2017 Types Of Orbits Brief Explanation

Types Of Orbits Brief Explanation
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r-04.txt)

1. Brief Explanation
Compiled By: Atiqa Ijaz Khan
Tuesday, 17 January, 2017
NOTE:
All textual content belongs to
Satobs.org
(http://www.satobs.org/faq/Chapte
r-04.txt)

2. 1
TABLE OF CONTENTS

1. Low Earth Orbit (LEO) ................................................................................................................... 2
1.1. Low-Inclination Orbits ............................................................................................................ 2
1.2. Polar orbits .............................................................................................................................. 2
1.3. Sun-synchronous orbits ........................................................................................................... 3
2. Geostationary Orbit ......................................................................................................................... 3
3. Geosynchronous Orbit .................................................................................................................... 5
4. Mid-Earth Orbit (MEO) .................................................................................................................. 5
5. What is a "transfer orbit"? What does GTO mean? ....................................................................... 6
6. High-Earth Orbit (HEO) ................................................................................................................. 6
7. Solar Orbit ....................................................................................................................................... 7
References ............................................................................................................................................... 8

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1. LOW EARTH ORBIT (LEO )
Any orbit, in which the satellite completes one full orbit around the earth (the "period") in less than 225
minutes, is called a "low earth orbit." In some documents these orbits are called "near earth" orbits.
The reason for the 225 minute definition is the factors which affect the satellite in orbit. Satellites so
low that they orbit in fewer than 225 minutes are far more susceptible to the earth's atmosphere and
earth gravitational anomalies than any other source of disturbance. Satellites with a period greater than
225 minutes are more likely to be affected by the gravitation of the sun, moon and planets, and the
earth's natural radiation belts.

1.1. LOW-INCLINATION ORBITS
This has to be one of the poorest choices of terms in the satellite industry. A "low-inclination" orbit is
whatever the term means to the user. The inclination of a satellite, defined as the angle between the
orbital plane of the satellite and the equatorial plane of the earth, manifests itself in the highest north or
south geographic latitude the satellite reaches in its orbit as viewed from the ground. The inclination
for a particular satellite is a particular number having a clear meaning and mathematical significance.
When referring to a "low-inclination orbit", there simply is no established definition or mathematical
significance. There is no military or civilian definition of a "low-inclination orbit". One finds a low-
inclination orbit can be somewhat arbitrarily defined as an inclination less than 45 degrees but no
accepted source of authority, including the USA Space Command (USSPACECOM), has defined the
term.
1.2. POLAR ORBITS
Strictly defined, a "polar orbit" is when the inclination is exactly 90 degrees. Some latitude (no pun
intended) is allowed so that any orbit within a few degrees of 90 is considered a polar orbit.

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1.3. SUN-SYNCHRONOUS ORBI TS
The sun-synchronous orbit is one of the special categories alluded to in the opening paragraph of this
section. All satellites, at any inclination other than exactly 90 degrees, are affected gravitationally by
the fact that the earth is not a perfect sphere. This "mass asymmetry" of the earth causes the orbit of
the satellite to change. The greatest effect is on the argument of the perigee, and the right ascension of
the ascending node. In simple terms, not only does the satellite go around the earth on its orbit, but the
orbit itself rotates, or "regresses", around the earth. This is "nodal regression" and is greatly dependent
on the satellite's orbital altitude and inclination. At 185 km (100 nm) altitude, 40 degrees inclination,
the nodal regression is about 6.8 degrees per day westward. For 555 km (300 nm) altitude, 130 degrees
inclination, the nodal regression is 4.7 degrees per day eastward.
One can take advantage of nodal regression and launch a satellite into an orbit where the nodal
regression nearly exactly cancels out the daily change in the position of the sun over any point on earth,
caused by the earth's orbit around the sun. This means that every day, when the satellite passes over a
point on earth, the position of the sun in relation to the satellite and the earth would be the same.
This is a very useful thing to do for a weather or surveillance satellite. The satellite always "sees" the
point on the earth, when the sun is shining on the earth from the same angle - the same "sun time". The
orbit which has this unique characteristic is called "sun-synchronous" and is an orbit where the
combination of orbit altitude and inclination causes a nodal regression of 0.98 degrees per day eastward.
2. GEOSTATIONARY ORBIT
A geostationary orbit is a special case of a geosynchronous orbit. Put the satellite in a very nearly
circular orbit (no eccentricity) and give it zero inclination and the satellite will stay over the same point
of the earth's equator – in other words, appear to be stationary in the sky. This is the ideal condition for
a communications satellite. One would simply point their ground antenna to the spot in the sky where
the satellite appears. Unfortunately, orbits are easily perturbed through natural causes, and a
geostationary satellite soon drifts from the position and must be forced back to position by firing

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thrusters. This uses up the satellite's fuel, and soon, with all fuel exhausted, the satellite drifts
permanently away from its geostationary point.

Communication satellite owners get around this problem by allowing the satellite to have a small
inclination and a very small eccentricity. Ground antennae "see" a large enough area of the sky that if
the satellite stays within that area, communications is retained. Thus, all "geostationary" satellites are
really allowed to be geosynchronous. They make tiny figure eights in the sky instead of staying in
exactly one place. It takes less fuel to let the satellite wander around a little, and the lifetime supply of
fuel on the satellite is extended. The communications industry somewhat whimsically refers to
geostationary communications satellites as "wobblesats."
All geostationary satellites must be located along the celestial equator as viewed from the earth. An
international commission "assigns" who gets to put a satellite on a particular longitudinal sub point.
Interestingly, a perturbation, caused by the Earth's oblateness, causes a longitudinal acceleration of a
satellite in geostationary orbit. The acceleration is zero at 75 degrees east longitude (over the Indian
Ocean) and 255 degrees east longitude (over the eastern Pacific Ocean). The lucky owners of these
slots get to put their satellites where they are least likely to need fuel to maintain position! All other
satellites must use fuel to retain their positions, or they will drift toward these two stable longitudinal
points!

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3. GEOSYNCHRONOUS ORBIT
A geosynchronous orbit is achieved when the satellite completes one orbit around the earth in one
sidereal day. (A sidereal day is the time it takes the earth to rotate once with respect to the stars (not
the sun). A sidereal day is 23 hours 59 minutes, 4.091 seconds, compared to a mean solar day which is
24 hours.) This gives the satellite an altitude of about 35,786 km (19,300 nm or 22,236 statute miles).
A satellite in a geosynchronous orbit, pretty closely matches the earth's rotation and "appears" from the
ground to stay overhead at all times.
It is only "pretty close" because, though the satellite has a sidereal period, nothing has been said about
the inclination or eccentricity. Give the satellite a nearly circular orbit, but some inclination, say 10
degrees, and the satellite will, over the course of an entire day, appear to inscribe a line in the sky - 10
degrees above the celestial equator to 10 degrees below it. Change the eccentricity a little and the
apparent path of the satellite can be changed to some rather odd shapes, from lopsided figure eights to
a circle.
Communications and surveillance satellites use geosynchronous orbits.

4. MID-EARTH ORBIT (MEO)
Mid-earth orbit is also known as Semi-synchronous. Satellites said to be semi-synchronous have a
period of 1/2 a sidereal day. Thus, they orbit the earth two times per day. Geopositioning and navigation
satellites, such as GPS and GLONASS, use this type orbit.

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5. WHAT IS A "TRANSFER ORBIT"? WHAT DOES GTO MEAN?
When it is desired to change the altitude of an orbit, to raise the perigee of a nearly circular orbit to a
higher perigee for example, it is accomplished through a "transfer orbit." Orbital maneuvers can take
place anywhere in an orbit, but you can accomplish the change using the least amount of propellant, if
the maneuver is performed at certain points on the orbit. To raise the satellite to a higher orbit, you fire
a thruster to increase its speed, wait for it to arrive at its new apogee, then fire a thruster to adjust the
satellite's speed to the new orbit. The satellite literally was put in a new orbit until the thruster was fired
again to achieve the desired end orbit. That temporary new orbit was the transfer orbit and was an
elliptical orbit which intersected the old orbit and the new orbit.

A special case of a transfer orbit is the Geosynchronous Transfer Orbit or GTO. The GTO is the
elliptical orbit needed from earth launch to geosynchronous altitude. At geosynchronous altitude, either
the payload or the final stage of the launch vehicle conducts a velocity change burn to correct the speed
of the payload to that needed for the geosynchronous orbit.
6. HIGH-EARTH ORBIT (HEO)
A high-earth orbit is any orbit greater than geosynchronous. Thus, if the period is greater than a sidereal
day, it is a high-earth orbit, also known as super synchronous. These orbits are often highly inclined

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and highly elliptical to get the satellite out of the earth's natural magnetosphere. Many satellites
designed for astronomical work are placed in HEO.

7. SOLAR ORBIT
The earth is in solar orbit. Any satellite given enough energy to leave earth orbit, but not enough energy
to leave the solar system, will enter solar orbit. Many science satellites are placed in a solar orbit -
Ulysses and Galileo for example. The two USA Pioneer spacecraft of 1972 and 1973 are in a highly
elliptical solar orbit. One special case is a "Halo" orbit. A Halo orbit relies on a gravitationally stable
point between the earth and the sun, one of the "Lagrange Points." A satellite placed at the Lagrange
point between the earth and sun, approximately 1.6 million kilometers from earth, will execute a three
dimensional elliptical orbit about the Lagrange point as the Earth, moon, and satellite system orbit the
sun. The satellite ISSE 3 was put in this orbit to detect solar wind products and thus provide an early
warning to observers on the ground when solar flare protons were heading toward the earth.

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REFERENCES
1. http://www.satobs.org/faq/Chapter-04.txt
2. http://www.radio-electronics.com/info/satellite/satellite-orbits/satellites-orbit-definitions.php
3. http://www.esa.int/Our_Activities/Launchers/Types_of_orbits
4. http://www.polaris.iastate.edu/EveningStar/Unit4/unit4_sub3.htm
5. https://www.esoa.net/technology/satellite-orbits.asp
6. http://www.daviddarling.info/encyclopedia/H/haloorbit.html
7. https://solarsystem.nasa.gov/basics/bsf4-1.php
8. http://calgary.rasc.ca/geo_orbits.htm