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]

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