Synthetic Aperture Radar (SAR) is a type of sensor that emits microwaves toward the Earth and captures images of the surface by receiving the microwaves reflected back from the target. Unlike optical sensors, SAR can conduct observations even under adverse weather conditions, as microwaves are largely unaffected by cloud cover. Additionally, because SAR is an active sensor that emits its own signal, it is capable of observations at night.

Conventional radar systems (real aperture radar) can measure the distance and direction to a target but cannot achieve high spatial resolution (the ability to distinguish between closely spaced objects) required for photographic-quality imaging. SAR, on the other hand, performs data processing on many signals received from different positions to generate high-resolution images comparable to photographs.

In conventional radar, resolution is determined by the width of the antenna beam. If two objects fall within the same beamwidth, they cannot be resolved. Achieving meter-level resolution therefore would require an antenna several kilometers in length, which is not practical. SAR overcomes this limitation by moving a physically small antenna (a few meters long) and then using data processing to synthetically create a large antenna, effectively replicating the performance of a real antenna several kilometers long.

As the small antenna traverses its path, it repeatedly transmits and receives microwaves at different positions, storing the microwaves reflected by the target from each position. The signals are then aligned so that their peaks and troughs match—a process that strengthens the signal and narrows the effective beam, achieving the same result as if a large antenna had been used.

This technique is called synthetic aperture processing, and it improves resolution along the direction of the antenna’s motion. For resolution perpendicular to the antenna’s motion, SAR uses a technique known as pulse compression, which is also commonly used in conventional radar systems. In pulse compression, a weak, long-duration microwave pulse is transmitted and received, then computationally compressed into a strong, short-duration pulse. Notably, the underlying computation is analogous to that of synthetic aperture processing.

In this way, SAR achieves high resolution in both the along-track and cross-track directions, enabling the capture of detailed, photograph-like images.

Related Links