Remote Sensing Fundamentals

How Sensors Work: The Basics

The light we see with our eyes can be thought of as having the properties of a wave and can be described by two related properties, the wavelength and the frequency. The wavelength is the distance between two wave peaks and the frequency is the number of waves that pass a point in one second. The colors of the rainbow each have a unique frequency and are just a small part of the electromagnetic spectrum.

Electromagnetic Spectrum
Sensors that collect electromagnetic energy, typically called radiometers, can measure the radiation reflected from, or emitted by, objects. Those measurements give information about the objects. As an example, your eye contains sensors (cones in this case) that are sensitive to red, green, or blue light. Looking at a red shirt will result in a high signal in the red sensitive cones and a low signal in the green and blue sensitive cones.

Sources of Electromagnetic Radiation

Everything is a source of electromagnetic radiation. The amount of radiation at each wavelength is primarily determined by the temperature of the object. The higher the temperature, the more radiation is emitted at any given wavelength. If an object is hot enough, it will emit visible light and you will be able to see it (e.g., the sun or a lightbulb). Cooler objects, such as a human or the earth, emit the most energy in the infrared part of the spectrum and emit very little visible light. Radiation based on the temperature of an object is called "blackbody radiation." There are other mechanisms for generating radiation, such as lasers, radio antennae, etc. These typically generate a single wavelength instead of a broad spectrum.

Radiation Interactions

As the electromagnetic waves travel through space, their energy interacts with matter. In a simplified view, one of the following interactions occur:

reflection off the object;
absorption by the object; or
transmission through the object.

The total amount of radiation that strikes an object is referred to as the incident radiation, and is equal to:

incident radiation = reflected radiation + absorbed radiation + transmitted radiation

Example of Reflected, Absorbed, and Transmitted Radiation
The relative amount of radiation that is absorbed, transmitted, or reflected is determined by the material and the wavelength of the radiation. Depending on the application, these interactions may be the source of the signal, or may confound the desired signal. Below are some examples:

Reflection as the signal: This is probably the most common remote sensing situation. This reflected radiation is what our eyes use to see objects, causes near-infrared film to record vegetation, and allows radar images of the earth to be created.
Absorption as the signal: Atmospheric gases, such as ozone, will absorb at specific wavelengths. By measuring sunlight passing through the atmosphere to the ground and calculating how much energy was lost by absorption, the concentration of a specific gas can be estimated.
Absorption as a confounding signal: Often absorption of light will introduce uncertainty in the desired measurement. Almost all remote sensing of the earth's surface from space is confounded by atmospheric absorption. For the visible part of the spectrum, absorption is only a serious problem when looking at relatively dark objects, such as the ocean, or if there are clouds. In these cases, the desired signal may be reduced to the noise level of the sensor, making interpretation impossible. For other parts of the spectrum, absorption may be so high that remote sensing at those wavelengths will not work, and only a few "atmospheric windows" through the atmosphere are available. If we wish to look at the temperature of the earth by measuring the radiation emitted by earth, we are limited to a small window between wavelengths of 10.5 and 12.5 micrometers. This window is used for sea surface temperature measurements.

Spectral Reflectance

Spectral reflectance is the portion of incident radiation that is reflected, as a function of wavelength, by a nontransparent surface. The fraction of energy reflected at a particular wavelength varies for different features. Additionally, the reflectance of features varies at different wavelengths. Thus, two features that are indistinguishable in one spectral range may be very different in another portion of the spectrum. This is an essential property of matter that allows for different features to be identified and separated by their spectral signatures.

A spectral signature is a unique reflectance value in a specific part of the spectrum. Displayed in the graph below are the spectral signatures for healthy green vegetation, stressed vegetation, and severely stressed vegetation. In the visible region on the electromagnetic spectrum, the three spectral signatures look similar. However, in the near-infrared region of the spectrum, the spectral signatures look very different from each other. The healthy vegetation has the highest reflectance value while the severely stressed vegetation has the lowest reflectance value.

Spectral Reflectance of Vegetation Over Different Wavelengths.

More About Radiation and Materials

The above simplified view of radiation interaction with matter neglects a number of other interactions that can be important for remote sensing. These include

Scattering;
Blackbody Emission; and
Fluorescence.

Only a short description of these interactions will be included here.

Radiation Interactions Including Scattiner and Emission

Example of Radiation Interactions Including Scattering and Emission

Scattering: Scattering is the change in the direction of electromagnetic energy. No energy is lost; the energy (or photons) simply travels in another direction. Two important sources of scattering are particle scattering and density-fluctuation scattering. Particle scattering occurs where the material is nonhomogeneous and if the particles are sufficiently large, can be considered similar to reflection. This would be the case in something like a pine forest, where the light reaching the ground has been scattered off pine needles. For small particles, such as phytoplankton in the ocean or aerosols in the atmosphere, the scattering properties follow more complex rules that depend on several properties, including particle size and photon wavelength. For remote sensing of the ocean, scattering provides the signal for determining what is in the water. Scattering in the atmosphere, especially by clouds and aerosols, confounds or completely obscures many remote sensing signals. When you cannot see the mountains in Los Angeles through the smog and haze, it is the scattering that obscures them, not absorption.
Density-fluctuation scattering occurs in liquids. Continual random motions of molecules cause localized microscopic density fluctuations. The fluctuations lead to scattering similar to the Rayleigh scattering of gases in the atmosphere, a process that scatters short wavelength light more than long wavelength light. This is part of the reason that both the open ocean and the sky look blue.
Emission: All objects emit electromagnetic radiation. The amount of radiation and its spectral shape are determined by the object's temperature and its emissivity. There is a simple equation describing how much energy at each wavelength a perfect object, a "blackbody," will radiate as a function of temperature. The emissivity is a measure of how well an object can radiate energy compared to a blackbody. These properties allow the use of remote sensing to measure the temperature of an object. Knowing the emissivity of an object and measuring the radiation emitted, the temperature can be derived. This mechanism is the basis for measurements of sea surface and cloud top temperature from satellite.
As a further example, the temperature dependence of the emission can be used for night vision instruments. The peak emission wavelength for a human is in the infrared at 9.4 micrometers, while an object at 15.5° Celsius (60° F) would have a peak at 10.1 micrometers. Using a detector sensitive to 9.4 micrometer radiation, a human or animal can easily be seen within a cooler nighttime environment.
Fluorescence: Fluorescence is the absorption of light followed by a reemmision of light at a lower energy (longer wavelength). There are many substances with this property, which allows the use of remote sensing to detect those substances. An example of fluorescence can be seen in plants. The chlorophyll in plants will absorb blue light and reemit red light.

Spatial Resolution

Most remotely sensed images are not captured with film in a camera (aerial photographs are the exception to this rule). Rather, images are captured with a digital sensor mounted to an aircraft or satellite. The sensor records energy reflected from the earth. This information is then transferred to users, where it can be processed with a variety of computer software applications.

Remote sensing software applications have been developed for users to see a pictorial representation of the image. Images displayed on a computer screen are composed of pixels (picture elements), the smallest unit of a digital image, that contain both spatial and spectral properties.

A pixel's spatial properties provide information about the resolution or area represented on the earth while its spectral properties provide information about the intensity of the spectral response collected from the sensor.

Spatial resolution describes the area of the earth that each pixel represents. For example, an image might have a spatial resolution of 3 meters. This means that each pixel in the image represents an area on the earth 3 meters by 3 meters. Such an image would be considered high-resolution imagery. High-resolution imagery allows details, like houses and cars, to be seen sharply and clearly. This type of imagery is often used for community and urban planning and for agricultural purposes. Generally, the higher the spatial resolution of the imagery, the smaller the region of earth covered in each image. In order to see a large area, such as a county or a state, numerous high-resolution images would be required—an expensive and time-consuming effort. If an organization is working on a regional scale, lower-resolution imagery, which covers a greater area of land, might be a better choice.

Comparison of a Landsat TM image and an Aerial Photo

The Scale of a Project Usually Determines What Type of Imagery Should Be Used.

Imagery of lower resolution can be used when studying or planning larger regions on the earth, such as a county, state, or even a country. Do not be fooled by the term "lower," it does not mean the imagery is of lesser quality. Rather, the term "lower resolution" means the spatial extent covered by each pixel in the image is large. Thus, this type of imagery can be used for identifying large features such as lakes, forests, and urban areas that cover a substantial amount of the earth's surface. Qualitative terms such as "high" and "low" resolution are often relative to the problem. A resolution of 30 meters might be low resolution for a city planner, but very high resolution for an oceanographer.

Satellite Images Provide Excellent Regional Information.

Temporal Resolution

Temporal resolution refers to how often an area can be imaged. In general, there is a trade-off between spatial resolution and temporal resolution. A sensor such as Landsat Thematic Mapper will provide 30 meter pixels, but can only image a given area once every 16 days. On the other hand, the Advanced Very High-Resolution Radiometer can image the entire earth every day, but has 1.1 kilometer pixels. Even higher temporal resolution is obtained by geostationary satellites such as the GOES weather satellites. The GOES satellites provide the same view of the earth every 30 minutes at 1 kilometer spatial resolution. As with spatial resolution, the required temporal resolution is dependent upon the application.

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