Geostationary orbit,Geosynchronous orbit,Kepler's laws,Low Earth Orbit,Polar Orbits,Hofmann Transfer Orbit

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Contents

Types of Orbits

1.   Kepler's laws of planetary motion

2.   Low Earth Orbit

a)    Why should satellites rotate?

b)   What is the speed required to keep a satellite in LEO?

c)    Advantages and Disadvantages of LEO

3.   Highly Elliptical Orbits

4.   Geosynchronous Orbits

5.   Geostationary vs. Geosynchronous

6.   Medium Earth Orbits

7.   Polar Orbits

8.   Parking Orbit

9.   Hofmann Transfer Orbit

Types of Orbits

·         These terms keep on appearing whenever there is a satellite launch.

·         So I decided to keep all the related concepts at one place.

·         Titbit: Russia's Sputnik, the world’s first artificial satellite, was launched in 1957.

Kepler's laws of planetary motion (applicable to satellites as well)

1.   The orbit of a planet is an ellipse with the Sun at one of the two focii.

2.   A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.

 

3.   The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

Low Earth Orbit

·         International Space Station (400 km), Space Shuttles, and the Hubble Space Telescope (560 km) and some observation satellites are all rotating the earth in Low Earth Orbit (commonly called "LEO" – 300-600 km. above the earth’s surface).

·         LEO is high enough to significantly reduce the atmospheric drag (friction due to atmosphere) acting on the satellite.

Why should satellites rotate?

·         Rotation gives centrifugal force (the object tends to move away from the center).

·         Higher rotational speed = Higher centrifugal force.

·         There are two important forces acting on the satellite. One, the gravitational force which will pull the satellite towards earth and two, the centrifugal force due to rotation which will counter gravitational pull.

·         So by varying the speed of rotation, we can make the satellite

a) fall back to earth [By decreasing the speed of rotation]

b) stay in its orbit [By giving it the required speed. It varies with distance from earth. Lower the orbit, highest should be the speed]

c) escape earth’s influence (By keeping the speed of rotation above the required speed).

 What is the speed required to keep a satellite in LEO?

·         18,000 mph roughly to place a satellite in orbit at 400 km.

·         The speed, however is dependent on the distance from the center of the Earth.

·         In the case of the space shuttle, it orbits the Earth once every 90 minutes at an altitude of 466 km (290 miles) above the surface of the Earth.

Advantages and Disadvantages of LEO

Advantages

·         Low Earth Orbit is used for things that we want to visit often.

·         Like the ISS [International Space Station placed in Upper Ionosphere (upper thermosphere) or lower exosphere], the Hubble Space Telescope and some satellites (usually spy satellites and other observation satellites).

·         This is convenient for installing new instruments, fixing things, experiments, and return to earth in a relatively short time.

Disadvantages

·         Atmospheric drag (friction offered by the scarce atmosphere)(this will lead to more fuel consumption and constant speed adjustments)(if the speed is not adjusted, the satellite will fall back to earth).

·         The most important disadvantage: A satellite traveling 18,000 miles per hour or faster does not spend very long over any one part of the Earth at a given time. (this doesn’t work for communication and weather observation and forecasting satellites which needs to keep an eye always on a specific location)

Solution

·         One solution is to put a satellite in a highly elliptical orbit (non-geosynchronous most of the time).

·         The other is to place the satellite in a geosynchronous orbit.

Highly Elliptical Orbits

·         Kepler's second law: an object in orbit about Earth moves much faster when it is close to Earth than when it is farther away.

·         Perigee is the closest point and apogee is the farthest (for Earth - for the Sun we say aphelion and perihelion).

·         If the orbit is very elliptical, the satellite will spend most of its time near apogee (the furthest point in its orbit) where it moves very slowly.

·         Thus it can be above a specific location most of the time.

·         With a highly elliptical orbit, the satellite has long dwell time over one area, but at certain times when the satellite is on the high speed portion of the orbit, there is no coverage over the desired area.

·         To solve this problem we could have two satellites on similar orbits, but timed to be on opposite sides of the orbit at any given time.

·         In this way, there will always be one satellite over the desired coverage area at all times.

·         If we want continuous coverage over the entire planet at all times, such as the Global Positioning System (GPS), then we must have a constellation of satellites with orbits that are both different in location and time.

·         In this way, there is a satellite over every part of the Earth at any given time.

·         In the case of the GPS system, there are three or more satellites covering any location on the planet.

Geosynchronous Orbits

·         Another solution to the dwell time problem is to have a satellite whose orbital period is equal to the orbital period of the earth (it revolution is in sync with the earth’s rotation)(it appears along the same longitude to an observer from earth).

·         The way we do this is to have the orbital period of the satellite exactly the same as the rotation period of the Earth, which is one day.

·         This is called a geosynchronous orbit, or GEO for short.

·         In this case, the satellite cannot be too close to the Earth because we already figured out that it would not be going fast enough to counteract the pull of gravity.

·         If you recall, the space shuttle, in order to stay aloft, must circle the planet every 90 minutes.

·         We can use Kepler's third law to figure out how far out a satellite must be to spend all its time over one part of Earth.

·         The answer is that a satellite has to be placed approximately 22,000 miles (36,000 km) away from the surface of the Earth in order to remain in a GEO orbit.

·         That is a lot farther than a low Earth orbit, or a relatively close to highly eccentric GPS-like orbit, so it costs more to get it there (This is why GSAT 16 is launched by Ariane Launch Vehicle. GSLV doesn’t have enough capability to launch 4 tonne class satellites to Higher Earth Orbit or GEO).

·         However, then you only need one satellite to do the job and it is on the job 24 hours per day.

·         By positioning a satellite so that it has infinite dwell time over one spot on the Earth, we can constantly monitor the weather in one location, provide reliable telecommunications service, and even beam television signals directly to your house.

·         If you have satellite TV at home, notice that the small dish antenna outside is pointing at the same location in the sky at all times. There is a geosynchronous satellite sitting 22,000 miles away in that direction sending the signal to your house!

·         The down side of a geosynchronous orbit is that it is more expensive to put something that high up and not possible to repair it from the shuttle.

·         When a satellite is in LEO, the shuttle can repair it if needed, as we have done with the Hubble Space Telescope several times. So you only put something in GEO if you really need to have it in the same location in the sky at all times.

Geostationary vs. Geosynchronous

Geostationary Orbits Geostationary Earth Orbit (GEO)	Geosynchronous Orbits Geosynchronous Earth Orbit (GEO)
Orbital period is the sidereal rotation period of the Earth which is 86,164.0909 seconds, or 23 hrs 56 min 4.0909 seconds (the time it takes the Earth to rotate once on its axis)	Same
Orbital path is circular.	Orbit is an inclined ellipse.
Orbital center point is the center of the earth	Orbital center point - the center of the Earth is at one of the two "foci" of the elliptical orbit
Orbital tilt is zero.	The orbital tilt is non-zero
Orbit and equator are coplanar (orbit of the satellite and the equator lie in the same plane).	Tilt is non-zero = not coplanar
Orbital height is 42,164 km (26,200 miles) always rotating exactly with the Earth.	Orbital height varies (the satellites will have an apogee different from its perigee.
Orbital position is always above a certain point on the Earth's equator (i.e. a specific longitude and latitude).	It moves along a specific longitude but not latitude.
an observer on the ground would not perceive the satellite as moving and would see it as a fixed point in the sky	Since the orbit has some inclination and/or eccentricity, the satellite would appear to describe a more or less distorted figure-eight in the sky, and would rest above the same spots of the Earth's surface once per sidereal day.
There are a limited number of positions available [traffic jam, interference of signals due to more satellites in the same orbit  and risk of damage due to space debris (Movie GRAVITY)] in this orbit due to safety and maneuvering limits. Retired satellites are often pushed slightly away from the precious exact positions.	There are more orbital planes and positions available to satellites using this technique
Line of sight transmission	Line of sight transmission
Can receive signals with a simple antenna as the satellite is in relatively same position (DTH, VSAT services). (Parabolic antenna is used to nullify the effect of atmospheric distortions)	Requires a parabolic antenna as the satellite’s position slightly changes longitudinally.
Steering the antenna is not required.	It may sometimes require steering the antenna to achieve line of sight
Because of the very high altitude of their orbits, geostationary satellites may have a very wide signal footprint covering up to 42% of the Earth's surface
Need more powerful launch vehicles to put them in place.	Same
Communications system needs higher power transmitters and more sensitive receivers because of the increased path loss.	Same
The satellite is first placed in Geosynchronous orbit and later maneuvered to Geostationary orbit.

Medium Earth Orbits

·         Medium Earth Orbits (MEO) range in altitude from 1,200 miles (2,000 kms) up to the geosynchronous orbit at 22,236 miles (35,786 kms) which includes part of the lower and all of the upper Van Allen radiation belts.

·         The Van Allen Radiation Belt is a region of high energy charged particles moving at speeds close to the speed of light encircling the Earth which can damage solar cells, integrated circuits, and sensors and shorten the life of a satellite or spacecraft.

·         Practical orbits therefore avoid these regions.

Polar Orbits

·         Satellites in these orbits fly over the Earth from pole to pole in an orbit perpendicular to the equatorial plane.

·         This orbit is most commonly used in surface mapping and observation satellites since it allows the orbiting satellite to take advantage of the earth's rotation below to observe the entire surface of the Earth as it passes below.

·         Many of the pictures of the Earth's surface in applications such as Google Earth come from satellites in polar orbits.

Parking Orbit

·         It is not always possible to launch a space vehicle directly into its desired orbit.

·         The launch site may be in an inconvenient location with respect to the orbit or the launch window may be very short, a few minutes or even seconds.

·         In such cases the vehicle may be launched into a temporary orbit called a parking orbit which provides more options for realising the ultimate orbit.

·         For manned space missions the parking orbit provides an opportunity to check that all systems are working satisfactorily before proceeding to the next critical stage.

Hofmann Transfer Orbit

·         The transfer orbit is the orbit used to break out of the parking orbit and break into the geosynchronous or geostationary orbit.

·         The Hofmann transfer uses two rocket engine impulses, one to move the spacecraft onto the transfer orbit and a second to move off it into a new orbit.

·         The inclination of the transfer orbit is the angle between the spacecraft's orbit plane and the Earth's equatorial plane and is determined by the latitude of the launch site and the launch azimuth (direction).

·         To obtain a geostationary orbit the inclination and eccentricity must both be reduced to zero.

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