What is Synthetic Aperture Radar (SAR)?

Demystifying this cool-sounding radar technology and it's advantages

What is SAR, and how does it differ from typical imaging radar?

Synthetic aperture radar is a specific type of active imaging radar that allows scientists to take higher-resolution images with smaller antennas. It has a variety of applications in the real world due to its vast data collection. 

SAR transmits and collects data in the same way as traditional radar imaging. First, signals are sent to Earth’s surface. These signals come in the form of pulses of varying wavelengths, most commonly as microwave energy. When the signals return to the satellite's antenna, specific measurements are recorded. This return signal is often referred to as ‘backscatter'. In SAR imaging, the amplitude and phase measurement of the backscatter are the key findings.

Visualization of the flight path and angle SAR Satellites take to collect amplitude and phase measurements. Courtesy of the UT Radar Interferometry Group.

The collected data can answer many important questions about the area we are surveying, such as:

• Is the terrain high or low?

• Is it wet or dry

• Is it rough or smooth?

• Are the surfaces straight up and down or slanted?

• Is the surface covered with grass or concrete?

It can also reveal important information about Earth’s complex systems, such as the global 

vegetation coverage, the extent of snowpacks, wetland areas, geologic features (rock types & their distribution), volcanic activity, ocean wave heights, and wind speed.

Here is a radar image I created using SAR data. It depicts the elevation on a region in Austin/San Antonio.

So, what makes SAR special? SAR refers to the technique of synthesizing a bigger antenna, which is the aperture in this case, by combining and storing the signals it receives as it moves through space. So SAR pretty much mimics a longer antenna using the motion of its spacecraft. A longer antenna is preferential when taking radar images because the resolution of an image is directly proportional to the length of the antenna; a longer antenna means better images. 

In addition to its ability to take high-resolution images with smaller antennas, SAR also gets points for its convenience and reliability. Unlike optical imagery or astronaut photography, SAR is operational in all light and weather conditions due to its active nature. This feature makes it the preferred choice by some scientists because it provides a much greater data range and can help scientists access hard-to-reach areas. 

Which satellites use SAR?

Here are a couple satellites and satellite series that rely on this versatile imaging technique:

• Sentinel 1 Satellite series

• Seasat

• Radarsat-2

• COSMO - SkyMed

Image of a Sentinel-1 SAR Satellite orbitting the Earth and collecting data. Courtesy of the European Space Agency.

Thanks for taking the time to read this blog!

-  Sophie Wassef

Sources:

• https://spaceplace.nasa.gov/review/topomap-clay/imaging-radar.html 

• Imaging Radar - NASA Facts 

• https://www.earthdata.nasa.gov/learn/earth-observation-data-basics/sar 

• https://www.earthdata.nasa.gov/s3fs-public/imported/Satellite_Property_Tables.pdf