The satellites using radar to peer at earth in minute detail

Synthetic aperture radar (SAR) allows satellites to bounce radar signals off the ground and interpret the echo - and it can even peer through clouds.

Clouds cover around two-thirds of the world at any one time, preventing conventional satellites from seeing much of the planet.

But now a declassified technology known as synthetic aperture radar (SAR) can be installed on satellites to "see" the Earth's surface in the dark, through the clouds (or the smoke of wildfires), to provide a constant unobscured view of our planet, and show changes on the Earth's surface in great detail.

Previously used to equip only a relatively small number of large commercial satellites, this technology is now being combined with constellations of inexpensive nanosatellites in low-Earth orbit by start-ups such as Iceye and Capella Space.

The goal is to provide round-the-clock observation of nearly anywhere on the planet for everyone from non-governmental organisations, to military customers.

"SAR satellites are capable of a wider coverage and higher-resolution images than their optical rivals, day or night. You don't have to wait for the clouds to clear, you don't have to wait for the rain to stop," says Holly George-Samuels, an associate scientist at security and defence contractors QinetiQ. "If you need the information, you can go and get it now, and it will be superior to an optical image.