The reason satellites keep missing Africa's informal settlements isn't resolution. It's that colour imagery can't tell a slum from an ordinary dense neighbourhood, and radar can. Getting them onto a map matters more than it sounds. Informal settlements are where a large share of urban Africa actually lives, and if a city can't see them, it can't plan water, sanitation, roads or disaster response for them. The obvious tool is satellite imagery, but the ordinary kind measures colour and brightness, and here that betrays you. A tightly packed formal neighbourhood of small concrete houses reflects light in much the same way as an informal settlement of tin-roofed shacks. To the sensor the two blur together, so the settlement gets folded into the surrounding city and vanishes from the count. A team from Sapienza University of Rome and the University of Pavia found a way to pull them back apart, and the trick is to stop relying on multispectral images and start listening to structure. Alongside the usual optical imagery they brought in radar from the Sentinel-1 satellites, which doesn't care what colour a roof is. Radar bounces off shape and material, off the angles and clutter of how buildings sit together, and informal settlements have a very particular texture from above: irregular, densely packed, metal-roofed, jumbled at odd angles rather than laid out in neat blocks. They fed that radar signal in through three escalating layers. First the raw strength of the return. Then the texture, a measure of how rough and disordered the radar pattern is across a patch of ground. Then a physics-guided index tuned to the specific fingerprint of a slum, the weak, scattered returns you get from small chaotic structures. To train and test it they used expert-drawn labels from very high-resolution imagery, roughly 426,000 pixels' worth, across Nairobi and Eldoret in Kenya, with a further check in Kigali, Rwanda. The payoff shows up exactly where the problem was. Judged on how well it finds informal settlements specifically, an optical-only map scored 0.44 in the dry season, roughly a coin toss dressed up as a classifier. Adding the radar layers lifted that to 0.67, and in the wet season from 0.52 to 0.68. Overall accuracy landed around 81.6% in the dry season and 80.7% in the wet, beating the standard reference product's 70.4%, and the mislabelling of informal areas as ordinary dense housing fell by about 7 percentage points. The seasonal detail carries more weight than it first appears. Optical imagery drifts with the weather, greener in the wet season, dustier in the dry, which is part of why maps built from it wobble month to month. The radar texture barely moved between seasons, so the settlement stayed visible either way. Obviously, a map you can only trust half the year is not much of a map, and this is one step to fixing that. link to full article: arxiv.org/abs/2607.07532