Basics of Satellite Orbits
Content
¾ Satellite orbit
¾ Geostationary orbit
¾ Polar orbit
¾ Sun-synchronous orbit
¾ Satellite repeat cycle
¾ Satellite ground swath
¾ Satellite ground receiving stations
Earth Orbit
Space
Satellite Orbits
¾ The path followed by a satellite in space is referred to as its orbit.
¾ A satellite always moves in a fixed plane. This is called the orbital plane, and in
the case of a satellite orbiting the Earth this plane always passes through the
center of the Earth.
¾ The orbit of a satellite can be elliptical or circular in shape, but remote sensing
satellites are usually put in circular orbits.
Polar orbit vs. Equatorial orbit
¾ If its orbit is inclined at more than 45 degree to the equatorial plane then a
satellite is in polar orbit.
¾ An orbit is less steeply inclined is called an equatorial orbit.
¾ Satellite orbits are designed according to the capability and objective of the
sensors they carry. Orbit selection can vary in terms of altitude and their
orientation and rotation relative to the Earth.
Orbit Velocity
¾ The law of gravity dictate that a satellite in a higher orbit travels more slowly
than one in a lower orbit
¾ A higher-orbiting satellite takes a good deal longer to circle the Earth than a loworbiting satellite.
Orbit period and height
Orbital height and perturbation
¾ Below 180 km, the Earth’s atmosphere is too dense for satellites to orbit without
burning as a result of frictional heating.
¾ Above 180 km there is still a small atmospheric drag on a satellite, causing its
orbit to spiral downward gradually until eventually it reaches thicker atmosphere
and burns.
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Types of Satellite Orbits
¾ Two general classes of circular orbits are widely used for meteorological and
environmental observations of the Earth: geostationary orbits and Sunsynchronous near-polar orbits (simply referred to as polar orbits).
Geostationary orbits
Characteristics
¾ The geostationary satellites, at altitudes of about 36,000 km, go around the Earth
at speeds which match the rotation of the Earth so they seem stationary relative to
the Earth's surface. Geostationary satellites complete a orbit in 24 hours.
¾ This allows the satellites to remain over specific areas and monitor and collect
information continuously and constantly.
¾ The orbit is circular, and its inclination is zero degrees, which means that it is
above the Earth's equator.
¾ Weather and communications satellites commonly have geostationary orbits.
¾ Due to their high altitude (almost three times the diameter of the earth, and some
geostationary weather satellites can monitor weather and cloud patterns covering
an almost entire hemisphere of the Earth.
¾ Since the satellite does not move in relation to the Earth, it can frequently and
repetitively observe and monitor the same portion of the Earth for the purpose of
detecting, tracking and predicting the weather or natural hazards.
¾ Making continuous observations over the same geographic areas permits
intensive study of both daily variability and changes over longer time periods, for
instance, tracking storms and hurricanes.
Geostationary satellites in orbit
¾ Global coverage needs a network of 5-6 geostationary satellites.
¾ Global coverage needs a network of 5-6 geostationary satellites.
GOES
Polar orbiting satellites
Characteristics
¾ Most of the remote sensing satellite platforms today are in near-polar orbits for
meteorological and geophysical applications.
¾ Polar orbiting satellites closely parallel the earth's meridian lines. They pass over
the north and south poles each revolution.
¾ Polar orbiting satellites can provide global or near global coverage of the
atmosphere and Earth surface.
¾ Polar satellites circle at a much lower altitude (~800km) providing higher quality
remote sensing data (more detailed information) than geostationary satellites.
¾ Most satellites with near polar orbits have altitudes ranging from 600 to 800 km,
with orbital periods of 98 to 102 minutes.
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Sun-synchronous orbits
¾ A sun synchronous orbit means that a satellite pass over each area of the Earth’s
surface at a constant local time of day called local solar time.
¾ To achieve this condition, the orbit cannot exactly follow a true north-south track
to go over the poles.
¾ Actually, the orbit must be slightly tilted with a steep inclination angle of about
98º. This introduces a slow precession in the orbital plane westwards over the
ground at a rate comparable with the Earth’s rotation, roughly one degree per day.
¾ Precession ensures that the equatorial crossing times of the satellites, in terms of
the local solar time, remain nearly constant throughout the year.
¾ Constant Equatorial crossing time
¾ Constant solar zenith angle for a given area
¾ Necessary for change detection
Ascending and Descending Passes
¾ Ascending passes of the orbit corresponds to that portion of the orbit when the
satellite is moving from south to north, while descending passes of the orbit
corresponds to north to south movement.
¾ Most sun-synchronous polar orbiters pass from north to south (descending passes)
over the sunlit hemisphere and return from south to north (ascending passes) over
the night-time hemisphere. In other words, the ascending pass is on the shadowed
side of the Earth while the descending pass is on the sunlit side.
Polar orbiting satellites in orbits
¾ Examples include POES, DMSP, Landsat, SPOT, IRS, etc.
Space Shuttles
Ground Swath of Satellites
¾ As a satellite revolves around the Earth, the sensor images a certain portion of the
Earth's surface. The area imaged on the surface is referred to as ground swath.
¾ Imaging swaths for space-borne sensors generally vary between tens, hundreds,
and thousands of kilometers wide.
¾ Since the orbital period is much less than 1 day, images of several ground tracks
can be acquired within 24 hours.
¾ As the satellite orbits the Earth from pole to pole, its east-west position wouldn't
change if the Earth didn't rotate. However, as seen from the Earth, it seems that
the satellite is shifting westward because the Earth is rotating (from west to east)
beneath it. This apparent movement allows the satellite swath to cover a new area
with each consecutive pass.
Inclination of the Landsat Orbit to Maintain A Sun-synchronous Orbit
Satellite Repeat Cycles
Repeat Cycles (Revisit Period)
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¾ If we start with any randomly selected pass in a satellite's orbit, an orbit cycle
will be completed when the satellite retraces its path, passing over the same point
on the Earth's surface directly below the satellite (called the nadir point) for a
second time.
¾ The revisit period is an important consideration for a number of monitoring
applications, especially when frequent imaging is required (for example, to
monitor the spread of an oil spill, or the extent of flooding).
¾ In near-polar orbits, areas at high latitudes will be imaged more frequently than
the equatorial zone due to the increasing overlap in adjacent swaths as the orbit
paths come closer together near the poles.
Ground Receiving Stations (GRS)
¾ Data obtained during airborne remote sensing missions can be retrieved once the
aircraft lands. It can then be processed and delivered to the end user. However,
data acquired from satellite platforms need to be electronically transmitted to
Earth, since the satellite continues to stay in orbit during its operational lifetime.
¾ There are three main options for transmitting data acquired by satellites to the
surface:
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The data can be directly transmitted to Earth if a Ground Receiving Station
(GRS) is in the line of sight of the satellite.
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If the satellite is not in the mask of a GRS, the data can be recorded on
board the satellite for transmission to a GRS at a later time. Tape recorders
use power and make the satellite heavier to launch, so not all remote-sensing
satellite carry them.
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Data can also be relayed to the GRS through the Tracking and Data Relay
Satellite System (TDRSS), which consists of a series of communications
satellites in geosynchronous orbit. The data are transmitted from one
satellite to another until they reach the appropriate GRS.
Suggested Reading:
¾ Chapter 2 in Jensen, J.R. 1996. Introductory digital image processing: a remote
sensing perspective. 2nd Edition, Upper Saddle River, NJ, Prentice Hall. 318pp.
Please see the class slides for the details.
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