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MODIS Data Product Non-Technical Description - MOD 02, 03

The 44 MODIS data sets, or products, as they are called, are somewhat like a biological food web. Products fall into one of four categories, with lower-level products being fed as input to the higher-level, more complex products. The base of the food web is formed from the minimally processed raw radiance counts provided in the Level 1A product. The Level 1A data product is a collection of the radiances MODIS detected reflecting back from the earth and its atmosphere – a stream of numbers, one after another, all day, every day. To make these raw radiance numbers useful for earth science research, two things have to happen: calibration and geolocation.

Calibration is the process by which scientists adjust the raw output of a measuring device--in this case, the MODIS sensor--for known sources of error or interference. Geolocation is the process by which scientists specify where on the Earth's surface or in the atmosphere a specific radiance signal was detected. These processes take place in the MOD 02 Level 1B and MOD03 data sets. Together, MOD 02 and 03 form the basis for every other MODIS product. So, if MOD 02 or 03 have any errors, all subsequent products that "feed" on these data will have the same errors.

It is precisely for this reason that calibration is so vitally important to MODIS. There are many factors that can affect the radiance signals that MODIS is detecting and can cause them to contain errors, for example broken or malfunctioning detectors, as well as normal degradation of the sensor over time as it is exposed to the harsh environment of space. Unless we take into account how these things affect the instrument, the data will be inaccurate. To counteract this, MODIS was designed with a number of built-in calibration instruments – the Black Body, the Solar Diffuser, and the Spectroradiometric Calibration Assembly.

The Black Body (BB) is designed to monitor the temperature inside the instrument and provide calibration data for the thermal (heat) bands. This is important because in the infrared (heat) wavelengths, every object emits photons (particles of light) according to temperature. Because the BB is black (a color that absorbs all light), it will have a known temperature against which the instrument can be calibrated. When MODIS looks at the BB and records a temperature, the difference between what is recorded and what the temperature should be will tell MODIS scientists how much they have to adjust the data to make them accurate.

Instead of calibrating the thermal bands, the Solar Diffuser (SD) provides calibration for the reflective bands. The white color of the SD reflects all light from the Sun. Because the Sun has a constant light output, MODIS scientists know exactly what MODIS should see when it looks at the SD. Differences in what MODIS actually sees and what MODIS should see will tell scientists that MODIS instrument and/or the SD are degrading, and that they have to adjust the data to make them accurate.

The Spectroradiometric Calibration Assembly (SRCA) is like a little self-contained laboratory inside of MODIS that monitors visible, near-infrared, and short wave infrared detection within the instrument, but it also monitors and calibrates itself to ensure that the calibration data it provides are not themselves inaccurate. The SRCA is quite important, because it monitors not just one region of light, but three. The data that the SRCA gathers are sent to MODIS scientists, who then adjust MODIS data accordingly.

Not only do these components allow MODIS scientists to ensure that the instrument itself doesn’t interfere with the data, but one calibration component, the Solar Diffuser, even has its own calibration monitor, the Solar Diffuser Stability Monitor (SDSM). The SDSM monitors a critical calibration component, the Solar Diffuser, and ensures that the MODIS data are that much more reliable. (For more information on MODIS’ calibration components, please refer to the components page.) We must also take into account that MODIS is on a spacecraft. The spacecraft occasionally has problems or experiences events that affect MODIS’ data, and those phenomena must also be taken into account in the calibration process.

On the ground, MODIS scientists incorporate the calibration data that they receive from the instrument into the data set production process. Calibration, then, allows scientists to produce useful products such as radiance, reflection, or temperature from the sensor's raw output after monitoring the instrument and its components for degradation or other events and phenomena that affect the data, and then adjusting for those factors. Calibration, therefore, is a process of both data conversion and quality control.

The more we learn about the technology of the instrument and the planet it observes, the more refined the data will become. Thus, to keep the data web up to date and increasingly reliable, there will be a major reprocessing effort every year or two. By ensuring that the base MODIS products are as high quality as possible, we ensure that all subsequent products and understanding drawn from them are dependable for the scientific community and the world.

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Curator: Brandon Maccherone
NASA Official: Shannell Frazier

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