EARTH ORBITS (I)
• Every object (natural, e.g. meteorites, asteroids, etc or artificial, e.g., satellites,
junks, and space stations) moves around the earth in a particular track - orbit.
•
• The orbit is defined by 3 factors.
– Shape – orbital shape can be either circular or elliptical.
– Altitude of the orbit - constant for a circular orbit but changes constantly for an
elliptical orbit.
– Angle the orbit makes with the equator. An orbit that brings the satellite over the
poles or close to it has a large angle.
• An orbit that makes the satellite stay close to the equator has a small angle.
• Orbits depend on the purpose of the mission, OST and the ITU regulations.
EARTH ORBITS (II)
• The following orbits can be defined:
• Low Earth Orbit (LEO)
Satellites in low earth orbit (LEO) orbit the earth at altitudes of less than 2000 km (1242
miles). Satellites in LEO can get much clearer surveillance images and require much less
power to transmit their data to the earth (e.g., ISS, IKONOS and most of the high spatial
res. Earth Observation Satellites)
Medium Earth Orbit (MEO)
At an altitude of around 10.000 km (6.000 miles) a satellite is in medium earth orbit (MEO).
This altitude balances the advantages and disadvantages of LEO and GEO. MEO’s are used
generally for navigational satellites and communications satellites (e.g., Iridium, GPS, etc).
• Geostationary Orbit (GEO)
Altitude is approximately 35,790 km . GEO orbits are used primarily for weather and
atmospheric monitoring. Intercontinental communication linking also make effective use of
the GEO orbit.
SPACEBORNE REMOTE SENSING: ORBITAL CLASSIFICATION (I)
• Polar Orbits
– Satellites go over the north and south
poles
– These satellites provide a more global
view of Earth, circling at near-polar
inclination (the angle between the
equatorial plane and the satellite orbital
plane (ideal polar orbiters are inclined
at 900)
– Orbiting altitude is between 500 and
800 km (e.g. Landsat, SPOT, etc).
– Very effective in covering areas of the
World that are very difficult to access
(e.g. the Antarctica,
Figure 14: Polar Orbit
– They are sun synchronous (i.e.,
movement coincides with the daily local
solar time of any imaging geographical
space/area)
SPACEBORNE REMOTE SENSING: ORBITAL CLASSIFICATION (II)
• Geostationary Orbits
– Orbit is one in which the satellite is always in
the same position with respect to the rotating
Earth.
– Satellite orbits at an elevation of
approximately 35,790 km (e.g. Meteosat,
Envisat, GOES, etc).
– By orbiting at the same rate, in the same
direction as Earth, the satellite appears
stationary (i.e., synchronous with respect to
the rotation of the Earth.
– Geostationary satellites provide a "big
picture" view due to its large swath (Figure
15) and effective in covering weather events
due to its high repetitiveness.
– Due to its permanence at the equatorial
plane, it provides distorted images of the
Figure 15: Geostationary Orbit
polar regions with poor spatial resolution.
INCLINED ORBITS
• Inclined orbits fall between Polar and Geostationary Orbits.
• They have an inclination between 0 degrees (equatorial orbit) and 90 degrees (polar orbit).
• These orbits may be determined by the region on Earth that is of most interest (i.e., an
instrument to study the tropics may be best put on a low inclination satellite), or by the
latitude of the launch site.
• The orbital altitude of these satellites is generally on the order of a few hundred km, so the
orbital period is on the order of a few hours.
• These satellites are not sun-synchronous, however, so they will view a place on Earth at
varying times
– Examples are
• Molniya
• Elliptical
• Sun Synchronous orbits = Geo Synchronous orbits
• Geo Synchronous orbits ≠ Sun Synchronous orbits
• Geo Synchronous orbits = Geostationary orbits
Figure 16: Illustration of an inclined orbit
ORBIT SELECTION CRITERIA
• User requirements
– Latitude coverage
– Desired spatial resolution
– Repeat cycle time
– Swath width
– Illumination (Sun Synchronous or not)
• Sensor type (Sensor for the GEO will be determined by the power requirement)
• Imaging mode (active or passive).
• Satellite design
– Power requirements
– Thermal control
– Air drag
– Attitude and Orbit Control Systems (AOCS), etc…..
GEO Vs. LEO Operational Orbits
• GEO LEO
Advantages Advantages
Ø Satellite appear stationary in the sky so Satellite in LEO are much closer to
the ground antenana pointing is easy. allowing low power ground
terminals such as hand-held
telephones.
Ø 42 % of the Earth’s surface can be covered
by one satellite.
Disadvantages
Ø Time delay (1/4 seconds) To provide full coverage of Earth’s surface, a
satellite constellation (e.g., DMC – Nigeria,
China, UK, Turkey and Algeria) will be
required.
