm of radar into a range of new areas, including the growing discipline of Earth Observation.

Synthetic Aperture Radar

Synthetic Aperture Radar (SAR) is a mode run by a conventional radar system that is moving to artificially create a representation of an extremely large antenna with the help of clever signal processing. SAR, an active system operating in the microwave domain of the electromagnetic spectrum has been widely used for Earth remote sensing for more than 30 years. SARs transmit microwave signals at an oblique angle, and measure the backscattered (in the direction of the sensor) portion of this signal to analyze the features on the surface. It provides high-resolution, day-and-night, cloud, and weather-independent images for a large number of applications ranging from geotechnical and climate change research, environmental and Earth system monitoring, 2D and 3D mapping, change detection, security and surveillance-related applications up to planetary exploration. Seasat, ALOS-1, Sentinel-1, Radarsat-2, NISAR, etc., are some of the many past, current, and upcoming spaceborne SAR sensors.

Exploring SAR with Open Data Cube

A major chunk of Earth Observation (EO) data remains underutilized mainly because of the big data challenges namely volume, velocity, veracity, and variety. This predicament  can be resolved satisfactorily by using Analysis Ready Data (ARD). Providing ARD directly to the user community, can utilize the potential of satellite data to a reasonable level. These pre-packaged and pre-processed bundles of data products are easier to analyze, and reduce the number of time users spend on data processing.

Open Data Cube (ODC) is a global initiative to enhance the use of free and open satellite data to tackle environmental, economic, and social challenges. ODC delivers information and applications that have a great impact at the local, regional, and global scales.

Free and open data provided by Sentinel-1A and 1B of the European Copernicus program, has opened up opportunities for greater exploration and application of SAR data globally. ODC has several ARD products which include radar backscatter (gamma nought), eigenvector-based dual-polarization decomposition, and interferometric coherence of Sentinel-1 Interferometric Wide(IW) swath mode data.

The processing workflows for producing these SAR ARD products make use of the European Space Agency’s (ESA) free Sentinel-1 Toolbox within the Sentinel Application  Platform (SNAP). SNAP is an open-source platform, allowing easy access and sharing of processing workflows with the capability of batch processing through its graph processing tool(GPT).

While each SAR ARD product requires its own processing workflow, there are some basic common procedures and parameters that could be reused for each product.

Observing the Earth using the SAR have a wide range of practical applications, such as:

On the ocean:

• Oil spill detection
• Ship/ship lane detection
• Detection of natural seepage from oil deposits
• Ocean features
• Ocean bottom features

On the land:

• Forestry and agriculture
• Detection of surface movements
• Geological and geomorphological applications
• Disaster management

To wrap up

With the growing volume of data from Earth Observation missions, Data Cubes are revolutionizing the way users can work with them. By utilizing the potential of all-weather  data from SAR along with the concept of ARD and ODC, we can bring new insights to EO data to laymen.