The spy in the sky that sees backwards in time

THE WAR in Ukraine has brought the topic of eyes in the sky to general attention, as the Ukrainian army in particular has put surveillance by drone to good effect in identifying and destroying targets in the here and now. But aerial surveillance can also reach backwards in time, by the expedient of indiscriminately recording everything that is going on in a particular neighbourhood, and then looking for useful patterns in the resulting footage. This technique, called wide-area motion imagery (WAMI), has been around since 2006. But improvements in both the recording equipment used and the means by which the images are analysed are making it more and more valuable.