An imager takes pictures of clouds and the surface; a sounder measures how temperature and moisture change with height, layer by layer through the atmosphere.
A weather satellite usually carries two kinds of instrument. INSAT-3DS has both: a 6-channel imager and a 19-channel sounder.
Think of the imager as a camera looking at the top of the weather. Think of the sounder as a probe that looks down through the air and tells you what is happening at different heights.
The imager records the Earth's surface and atmosphere in a few broad channels. ISRO says the INSAT-3DS imager helps capture clouds, aerosols, land surface temperature, vegetation health and water-vapour distribution. Its pictures are what you see in TV weather reports: swirls of cloud moving across the map.
A sounder has many more, narrower channels. ISRO explains that the INSAT-3DS sounder's channels are chosen to catch radiation from particular parts of the atmosphere. Some are tuned to water vapour, ozone and carbon dioxide. Others are designed to measure temperature at different layers.
From these, scientists build temperature profiles, water-vapour profiles and total ozone. A forecaster can then see whether the air is warm and moist near the ground and cold higher up, which is the recipe for thunderstorms.
The Indo-French Megha-Tropiques satellite showed the same split. It carried MADRAS, an imaging radiometer, alongside SAPHIR, a sounder that profiled humidity.
Weather models start from a snapshot of the whole atmosphere. Ground stations and weather balloons give profiles at a few places. A sounder fills the huge gaps between them, especially over the oceans around India. The imager then shows how clouds actually develop, so forecasters can check whether the model is right.
Find INSAT-3DS: from geostationary orbit its imager and sounder look down on the same view of India all day. See INSAT-3DS and its sister satellites
How can a satellite looking straight down tell what is happening at 3 km and at 10 km? The answer is absorption.
Pick a wavelength where carbon dioxide absorbs strongly. Radiation from the ground cannot get through, so what reaches the satellite comes only from the top of the atmosphere. Now pick a nearby wavelength where CO₂ absorbs a little less. The satellite can see a bit deeper. A wavelength that is barely absorbed lets the satellite see almost to the ground.
Each channel therefore has a weighting function: a curve showing which layer of air contributes most of the radiation it measures. A sounder chooses its channels so that their weighting functions peak at different heights, from near the surface to the stratosphere.
Because CO₂ is evenly mixed in the air, the radiation in CO₂ channels depends mainly on temperature. Inverting the set of measurements gives a temperature profile. Water-vapour channels, once temperature is known, give a humidity profile. Ozone channels give total ozone.
A rough numerical idea: if a sounder has 18 infrared channels with weighting functions spread over 0–20 km, each channel represents a slice of roughly 1 km on average. The slices overlap, so a sounder's vertical resolution is coarser than a balloon's. But it covers the whole disc of Earth it can see, which no balloon network can.
An imager needs none of this. Its few channels are chosen to be clear windows (to see the surface and cloud tops) or broad absorption bands (to see water vapour).
A 6-channel imager and a 19-channel sounder.
A curve showing which height in the atmosphere contributes most of the radiation measured in a given sounder channel.
CO₂ is evenly mixed in the air, so the radiation it emits depends mainly on temperature, not on how much CO₂ is present.