How Do SAR Satellites Collect Data from Earth?

Synthetic Aperture Radar (SAR) satellites are among the most sophisticated tools for Earth observation. SAR satellites are different from traditional optical satellites. These radar systems can see through clouds, rain, and darkness. This means they provide clear and accurate data no matter the weather or time of day.

But how exactly do these technological marvels collect data from Earth?

InSAR output for displacement in the Himilayan village of Joshimath.

The Basics of SAR Technology

SAR operates by emitting radar signals (radio waves) toward Earth and then capturing the signals that bounce back. These signals carry information about the surface they interacted with, enabling detailed examination of terrain, structures, and natural phenomena.

Step-by-Step Process

1) Emission of Radar Pulses

SAR satellites emit microwaves using a radar antenna. These pulses travel to Earth and return from the surface; a process of transmitting and receiving.

2) Interaction with the Earth's Surface

Radar waves bounce back differently depending on the surface material, roughness, and moisture content. For example, smooth surfaces, like calm water, reflect most signals away from the satellite, appearing dark in SAR imagery. Rough surfaces, like forests or urban areas, scatter signals, creating brighter return.

3) Receiving Reflected Signals

The satellite's antenna collects the reflected signals and records their intensity and phase (the wave's position in it's cycle). These data points are critical for generating detailed images and measurements.

4) Synthetic Aperture Formation

SAR uses the motion of the satellite to stimulate a much larger antenna, effectively creating a "synthetic aperture". This technique enhances resolution, allowing SAR to capture fine details over large areas.

Data Processing and Interpretation

The raw radar signals collected by SAR satellites undergo extensive processing to create meaningful data:

• Backscatter analysis determines the reflectivity of surfaces, revealing characteristics like soil moisture, vegetation density, or surface roughness.

• Interferometric Synthetic Aperture Radar (InSAR) compares several SAR images over time. It measures surface movement with millimeter accuracy. This technology is helpful for finding ground subsidence or landslides.

As well as ground displacement, InSAR analysis can be combined with other datasets to build susceptibility maps to landslides.

The Future of SAR

Advancements in remote sensing technology, such as higher resolution sensors and AI-based analytics, promise even greater capabilities. Upcoming missions, like NASA-ISRO (NISAR), aim to provide unprecedented insights into the Earth's changing surface.

Want to learn more about the accuracy of InSAR? Read this article next.