Geospatial Data / Remote Sensing Tutorial / Introduction to Satellites and Orbits
Introduction to Satellites and Orbits
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]
Shape
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.
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.
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.
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.
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 0º 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 90º
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 90º.
Polar orbiting satellites
While a true polar orbit has an inclination of 90º, many satellites
orbit the earth with inclinations that are close to 90º. 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 km. 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.
As a polar orbiter 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 0º.
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 orbiters
Highly Elliptical Orbits (HEO) satellites orbit the Earth in an
orbital plane with an inclination between 50 and 70º. 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.
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.
References
- NASA
Observatorium Education-Reference Module: Satellite Orbits
- http://observe.arc.nasa.gov/nasa/education/reference/orbits/orbits.html
| 
