An instrument on INSAT-3D, INSAT-3DR and INSAT-3DS that measures how temperature and humidity change with height in the air, the first such sounder on an Indian geostationary satellite.
An imager makes a flat picture. A sounder measures the atmosphere in layers. Each of its narrow infrared channels senses a different height in the air. Combining them gives a profile of temperature and humidity from the ground up, a three-dimensional view of the atmosphere.
The sounder has 19 channels: 18 in the infrared and one visible. A two-axis mirror points its view anywhere on the Earth disc. Each reading covers about 10 km by 40 km of ground directly below. It works only where the sky is clear, because thick cloud blocks the infrared signal from below.
ISRO designed the sounder as a new instrument for the INSAT series. It first flew on INSAT-3D in 2013, giving round-the-clock temperature and moisture profiles over the Indian Ocean region. INSAT-3DR's sounder covers the Indian land region many times a day. INSAT-3DS (2024) carries the same type of sounder.