MODIS Calibration Overview
Table of Contents
General IntroductionFor the last three decades, news reports have painted gloomy pictures of the declining health of our environment. We read or hear about deforestation, loss of biodiversity, acid rain, ozone depletion, and global warming, but it is difficult to understand what these terms mean on a global scale. What are their causes and what long-term effects will they have on the environment? We wonder what the quality of life will be like on Earth for our children, and their children. Unfortunately, too few data currently exist to adequately answer those questions.
In the early 1980s, NASA began planning the Earth Observing System (EOS), the primary initiative in its Mission to Planet Earth, to gather much-needed global climate change data. The EOS objective is to launch a series of satellite sensors that will look back at our planet to observe and measure global dynamics. The flagship of these satellite sensors--the MODerate-resolution Imaging Spectroradiometer, or MODIS (see Figure 1) will be launched first in 1999 aboard Terra (formerly EOS AM-1) , and in later years aboard subsequent EOS satellites. Every two days, for at least a 15-year period, MODIS instruments will collect data over all regions of the Earth's lands, oceans and atmosphere. These data will consist of measurements of visible and infrared light that is either emitted or reflected by the Earth. Particular care is being taken to characterize the performance of the instrument throughout the life of its mission by including specially-designed onboard calibration devices.
Energy from the SunOur planet is continually bathed in electromagnetic radiation from the sun, which directly or indirectly drives most of the Earth's surface dynamics. Billions of the sun's basic energy units (called photons) bombard the atmosphere every second and are either reflected back into space, absorbed by suspended atmospheric gases and particles, or transmitted to the Earth's surface. At the surface, the photons are either reflected by the land or ocean back up through the atmosphere, or absorbed and stored as energy, which will be emitted later in the form of heat (infrared photons) or used to evaporate surface water.
Remote SensingDepending upon their physical and chemical properties, all objects reflect, absorb, or transmit visible and infrared light in characteristically different ways. For example, from across the room we can tell the difference between a rotten banana and a ripe one, simply by observing them in visible light. The rotten banana appears black and leathery, whereas the ripe one is yellow-green and has a smoother texture. Using your eyes, you can immediately tell whether a wheat field is barren, growing, or mature. You could make such an observation firsthand with your own eyes, or remotely using a device such as a camera or telescope. (For a more thorough tutorial on the science and engineering of remote sensing, see The Remote Sensing Tutorial by the Applied Information Sciences Branch, Code 935, at NASA's Goddard Space Flight Center.)
With more sophisticated sensors, such as MODIS, scientists can remotely sense light not only in the visible region of the spectrum, but also in the infrared regions that our eyes can't see. By observing the reflection, absorption, emission, or transmission of photons by an object, scientists can quantitatively infer certain attributes of that object. For example, understanding the characteristic ways in which the chlorophyll in different species of plants absorb and reflect light enables scientists to identify plant biomes using remote sensors.
In another example, the pigments in certain species of phytoplankton (a basic link in the oceanic food chain) preferentially absorb red and blue light, and reflect green. From space, MODIS will remotely sense the light reflected from the ocean, enabling scientists to accurately estimate the abundance of this microscopic plant life in the oceans.
HOW MODIS WORKS
Photons to DataThis illustration and this flow diagram show how MODIS will convert incoming photons to data. Light that is reflected or emitted by the Earth back to outer space will pass through MODIS' scan aperture, enter into the scan cavity, and hit the scan mirror. A constantly spinning, double-sided scan mirror will reflect the incoming light onto MODIS' internal telescope, which in turn focuses the light onto four different detector assemblies.
En route to the detector assemblies, the light passes through dichroic beamsplitters and spectral filters that divide the light into four broad wavelength ranges. To be more precise, visible light is allowed to pass through the dichroic beamsplitter and visible light filter to reach the visible light detectors, whereas all other wavelengths of light are reflected or absorbed. Likewise, there are three additional detector groups sensitive to near infrared, shortwave/midwave infrared, and longwave infrared wavelength regions, respectively. Here is another illustration of how light passes through MODIS to reach one of four different detector assemblies.
Each time a photon strikes one of the four detector assemblies, an electron is created. Resulting electrons are collected in a capacitor, where they are allowed to accumulate until they can be "dumped" into the onboard preamplifier. Passing through the preamplifier, the electrons are routed to a Digitizer which converts the electrons from an analog signal to digital data. These digital data are stored onboard MODIS until they can be transmitted to ground receiving stations, where the data will be processed and stored by computers. Using specially developed computer programs, scientists may then translate the data into meaningful images-such as the distribution of phytoplankton blooms in the Atlantic Ocean-that help them interpret conditions on Earth. This graphic illustrates the data sequence for one MODIS scan.
The Harsh Environment of Outer SpaceBecause outer space is such a harsh environment-eroding even the most stable materials with constant bombardment by cosmic rays, ultraviolet radiation, micrometeorites, and extreme temperature shifts-the performance of all satellite sensors degrades over time. Historically, once an instrument is launched into space, it is eroded by the elements in ways we cannot accurately predict, so that, over time, errors and uncertainties are introduced into the collected data.
In anticipation of the harsh environment of outer space, MODIS will have unprecedented onboard calibration systems enabling engineers on the ground to characterize its performance throughout the lifetime of its mission and correct for the errors introduced into the data by system degradation.
Solar Diffuser and Solar Diffuser Stability MonitorThe Solar Diffuser and Solar Diffuser Stability Monitor (or SDSM) will have clear "views" of the sun in order to constantly monitor its output of photons. The solar diffuser is a pad made of a reflective material similar to white paint. Photons, scattered in all directions off of the Solar Diffuser, will in turn be reflected off of the scan mirror and then onto the telescope and the SDSM. From the telescope, the sunlight passes through the dichroic beamsplitters and completes the same process decribed above.
Measuring the photons coming directly from the sun will enable engineers on the ground to calibrate the responsiveness of the entire MODIS system. Since the sun has a constant output, any change in MODIS output must be due to degradation of either MODIS itself, or the Solar Diffuser. The SDSM will characterize the degradation of the Solar Diffuser over time.
Spectroradiometric Calibration Assembly (SRCA)The Spectroradiometric Calibration Assembly (or SRCA) is a light source that emits wavelengths of light at known intensities, which allows further testing and calibration of the system. Light emitted from the SRCA reflects off of the scan mirror onto the telescope, and proceeds onward through the system as illustrated in SRCA structural layout diagram. Specifically, the SRCA allows engineers on the ground to check the radiometric, spectral and geometric properties of the MODIS system.
Blackbody AssemblyIn the far infrared, photons are emitted by all objects corresponding to their temperatures. To calibrate its thermal bands, MODIS will view a device called a blackbody, which is designed to be non-reflective (hence black) and kept at a precise temperature. Photons "seen" by MODIS while viewing the blackbody will then be a measure of its temperature. This calibration will be used to very precisely determine the temperatures of the Earth's surface and clouds, as illustrated by this graphic.
Views of the Moon and Deep SpaceTwo additional calibration techniques that MODIS will use are views of the moon and deep space. The advantage of "looking" at the moon is that it enables MODIS to view an object that is roughly of the same brightness as the Earth. Like the onboard Solar Diffuser, the moon is illuminated by the sun; however, unlike the Solar Diffuser or the Earth, the moon is not expected to change over the lifetime of the MODIS mission. "Looking" at the moon provides a second method for tracking degradation of the Solar Diffuser.
"Looking" at deep space provides a photon input signal of zero, which will be used as an additional point of reference for calibration. (Stars are too dim to be "seen" by MODIS.)
ConclusionThe MODIS series of instruments will be the best calibrated Earth remote sensors ever launched. They will provide much-needed spectral data from every region on Earth for at least 15 years. Ultimately, these data will help scientists better understand the Earth as a whole, integrated system so that they may assist policymakers worldwide to manage and protect our natural resources more effectively and more efficiently.
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