In remote sensing, the basic measure of the spatial quality of an image is its “resolution.” The term “spatial resolution” refers to the ability of our radar (or any sensor) to differentiate between objects. It is a measure of finest detail our radar can see and is typically measured in meters. There’s a measure of resolution for both dimensions of the radar image: “azimuth resolution” and “range resolution,” which refer to how well the radar can resolve objects in either direction.

Synthetic aperture radar (SAR) exists because conventional radar suffers from poor azimuth resolution—in other words, conventional radar is not very good at distinguishing objects in the direction along the aircraft’s flight direction. The azimuth resolution of the radar is dependent on the physical dimensions of the radar’s antenna—in fact, they have an inversely proportional relationship. This means that, as the radar’s antenna size grows, the resolution improves, though it also depends on other variables, like the radar’s transmitting wavelength and the distance to the target or scene in question.

How do we make the azimuth resolution better? Based on the relationship I’ve just described, we can guess a few ways this can be done: 1) increase the along-track antenna length, 2) increase the antenna’s transmitting wavelength, or 3) get the aircraft closer to the target.

It’s clear that the last option is not viable, especially for a SAR on a spacecraft. The second option is also not viable, because as we increase the wavelength of the radar’s transmitted signal, the radar requires more power to generate that signal, and we’d like to keep the power requirements for our radar as low as we can.

That just leaves us with the first option—and it’s the key behind SAR. But adding an antenna that might be 25 to 100 meters long to an aircraft or spacecraft is generally impractical, so what do we do? We use computers to simulate a long antenna using a conventional antenna, which might be just a few meters long, instead.

How is a single SAR antenna able to emulate a very large antenna? The basic concept is as follows: the SAR transmits a signal, receives the echo of its transmitted signal from a target—while the SAR is moving—and maps that received signal to a specific position. In order to effectively map the positions of whatever the received signals have bounced off of (the targets), SAR exploits the motion of its platform to determine the direction of the signal, also known as the signal’s “phase”. Without the relative motion between the SAR and the target or scene, the SAR would not be able to locate the target(s) in question.