Earth observation satellites are small satellites, flown together as a constellation in low orbit, that carry optical cameras, radar that sees through monsoon cloud, or hyperspectral sensors. They photograph the Earth so that governments and companies can see what is happening on the ground.
Each kind of sensor sees something different. An optical camera needs daylight and a clear sky. A radar sends out radio waves and reads their echo, so it sees through cloud and works at night. See synthetic aperture radar. This matters in India, where much of the country is under cloud during the monsoon. A hyperspectral sensor sees hundreds of narrow bands of colour, so a diseased crop or a mineral shows its own signature. See hyperspectral imaging.
The satellites fly in low Earth orbit, a few hundred kilometres up, as explained under LEO and GEO satellites. A single satellite goes round the Earth in about 90 minutes but sees each place only briefly. So companies fly many together, a constellation, to see the same place often. The more satellites, the shorter the gap between two pictures of the same place. Besides the sensor, each satellite carries solar panels for power, a radio to send the data down, and thrusters to adjust its orbit.
Say a constellation of radar satellites passes over a district in Bihar the morning after the Kosi river floods its banks. One satellite photographs the flooded land through the clouds. The picture is held in the satellite's memory until it passes a ground station, an antenna on the Earth that receives the data. It can also be sent through a satellite data relay, or over laser links between satellites. Then satellite imagery analytics turns it into a flood map.
Satellites have become small and cheap, some the size of a suitcase. They can share a rocket with others, which cuts the cost of launch. The images are used for crop monitoring, vegetation management on power lines, methane monitoring, drone surveys and defence. Their positions are tracked by space situational awareness. These satellites differ from communication satellites, which carry phone and internet signals and do not take pictures.