In this figure, several different light pathways in the atmosphere are illustrated. a) The light path of the water-leaving radiance; b) attenuation of the water-leaving radiance; c) scattering of water-leaving radiance out of the sensor's field-of-view (FOV); d) Sun glint (reflection from the water surface); e) Sky glint (scattered light reflecting from the surface; f) scattering of reflected light out of the sensor's FOV; g) reflected light which is also attenuated towards the sensor; h) scattered light from the Sun which is directed toward the sensor; i) light which has already been scattered by the atmosphere, which is then scattered toward the sensor; j) water-leaving radiance originating out of the sensor FOV, but scattered toward the sensor; k) surface reflection out of the sensor FOV which is then scattered toward the sensor; Lw , total water-leaving radiance; Lr , radiance above the sea surface due to all surface reflection effects within the IFOV; and Lp , atmospheric path radiance. (This figure is adapted from Robinson, I.S., 1983: Satellite observations of ocean colour, Philo. Trans. Royal Soc. of London, Series A, Volume 309, 338-347.)
The simplest path a photon can take is the most direct one. The photon enters the atmosphere, hits a surface, is reflected, and bounces right back out into space, where it encounters the detector of a remote-sensing instrument. For ocean color remote sensing, this path presents a problem. Imagine the way that sunlight sparkles on the surface of water on a lake. Those sparkles are the direct reflections of light from the Sun into your eyes. Though such reflections are pretty, reflected light doesn't provide any information on what is actually in the water. When there is too much direct reflection, no information on what is in the water can be derived. For that reason, areas with too much reflection (called sun glint) are masked out of the data. Most ocean color sensors are designed to be tilted so that fewer directly-reflected photons will find their way to the detector.
The next path is the main one of interest to science. A photon enters the atmosphere, encounters a surface, is modified in some way, and then is radiated up to the detector in space. The example of a tree leaf was already given above. In water, photons enter the ocean, and some wavelengths are absorbed while others reflect off particles suspended in the water. The most important path in the ocean is the absorption of specific wavelengths of light by the chlorophyll present in phytoplankton cells, so that the remaining radiation is an indication of how much light was absorbed. The net result is that a only small percentage of the light that enters the water (the downwelling irradiance) is redirected back toward the surface (the upwelling radiance). If the upwelling radiance actually leaves the surface and heads toward space—even though all of it doesn't get there—it is termed the water-leaving radiance. Water-leaving radiance is what ocean color sensors are specifically designed to measure.
Those two paths are fairly direct. However, quite a bit of the light that enters the atmosphere and ocean is scattered, because it interacts with air molecules, dust and other substances suspended in the atmosphere, or substances and particles in the ocean. Light scattering (particularly the preferential scattering of higher frequency light) is what causes the light blue color of the sky, or the intense blue of very clear, deep water. There are numerous scattering paths that photons can take. The path a photon takes before encountering a surface isn't important; the important path is the one it follows after leaving the surface. For example, many of the water-leaving radiance photons will be scattered by the atmosphere and never make it to the detector on the satellite. Many more photons won't even reach the ocean, but will be scattered by the atmosphere back to the detector (or they will reflect off clouds). Aerosols or haze in the atmosphere will also partially interfere with the photons radiating toward the detector, perhaps modifying their wavelength by absorption and re-radiation, or by scattering the light even more.
The net result of all these interactions is that for an orbital sensor aimed directly at the ocean, about 10% of the total light it detects is water-leaving radiance. The other 90% of the light is due to atmospheric effects. (Since land is more reflective than the ocean, a sensor aimed at land receives a greater percentage of light from land surfaces and a lesser percentage from the atmosphere.) Corrections must be applied to the data to remove this atmospheric radiance, allowing accurate measurement of the amount and color of light exiting the ocean surface. This is where data processing comes in. An optical model of the atmosphere above the ocean can be formulated, using such inputs as the surface pressure and the transmission of light through the atmosphere at certain wavelengths. Using this optical model, the radiance the satellite "sees" can be corrected for the influence of the atmosphere, theoretically leaving only the water-leaving radiance!
Data Product Levels
In order for remote-sensing data to be useful, the data is processed through several "levels". The definitions of the data levels were agreed upon by the National Academy of Sciences Committee on Data Management, Archiving, and Computing (CODMAC). For precisely worded definitions, the EOS Data Product Levels show below were formulated. (Note that the definitions are not specific to the Earth Observing System (EOS), but are applicable to all types of remote sensing data.)
Level 0
Level 0 data products are reconstructed, unprocessed instrument/payload data at full resolution; any and all communications artifacts, e.g. synchronization frames, communications headers, duplicate data removed.
Level 1A
Level 1a data products are reconstructed, unprocessed instrument data at full resolution, time-referenced, and annotated with ancillary information, including radiometric and geometric calibration coefficients and georeferencing parameters, e.g., platform ephemeris, computed and appended but not applied to the Level 0 data.
Level 1B
Level 1A data that have been processed to sensor units (not all instruments will have a Level 1B equivalent).
Level 2
Level 2 data products are derived geophysical variables at the same resolution and location as the Level 1 source data.
Level 3
Level 3 data products are variables mapped on uniform space-time grid scales, usually with some completeness and consistency.
Level 4
Level 4 data products are model output or results from analyses of lower level data, e.g. variables derived from multiple measurements.
Before there has been any data processing, the data is termed raw data. Raw data simply consists of the electronic signal that is produced when photons of light are detected by the instrument. Depending on how the instrument looks at the Earth (which is determined by the way the instrument works), the signals are assigned to picture elements, or pixels, the basic pieces of a remote sensing image.
The first level above raw data is Level 0. Navigational data and other relevant information from the satellite are assigned to the detected signal. This data ensures that the corresponding region on Earth that was being scanned from space is known. The electronic signal has not yet been converted to measured radiances.
To produce Level 1 data, the electronic signal from the detector is converted to radiances measured at the satellite, and information from the satellite's on-board calibration routine is added to the data. There are many different ways to maintain the accurate calibration of a satellite instrument. One of the most common calibration methods has the sensor scan a "source" possessing a known, consistent radiance. The Sea-viewing Wide Field-of-view Sensor (SeaWiFS), NASA's ocean color instrument that followed the CZCS, scanned a solar (sunlight) diffuser possessing a known radiance, and the sensor also performed a lunar calibration by viewing the Moon at a certain phase. (SeaWiFS, like most other remote-sensing instruments, was accurately calibrated before launch. On-board calibration methods such as these strive to insure that the calibration of the instrument is known throughout the mission.) Once the radiances are determined, the navigation data can be used to generate an image. However, more information must be used to make this image relevant to conditions at the surface of the Earth.
The conversion from Level 1 data to Level 2 data products applies sensor calibration data and atmospheric correction to calculate Earth surface radiances from the radiances measured at the satellite. The ongoing calibration routines ensure that the radiances will always represent the same absolute radiance, despite possible changes in the optical system of the instrument. Other checks on the quality of the data will also be applied here. Based on the radiances measured at the satellite, masks indicating the presence of clouds, land, and perhaps sea ice will be added to the data stream. Flags may also be added to indicate unusual conditions or anomalous data.
Several different kinds of data are used to derive the most accurate geophysical parameters. Data from sources other than the satellite itself is termed ancillary data. For ocean color, examples of ancillary data are wind speed (used to calculate sun glint masks and the presence of whitecaps), ozone (used for atmospheric correction, as ozone absorbs some light), and atmospheric pressure.
Once the surface radiances are calculated, new analytical routines can be applied that convert this information into different types of geophysical parameters, or products. For land surfaces, the radiances may indicate the different types of surfaces or the amount of land covered by vegetation. For ocean color, the radiances can be used to calculate the concentration of chlorophyll in the water, or the amount of suspended sediments. Thus, Level 2 data includes both Earth surface radiances and calculated geophysical parameters.
The conversion of radiances to geophysical products employs algorithms developed by painstaking research. Highly accurate measurements of radiation are made, using either radiometers or spectrometers, to characterize the radiative signature of a particular environment. For ocean color data, such radiometers must be immersed in the open ocean, and they will measure both the incoming (downwelling) and outgoing (upwelling) radiation. The instruments measure the variability of light at many different wavelengths (particularly those wavelengths that the sensor in space has been designed to measure). At the same time, samples of the environment, which may mean vegetation or soil on land, or water samples from the ocean, are examined.
In seawater, the concentrations of phytoplankton and their chlorophyll will be analyzed, and these concentrations will then be correlated with the measured radiances. As these measurements are made, researchers hope to find consistent relationships between the radiances and the surface variables that are being measured, which will allow them to construct an algorithm. The algorithm will calculate a specific variable, such as chlorophyll concentration, based solely on the radiance data. Satellite data is then used in these algorithms to calculate the geophysical parameters over large areas of the Earth.
