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Detection of Electromagnetic Radiation

Radiometers are instruments that are sensitive to varying amounts of electromagnetic radiation. Radiometers are designed to "sense," or 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.

Comparison of the Spectral Signatures (Reflectance) for Four Surface Types in Visible Light
Figure 1. Comparison of the Spectral Signatures (Reflectance) of Four
Surface Types in Visible Light

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 re-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 (TM), the primary sensor on the Landsat 5 and 7 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 NASA's 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.

Three-Dimensional Slice of a Hurricane from the TRMM Precipitation Radar
Figure 2. Three-Dimensional Slice of a Hurricane from the TRMM Precipitation Radar

Satellite Orbits

Satellites orbit their primary body in a shape that is called an ellipse. An ellipse can be thought of as a circle that is somewhat "out of round," although the technical definition of an ellipse is "a closed plane curve generated by a point moving in such a way that the sums of the distances from two fixed points is a constant." The characteristics of an ellipse are probably best understood when compared to a circle.

Circle v. Ellipse
Figure 3. Circle v. Ellipse

Shape. A perfect circle has a single point in its center. Each point on the circle is an equal distance from this point, known as the focus of the circle. Any line that connects two sides of the circle and passes through the focus are equal in length.

An ellipse is a circle that is slightly stretched in one dimension. An ellipse has two focal points. The sum of the distances from any point to each focal point will always remain constant. A line that connects two sides of the ellipse and passes through both focal points is called the major axis of the ellipse. A line perpendicular to the major axis that passes through the point directly between the focal points is the minor axis of the ellipse.

Measurements of an Ellipse
Figure 4. Measurements of an Ellipse

The degree to which an ellipse is stretched is described as the eccentricity of the ellipse. The eccentricity can be described as the ratio between the length of the major axis and the distance between the foci of the ellipse. The foci in highly eccentric ellipse are spread farther apart than those of an ellipse with a lower eccentricity. The value of eccentricity ranges from 0 (a perfect circle) and approaches a value of 1 as the ellipse becomes more eccentric.

Elliptical Eccentricity
Figure 5. Elliptical Eccentricity

A satellite's orbit around the Earth is in the shape of an ellipse, and the Earth's center of mass is at one of the focal points of the ellipse. A satellite orbit can be described by the eccentricity of the orbit. Satellite orbits range from nearly circular orbits (with a low eccentricity) to very highly elliptical orbits (with a high eccentricity). A satellite with a nearly circular orbit maintains a relatively constant altitude above the Earth's surface, while the altitude of a satellite with a very highly elliptical orbit is constantly changing.

Size. Another of the orbital elements used to describe a satellite's orbits is the semi-major axis, which is defined as half the distance of the major axis. In general, the larger the semi-major axis, the larger the orbit. The larger the orbit, the greater the amount of energy required to place the satellite into the orbit. Thus, satellites with larger orbits with higher altitudes above earth are much more expensive to launch and maintain.

Velocity. As a satellite orbits the Earth in an elliptical orbit, its distance from the Earth's surface changes. The point in its orbit at which the satellite is closest to the Earth is called perigee. The point opposite to perigee, when the satellite is at its furthest point from Earth, is called apogee. As a satellite approaches perigee, its orbital velocity increases. At perigee, the satellites velocity is at its maximum. As it approaches apogee, its orbital velocity decreases. At apogee, the satellites velocity is at its minimum. Thus, satellites with a nearly circular orbit maintain a nearly constant orbital velocity, while satellites with highly elliptical orbits have a wider range of orbital velocities, speeding up as they get closer to Earth and slowing down as they move further away.

Apogree, Perigee, and Velocity
Figure 6. Apogee, Perigee, and Velocity

Period. The period of an orbit is the amount of time it takes for a satellite to complete one full orbit around its primary body. A general rule of orbital mechanics states that the closer an orbiting object is to its primary body, the higher its velocity. In addition, the closer a satellite is to the Earth, the less distance it must travel to complete a single orbit. The result is a general relationship between a satellite's altitude and its period: the lower the altitude, the shorter its period. The lowest satellites orbit the earth with a period of approximately 87 minutes per orbit (if a satellite were placed any lower in the orbit the atmosphere would interfere so much that it could not maintain its orbit). Other satellites at higher altitudes have orbital periods that are longer than a full 24 hour day.

Inclination. Inclination of a satellite orbit describes the tilt of the orbit plane with respect to the equatorial plane. An orbit with inclination angle of 0 would orbit the Earth in the same plane as the Equator. This is known as an equatorial orbit, and a satellite in this type of orbit follows the Earth's equator. An orbit with an inclination angle of 90 would orbit the Earth crossing the North and South Poles in a plane that is perpendicular to the equatorial plane. This type of orbit is known as a polar orbit. Other satellites are in orbits with inclinations between 0 and 90.

Polar Orbiting Satellites. While a true polar orbit has an inclination of 90, many satellites orbit the earth with inclinations that are close to 90. These form a class of satellites known as polar orbiting satellites. These satellites orbit the Earth in an orbital plane that goes nearly from pole to pole. They are considered Low Earth Orbiters (LEO), which orbit the Earth at an altitude of approximately 300 kilometers. Polar orbits are usually nearly circular and the satellites have a constant height above the planet. They generally have a period around 90 minutes maintain a constant orbital velocity.

Polar Orbiting Satellites
Figure 7. Polar Orbiting Satellites

As a polar orbiting satellite circles the planet, and as the Earth rotates underneath, the satellite crosses a different strip of the Earth with each orbit. The effect is that a polar orbiting satellite can scan the Earth in strips, and over the course of several orbits, it can collect data over a significant portion of the planet. The lower altitude of the polar orbits can allow the sensors to study the Earth in greater detail than a higher altitude craft, and it is far less expensive to build, launch, and maintain than a higher altitude satellite.

This type of orbit is primarily used for surveillance, environmental monitoring, and space related research. Examples of polar orbiting satellites are the Landsat satellites, the TIROS-class meteorological satellites, the space shuttle, and the Mir space station.

Geostationary Satellites. A geostationary satellite orbits the Earth in an equatorial orbit at an altitude where its period is equal to that of the Earth's rotation (24 hours). The result is that the geostationary satellite turns with the Earth and remains over the same fixed point of the planet at all times. A geostationary orbit is usually circular with an inclination of 0°.

Geostationary Satellites
Figure 8. Geostationary Satellites

The fixed nature of a geostationary satellite with respect to a given point on the Earth makes them very useful for surveillance, communications and broadcasting, and environmental monitoring. Satellite television broadcasts make use of geostationary satellites, as do many of the telecommunications companies around the world. The GOES meteorological satellites operated by the U.S. provide constant satellite coverage of the entire hemisphere from a geostationary orbit.

One limitation of a geostationary satellite is that the platform is only useful from the equator up to a latitude of about 70 degrees north and south of the equator. Therefore, to provide communications or other satellite support to higher latitudes, either polar orbiter or highly elliptical orbiters must be used.

Highly Elliptical Orbits. Highly Elliptical Orbit (HEO) satellites orbit the Earth in an orbital plane with an inclination between 50 and 70. The period of a HEO satellite is approximately 12 hours and the shape of the orbit is highly elliptical. During the HEO satellite's orbit, it comes very close the planet for part of its orbit, causing its velocity to increase. Then it travels very far away from the Earth and its orbital velocity decreases. As a result it spends much of its time in the portion of the orbit that is at a very high altitude.

High Elliptical Orbit Satellites
Figure 9. High Elliptical Orbit Satellites

A HEO satellite is placed in an orbit in such a way that it will spend the greatest amount of time over a specific area of the planet. Thus, if a HEO communications satellite is launched to provide communications in the arctic region, its orbit would be configured so that is spends the bulk of its time in orbit above these latitudes. This is especially useful in providing communications and navigation services from a satellite. One example of HEO satellites is the Global Positioning System, which uses a fleet of HEO satellites to provide constant coverage of the entire planet and which are capable of providing precise locational data to any point on the surface of the Earth.


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