Lecture 5: Orbital Perturbations, launch and orbital effects
Spring 2021
Orbital Perturbations
v Under ideal conditions, we consider the orbit as an ellipse
whose properties are constant with time.
v In practice, the satellite and earth respond to many other
influences including asymmetry of earth’s gravitational field, the
gravitational fields of sun and moon, and solar radiation
pressure.
v For LEO satellites, atmospheric drag can also be important.
v All of these interfering forces cause the true orbit to be
different from ellipse.
v The orbital disturbances are assumed to cause the orbital
elements to vary with time
Orbital Perturbations
There are two types of Orbital Perturbations:
1) Gravitational, when considering third body interaction and the
non-spherical shape of the earth.
2) Non-gravitational like atmospheric drag, solar-radiation
pressure and tidal friction.
v These types can also be classified as conservative or
non-conservative disturbances forces where:
1) Conservative forces depend only on the position
2) Non-conservative forces depend on both position and velocity.
Sources of orbital perturbations
v Major sources of orbital perturbations
1) Perturbations due to non-spherical Earth
2) Third body perturbations
3) Atmospheric drag
4) Solar radiation and solar wind
The Non-Spherical Earth
v The earth is neither a perfect sphere nor a perfect ellipse.
v The earth is flattened at the poles; the equatorial diameter is about
20 km more than average polar diameter.
v The equatorial radius is not constant (does not vary more than 100m
around equator)
v There are some regions on earth where the gravitational attraction
is higher, referred as regions of mass concentration or MASCONS.
v These nonregular features of the earth lead to nonuniform
gravitational field around earth.
The Non-Spherical Earth
v One depends on the rotation, making the radius from center
of the earth to the equator larger than from the radius from
the center of the earth to the poles:
Ø Equatorial radius: ~ 6,378 km
Ø Polar radius: ~ 6,356 km
Ø Equatorial radius not constant
(small variations ~ 100m)
v Earth mass distribution
Ø Earth mass distribution not uniform
Ø Regions of mass concentrations: MASCONS
v For LEO and MEO satellites this effect is not very significant
Third-body perturbation (SUN & MOON)
• Motion of the satellite is not a “two-body” problem
• Satellite experiences gravitational pull from Sun and Moon
• The orbital relationship is complex and time dependent
• Gravitational forces from Sun and Moon tend to move
satellite out of the orbit
• Under these conditions the orbit will precess and its
inclination will change (up to 1°/year)
Orbital position of a satellite the Earth, the Sun and the Moon
Atmospheric drag
• Drag is the resistance offered by a gas or liquid to a body
moving through it.
• Significant for satellites altitude below 1000km such as LEO
• A spacecraft is subjected to drag forces when moving through
a planet's atmosphere. This drag is greatest during launch and
re-entering planet's atmosphere.
• However, even a space vehicle in Low Earth Orbit (LEO)
experiences some drag as it moves through the Earth's thin
upper atmosphere.
Atmospheric drag
• In time, the action of drag on a space vehicle will cause it to
spiral back into the atmosphere, eventually to disintegrate or
burn up.
• Drag reduces the velocity of the satellite
Ø Semi-major axis is reduced
Ø Eccentricity is reduced
Satellite launch
vTwo classification of satellite launches
1. Expandable launch vehicles:
Ø Ariane (EU)
Ø Atlas, Delta (US)
Ø Soyuz (Russia)
2. Reusable launch vehicles:
Ø Space Shuttle (STS)
Ø Dragon, Falcon 9 (Space X)
Ø Buran, Orion
Reusable Launch Vehicle (RLV)
v Reusable launch vehicle (RLV) is a space launch vehicle
which can be used for several space missions.
v The main objective of RLV is to return the first
stage/boosters of a launch vehicle at the launch site.
v The main advantage of RLV is that it can be used multiple
times, hopefully with low servicing costs.
v A successful RLV would will decreases the missions' cost and
make space travel more accessible.
Reusable Launch Vehicle (RLV)
Expandable Launch Vehicle (ELV)
v Expandable Launch Vehicle (ELV) is a single-use launch
vehicle usually used to launch a payload into space.
v ELV typically consist of stages which are discarded one by
one, in order not to have to carry and accelerate parts that
are no longer needed.
v The vehicles used in ELV are designed to be used only
once (i.e. they are "expended" during a single flight), and
their components are not recovered for re-use after
launch.
Expandable Launch Vehicle (ELV)
Satellite launch
v Launches are usually done in several stages.
v Due to Earth rotation – the launch is easiest from equator.
v Launches from sites that are not on the equator place
satellites in inclined orbits.
Example of GEO launches: French Guyana and KSC
Launch from French
Guyana
Launch from
Kennedy Space Center (KSC)
at Cape Canaveral , FL
Rocket Fuel
Some rockets use airplane fuel (kerosene). However, other
types of fuel are often used. Several types of fuel are used in
rockets including:
1. Solid Propellants
2. Liquid Propellants
Orbital Effects
v Motion of the satellite has significant impact on its performance
v Some of the orbital effects to take into account
1) Doppler shift
2) Range Variation
3) Solar eclipse
4) Sun transit outage
Doppler Shift
v Doppler shift: the relative movement is by far faster than
on-ground hence some connection problems could be
present.
v Important in case of LEO and MEO satellites
v Negligible for GEO satellites
Range Variation
v Generally, when satellite revolves around the earth, it
undergoes some form of variation in its position during
a cyclic daily variation.
v This variation in the position heads to the variation in the
range between satellites and user terminal.
Solar eclipse
v Eclipse: sunlight fails to reach satellite due to obstruction from
either Earth or Moon
v More common type of eclipse – eclipse due to satellite coming in
the shadow of the earth
v For GEO satellite, the solar eclipse occurs for about 82 days
every year
Solar eclipse
v When the satellite passes through eclipse
1) It loses its source of power (relies on battery power)
2) It may need to shut off some of its transponders
3) Experiences cooling down in temperature
4) Transients due to switching of the equipment and
temperature variations may cause equipment failure
Sun Transit Outage
v Sun transit outage: occurs when the satellite passes in front of the
Sun
v Sun is a source of electromagnetic radiation
v Sun is a “hot” microwave source with an equivalent temperature of
about 6000 to 10000k.
v The earth station antenna will therefore receive not only the signal
from the satellite but also the noise temperature transmitted by sun.
v For satellite system operators with more than one satellite at their
disposal, traffic can be off-loaded to satellites that are just out of , or are
yet to enter, a sun outage.
Sun Transit Outage
v During sun transit outage – communication lost
v Duration of the outage as much as 10 min/day
v For GEO satellites outages occur 0.02% of the time
