NASHUA, N.H. — A satellite passes hundreds of miles above Earth while an aircraft flies over terrain hidden beneath clouds. Neither needs a clear view of the ground to produce an image of what is below.

Synthetic aperture radar (SAR) uses radio waves rather than visible light to image the Earth's surface. The radar sends signals toward the ground and records the energy that returns after those signals interact with objects such as terrain, buildings and vehicles.

As the aircraft, satellite or other platform carrying the radar moves, it observes the same area from multiple positions. Signal processing combines those measurements to produce a detailed image.

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What is synthetic aperture radar?

A basic radar transmits radio-frequency energy and listens for a return. When that energy encounters an object or surface, some of it reflects toward the radar. Measuring how long the signal takes to travel out and return helps determine how far away the reflecting surface is.

SAR starts with the same principle, but it collects many measurements as the radar moves. The word "aperture" refers to the part of an antenna involved in transmitting or receiving electromagnetic energy. A larger antenna aperture can generally provide finer angular resolution, so a radar can distinguish objects that are close together.

Putting a very large antenna on an aircraft or spacecraft is not always practical. SAR instead uses the movement of the platform and signal processing to create, or synthesize, the effect of a much larger aperture.

Why does synthetic aperture radar need to move?

Picture a SAR sensor mounted on an aircraft. As the aircraft moves forward, the radar repeatedly transmits signals toward an area on the ground. The same object may remain within the radar's field of view for many of those transmissions. Its position relative to the radar changes as the aircraft moves. That means each return contains slightly different information about the same part of the ground.

SAR processing combines those returns rather than treating each one as an unrelated measurement. Differences in the signal’s phase and Doppler characteristics help the system determine where reflected energy came from along the aircraft's path.

The distance the platform travels while collecting useful measurements then becomes part of the radar's synthetic aperture. This allows SAR to achieve finer resolution in the direction of travel than the physical antenna alone could provide. The same basic process works from space. Instead of an aircraft moving along a flight path, a satellite collects measurements as it travels through its orbit.

How does SAR turn radar returns into an image?

A radar return by itself is not an image because the system has to determine where the reflected energy came from and organize that information spatially. SAR images have two primary dimensions: range and azimuth.

Range describes distance in the direction between the radar and the area it is imaging. The radar can distinguish targets at different ranges by measuring the timing of returned signals. Using signals with sufficient bandwidth allows it to separate objects that are relatively close together in range.

Azimuth runs along the direction the platform is traveling. Resolving objects in this direction is where the synthetic aperture becomes particularly important. SAR processing uses the phase and Doppler information collected across the aperture to separate objects along the platform's path and place them in the image.

Finally, the processor combines measurements from across the synthetic aperture and focuses them into image pixels. Precise information about the platform's position and motion is important because errors in that information can reduce image quality.

Why does a SAR image look different from a photograph?

The brightness in a SAR image does not represent visible light. Instead, bright and dark areas generally indicate differences in the strength of the radar energy returning to the sensor.

A smooth surface can direct most of the transmitted energy away from the radar, producing a relatively dark area. Rough surfaces can scatter more energy back toward the sensor. Buildings and other structures can also create strong returns depending on their shape, orientation and surrounding surfaces.

The radar's viewing angle is also very important. Tall terrain or structures can block the radar signal from reaching areas behind them, producing radar shadows. Other geometric effects can change how slopes and elevated objects appear.

Because of those effects, interpreting a SAR image is different from looking at a conventional photograph.

Can synthetic aperture radar see through clouds and at night?

SAR does not need sunlight to create an image. It transmits its own RF energy and measures the return, allowing it to operate during the day or at night. Radar wavelengths can also pass through clouds and haze that would obstruct an optical camera, making SAR useful when weather limits visible or infrared imaging.

But radar cannot see through every material. How a signal interacts with rain, vegetation, soil and other materials depends partly on its wavelength, the material itself and environmental conditions. Different radar frequency bands therefore suit different sensing applications.

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What determines SAR image resolution?

Resolution describes the radar's ability to distinguish separate features that are close together. In the range direction, resolution depends heavily on the bandwidth of the transmitted signal. Greater bandwidth allows the radar to differentiate objects separated by smaller differences in range.

In the azimuth direction, resolution depends on the synthetic aperture and the processing of measurements collected as the platform moves. Antenna characteristics, platform motion and signal-processing methods also affect the resulting image.

Designers have to balance image resolution against factors such as coverage area and the capabilities of the aircraft or spacecraft carrying the radar.

Where is synthetic aperture radar used?

SAR can provide imagery for military intelligence, maritime monitoring and mapping. Space-based systems can repeatedly image large areas to track changes on the ground, while airborne systems can collect imagery over specific areas of interest.