All satellite borne sensors measure the intensity of electromagnetic radiation arriving at the satellite. As such, for us to understand how a satellite can measure the concentration of sea ice in a given region, we must define electromagnetic radiation and describe the processes that affect the electromagnetic radiation as it leaves Earth's surface and travels toward the satellite.
Electromagnetic radiation refers to all energy that moves with the speed of light in a harmonic wave pattern. The electromagnetic spectrum (Figure 2.01) is a representation of the continuum of wavelengths, from long radio waves (left side of Figure 2.01) to the short x-rays and gamma rays (right side of Figure 2.01). Note that the portion of the electromagnetic spectrum we can detect with our eyes (the visible portion) constitutes a very small portion of the electromagnetic spectrum. The ability to infer sea ice concentration is based on measurements in the microwave portion of the spectrum, wavelengths of approximately 10-2m, longer than visible wavelengths.
All objects whose temperature is greater than absolute zero emit electromagnetic radiation. This electromagnetic radiation is commonly called thermal emission. The temperature of the radiating body determines the intensity and characteristics of the radiation it emits. Two electromagnetic radiation principles describe the relationship between a radiating body's temperature and the radiation it emits.
To visualize this, imagine an iron poker. At room temperature it's black. Once put into a fire, it begins to glow red. This is because as its temperature increases, it emits electromagnetic radiation in the visible part of the spectrum. As its temperature continues to increase, it will appear yellow-white. This is because the wavelengths of its maximum emissions moved from the longer red light to the shorter yellow light. Once the iron poker is removed from the fire, its color will become red again as its temperature decreases. As it cools further, it will return to its original black. However, if you touch it now, it will still be warm. As its temperature decreased, the wavelengths of its maximum emissions moved from the shorter visible light to the longer infrared (IR) wavelengths. At these wavelengths, we cannot see its emissions.
Now think about the Sun. We can see our Sun as yellow because it is so hot (6000 K) that it emits most of its energy in the visible portion of the electromagnetic spectrum (around 0.5 x 10-6m (see Figure 2.01). Based on Earth's temperature (~300 K), most of Earth's emitted radiation (from land, the ocean, and clouds) is in the longer thermal IR portion of the electromagnetic spectrum (around 10-5m, see Figure 2.01). Although we cannot see this radiation (because our eyes only detect visible light), satellite sensors can measure it and, from the amount of emitted radiation, infer the temperature of the land, ocean, and atmosphere.
The Sun and Earth also emit energy in the microwave portion of the electromagnetic spectrum. Since Earth is much colder than the Sun, however, more radiation will arrive at the satellite directly from EArth than from solar emissions reflected of Earth's surface.
Different natural bodies and surfaces will emit radiation differently based on their chemical composition and structure, even if they are at the same temperature. Emissivity is defined as the rate at which a natural body emits radiation relative to the rate of a perfect emitter (called a blackbody). The emissivity of a body or surface is independent of temperature and is a physical property of it. Because different bodies and surfaces have different emissivities, and therefore emit different amounts and types of radiation, they can be distinguished from space.
Through the application of the Rayleigh-Jeans approximation, the microwave radiation emitted by a surface can be expressed as a brightness temperature as follows:
TB = eTs
The ability to use brightness temperatures at microwave wavelengths for distinguishing ice covered waters from open ocean results from the wide contrast between the emissivities of ocean and sea ice, which causes a large contrast in the emitted brightness temperature of open ocean and sea ice. In Figure 2.02, the ice emissivities for both the horizontal and vertical polarization are greater than 0.9 at all wavelengths, and exhibit little spectral variation. The emissivities of water, on the other hand, are considerably lower, decrease with increasing wavelength, and have a greater degree of polarization.
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