Sensors

• To describe how sensors are used to detect the spectral characteristics of a remote sensing target
• To describe how spectral signatures are important in the design and purpose of a sensor.
• To differentiate between active and passive detection sensors

Detection of Electromagnetic radiation

Radiometers are instruments that are sensitive to varying amounts of electromagnetic radiation. Radiometers are designed to measure energy levels in well-defined ranges of wavelengths known as channels. A channel is a relatively narrow band of wavelengths within a portion of the electromagnetic spectrum. Radiometers are engineered to use specific channels based on the information about the target provided by the channel. Multi-spectral remote sensing makes use of a radiometer that is comprised of an array of sensors, each tuned to a particular channel or band of wavelengths, in order to provide spectral data about a target across a range of energy levels.

Radiometers on aircraft or satellites scan the Earth and measure the levels of radiation that is reflected off or emitted from the materials on the surface or in the atmosphere. This information is transmitted back to Earth and usually converted into an image. Since each type of surface material on earth and each type of particle in the atmosphere has its own unique spectral characteristics (or spectral signature) these data can be used to discern a great deal of information about the nature of the target.

For an illustration, refer to the graph above, which compares the spectral signature of four surface types in the visible light spectrum. The curves on the graph illustrate the percent of energy reflected by each surface at each wavelength. Thus, a sensor can be designed to detect energy in specific wavelengths to provide known information about the surface type being scanned.

For example, weather satellite sensors are designed to detect energy in the visible, near infrared, and thermal infrared portions of the electromagnetic spectrum. The visible and near infrared channels measure the intensity of reflected solar radiation. The thermal channels measure the amounts of heat energy emitted from the various surface materials and atmospheric components. Together, the combination of data from each channel offers a deep set of information about the state of the atmosphere at any given time.

The radiometers on land use satellites such as Landsat and Spot are engineered to provide multispectral data that aids in measuring the spectral differences between varying surface materials. Different land surface types such as concrete, asphalt, crops, meadow, forest, water, and desert all exhibit unique spectral signatures. Even within one category of land use, differences exist. For example, corn, soybean, and wheat can be classified as crop land, but each will exhibit a unique spectral pattern when imaged with a multispectral radiometer. These differences can be extended even further. For example, a healthy crop of soybeans will exhibit a different spectral signature than one that is suffering from drought or a pest infestation.

All of the varying materials on the Earth's surface and in its atmosphere interact differently and uniquely with electromagnetic radiation. Through the use of satellite remote sensing technologies, these differences can be detected and measured from space, providing us with a very rich set of tools with which we can better monitor and understand our environment.

Passive Detection

Many forms of remote sensing use passive detection, in which sensors measure levels of energy that are naturally emitted, reflected, or transmitted by the target object. Passive sensors are those which are designed to detect naturally occurring energy. Most often, the source of radiative energy is the sun. The sun's energy is either reflected, as it is for visible wavelengths, or absorbed and then red-emitted, as it is for thermal infrared wavelengths.

Passive detection can only work when the naturally occurring energy is available. Detection of reflected solar energy, for example, can only proceed when the target is illuminated by the sun, thus limiting visible light sensors on satellites from being used during a nighttime pass. The amount of solar radiation present at polar latitudes is often insufficient for visible light sensors, limiting the use of passive detectors to lower latitudes. Clouds, dust, smoke, and other particles in the atmosphere can block reflected energy from reaching a sensor.

The problems associated with passive sensing can be overcome when designing a remote sensing system. One common method is to use a sensor that is capable of detecting radiation in several different portions of the electromagnetic spectrum. For example, by using a combination of visible and thermal infrared channels, weather satellites can provide imagery of the Earth's cloud patterns during both day and night hours. A combination of visible channels and reflected infrared channels can also be used to mathematically correct an image for atmospheric interference, which is caused by energy interacting with and being absorbed by particles in the atmosphere before it reaches a sensor.

The Thematic Mapper, the primary sensor on the Landsat satellites, is a good example of a passive sensor. This sensor has seven bands, or channels, each being sensitive to a different range of electromagnetic radiation. The sensors on the Thematic Mapper are sensitive to narrow portions of the visible and near infrared portion of the spectrum, with one band sensitive to thermal infrared. The selected range of wavelengths are specifically designed to detect differences in plant production, soil moisture, and mineral content in soils, providing a useful tool in assessing and monitoring land use practices. The sensors depend on available reflected solar energy, so the Landsat satellite is placed into an orbit that ensures that the satellite will pass overhead at the time when the amount of solar radiation is optimal for the sensor.

Active Detection

Other forms of remote sensing provide their own energy source for illumination of the target. These devices, known as active sensors, direct a burst of radiation at the target and use sensors to measure how the target interacts with the energy. Most often the sensor detects the reflection of the energy, measuring the angle of reflection or the amount of time it took for the energy to return. Active sensors provide the capability to obtain measurements anytime, regardless of the time of day or season. They can be used for examining energy types that are not sufficiently provided by the sun, such as microwaves, or to better control the way a target is illuminated. However, active systems require the generation of a fairly large amount of energy to adequately illuminate targets.

Doppler radar is an example of an active remote sensing technology. A Doppler radar device is a ground-based system that emits radio energy in a radial pattern as the transmitter rotates. A sensor measures the reflection, or echoes, of this energy off such atmospheric particles as dust, raindrops, and even birds! These echoes, when plotted on a regional map, assist a meteorologist in determining the exact location of storm centers, measuring the speeds in the wind field of a storm, and notifying the public of areas of potentially severe weather.

Another form of active collection is the atmospheric sounder, which uses various forms of energy, including lasers, microwaves, and radar, to take measurements of the density of the atmosphere at certain altitudes, thus providing detailed data about a wide variety of phenomena that includes wind speeds, pollution levels, and atmospheric composition. Sounders can be ground-based and measure from the ground up, or they can be mounted on an airborne or satellite platform and measure down through the atmosphere. Data from sounding equipment can be used to construct 3-dimensional models of the state of the atmosphere and often form the basis of prediction models used to determine future weather patterns.

The following image is an example of the type of data that can be generated from an active sensor flown on a satellite. The image was produced from data gathered with the Precipitation Radar, flown on the Tropical Rainfall Measuring Mission (TRMM) satellite. This is the first spaceborne instrument designed to provide three-dimensional maps of storm structure. This is accomplished using a narrow beam radar that is transmitted from the satellite through the atmosphere. When the radiation strikes raindrops in the atmosphere it is echoed back up to the satellite. The size and height of the raindrops is discerned from the pattern of the returned radar pulses. The measurements have yielded invaluable information on the intensity and distribution of the rain, on the rain type, on the storm depth and on the height at which snow melts into rain. The estimates of the heat released into the atmosphere at different heights based on these measurements can be used to improve models of the global atmospheric circulation.

[3D slice of a Hurricane from TRMM precipiation radar]

Reference