Each satellite carries sensors which measure the amount of visible light, thermal (infra-red) radiation, and radiation from other parts of the electromagnetic spectrum coming from the earth. (Note: at this point we make no distinction between reflection and radiation).
Each sensor is only sensitive to a small part of the spectrum, so multiple sensors are fitted to each scanner. For example, the AVHRR scanner has two sensors in the visible region and two in the infra-red region (plus two in between).
For more details see the electromagnetic spectrum document.
The sensors which are of interest to us primarily are sensitive to visible and infra-red radiation. They include:
This diagram shows the sensors on a selection of satellites. The spectrum goes horizontally along the X axis from short wavelengths (visible) on the left, to long wavelengths (infra-red) on the right. Scanners are shown vertically on the Y axis. Each scanner has multiple sensors, and the sensor number increases vertically. For example, reading upwards for AVHRR you will see sensor 1 is in the red part of the visible spectrum, sensor 2 is just outside the red, and sensor 6 is the far end of the infra-red.
You can see that VIIRS, MODIS and SeaWiFS have quite a number of sensors spread across the visible part of the spectrum. This makes them useful for monitoring ocean colour and thus the health of our oceans.
Zoom in on the diagram using the right mouse button (in combination with the Control key on Apple Macintosh computers).
Note that you will need the Flash player installed to see this image
(To see this graph larger please use this page: Larger Sensor Comparison).
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