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MODIS Data Product Non-Technical Description - MOD 1

Scientists studying the Earth as a system have a staggering array of questions. What role do clouds play in global warming? How much of the world is being deforested, and what effect will that have on our climate? Is the hole in the ozone getting bigger or smaller? What will happen to the billions of tons of carbon dioxide we generate with our cars and our power plants? Scientists are asking a lot of complicated questions, and they’re hoping that MODIS can provide some answers.

While scientists have numerous, complex questions, MODIS itself really has only one—one question that it asks over and over as it orbits the Earth: How much light (electromagnetic radiation) is there?
As MODIS orbits the Earth, its scan mirror rotates round and round, continually diverting electromagnetic signals from Earth onto its photon, or light particle, detectors. Like a small child on a long car ride asking every few minutes, “How far it is now? How far is it now?”, MODIS continuously looks down at the Earth’s surface and asks, “How bright is it now? How bright is it now?”

How much radiation MODIS sees depends on how much light, or solar radiation, the Earth is reflecting back into space and on how much heat, or thermal radiation, the Earth is giving off. Different features of the Earth’s surface and atmosphere reflect or emit radiation in different ways, namely at different wavelengths. MODIS has detectors that allow it to distinguish 36 spectral bands, or groups of wavelengths. These bands represent a portion of the electromagnetic spectrum, including visible light and infrared radiation. When light of a certain wavelength hits a detector that is specially designed to recognize that particular wavelength, the detector registers that it has been hit by creating either an electric current or voltage. These analog signals are translated into digital signals, which a satellite relays down to Earth.

Scientists call the readings from the detectors radiance counts because the voltages and currents that are the basis for the digital signals are directly proportional to the number, or count, of photons of radiation that hit the detectors. Radiance counts are the first level of data scientists on Earth receive from MODIS. These data come down from MODIS along with other important information: where the spacecraft was in its orbit when the readings were made; whether some data points, called pixels, are missing; and information from onboard calibration devices.

How much radiation is coming from Earth? MODIS answers that question nearly 500 million times a day. What does it all mean for our planet’s future? Scientists will be working on the answer to that question for years to come.

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Curator: Brandon Maccherone
NASA Official: Shannell Frazier

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