How SAR helps NISAR ‘see’ earth in any weather, day or night?

Satellites often use optical sensors that need visible light to take images of the Earth. But that means they can’t see through clouds or take pictures in the dark. That’s where Synthetic Aperture Radar (SAR), like the one used in the NISAR satellite, comes in.

SAR uses microwaves instead of visible light, allowing it to work in any weather, day or night. This makes it a powerful tool for monitoring natural events like earthquakes, ice movements, and forest changes.

What’s the difference between normal radar and SAR?

Traditional radar systems send out microwave pulses. These waves bounce off the Earth's surface and return to the radar system. By looking at how long the signal takes to come back, its strength, and any changes in frequency, radar systems can learn how far away something is, whether it’s moving, and what its surface is like. However, the images from basic radar aren’t very detailed.

SAR improves this by using the movement of the satellite. As the satellite moves, its radar keeps sending out pulses and collecting echoes.

Since each signal comes from a slightly different position, there are small changes in the returning signal’s frequency. This is called the Doppler shift, the same effect that makes a siren sound higher-pitched when coming towards you and lower when it moves away.

Why can’t we just use a bigger antenna?

To get a high-resolution radar image using traditional methods, you’d need a very large antenna, sometimes many kilometres wide, which simply isn’t possible to launch into space. For example, to get a 10-metre resolution image using NISAR’s L-band radar in the traditional way, you’d need a 19-kilometre-wide antenna.

SAR solves this by using a smaller antenna that moves with the satellite. NISAR’s antenna is 12 metres long when opened, about the size of a city bus.

As it travels, the radar collects many signals and combines them using clever processing. This creates the same effect as using a much larger antenna. In simple terms, SAR “fakes” a big antenna by collecting lots of data from a small one as it moves.

How does SAR create images?

Once SAR has collected raw data, this information can be processed into different image types that scientists can study.

Interferometry (InSAR): This method compares two SAR images of the same place taken at different times. The result is a colourful image called an interferogram. These maps show small changes in elevation. If the coloured lines are close together, it means the ground has shifted more. This is useful for tracking earthquakes, landslides, or melting glaciers.

Polarimetry: This looks at the direction of the radar waves when they bounce back. For example, straight structures like buildings usually reflect waves in the same direction. In contrast, bumpy areas like forests scatter the waves. This helps researchers tell the difference between land types and assess damage after natural disasters like floods or storms.

Why is SAR so important?

Because SAR doesn’t rely on clear skies or daylight, it can monitor the Earth all the time. This makes it essential for studying how the planet is changing. SAR data can show how far a glacier has moved, how much soil moisture has dropped during a drought, or whether land near a fault line has shifted after an earthquake.

As the former director of NASA’s Jet Propulsion Laboratory, Charles Elachi, said, SAR lets scientists measure things very precisely. By regularly capturing detailed images of the same areas, missions like NISAR help scientists track change over time.