Synthetic aperture radar (SAR) is a way for a satellite to take pictures using radio waves, so it can see through cloud and at night. It is used to watch ships, floods and land. It is one of the sensors on an Earth observation satellite.
A radar sends out pulses of radio waves towards the ground and reads the echo that comes back. Smooth water reflects the waves away from the satellite, so it shows up dark. Buildings, ships and rough ground reflect them back strongly, so they show up bright. Because radio waves pass through cloud and need no sunlight, the picture is the same by day or night. This matters in India during the monsoon, when an optical camera sees only cloud.
The word synthetic comes from how the picture is sharpened. A real antenna big enough to take a sharp picture would not fit on a small satellite. Instead, the satellite keeps sending pulses as it moves, and software combines the echoes from many positions along its path, as though they came from one very large antenna. The radio part is often controlled by software, as in software-defined radio.
Imagine a radar satellite passing over the Arabian Sea at night. The sea is dark, and a ship shows up as a small bright spot, because its metal hull reflects the pulses strongly. A ship that has switched off its transponder, the device that broadcasts its position, still appears. That is how radar helps to watch fishing and shipping.
The flood in the box is the other main use. Software compares a new picture with an older one, and geospatial AI marks the areas that turned dark. Relief agencies use the flood map. Insurers use it too, with catastrophe modelling, to estimate the losses.
A radar picture has no colour, and it looks unfamiliar, because it shows how strongly each place reflects radio waves. So a radar picture is often combined with an optical one.