17 Apr

Some Basics About Satellite Orbits

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Content

• Orbit
• Types of Orbits: 1. Circular (LEO, MEO, GSO) 2. Elliptical Orbits
• Highly Elliptical Satellite Orbits
• Transfer Orbits
• Some other basics (Class Discussion)

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1. SOME BASICS ABOUT SATELLITE ORBITS

1) ORBIT:

• An orbit is the curved path that an object in space (such as a planet, moon, star etc.) takes around another object due to gravity.
  • Objects of similar mass orbit each other with neither object at the centre, whilst small objects orbit around large objects. In our Solar system, earth revolves around sun, Moon revolves around earth.
• Satellite Orbits: The path that satellite takes to revolve around a planet due to force of gravity is called satellite orbit.
• Gravity and Speed of Satellite in an orbit
• How are satellites placed in Orbit –Circular vs Elliptical Orbit – Detailed Class Discussion
• Orbital Velocity – Circular vs Elliptical
  • For a circular orbit, it is always the same.
  • However, in the case of an elliptical one this is not the case as the speed changes dependent upon the position in the orbit. It reaches the maximum when it is closest to the earth and it has to combat the greatest gravitational pull, and it is at its lowest speed when it is furthest away.

2) TYPES OF ORBITS: 1. CIRCULAR (LEO, MEO, GSO) 2. ELLIPTICAL ORBITS

A) LOW EARTH ORBIT (CIRCULAR ORBIT)

• A low earth orbit is an orbit around earth with an altitude between 160 kilometers and 2000 Kilometers. Objects below approximately 160 Kilometers will experience very rapid orbit decay and altitude loss.
• It is used for vast majority of satellites.
  • Most satellites
  • All human space flights (except manner lunar flight of the Apollo program);
  • All space stations.
• Main Characteristics
  • Low orbital period
  • Satellites closer to earth -> better visibility -> earth observation/remote sensing satellites.
  • Easier placement of satellite in orbit
  • Lower latency in communication -> less round-trip time.
  • Satellites face lower radiations when compared to satellites at higher altitudes.
• Applications
  • Earth Monitoring Satellites
    • As they are able to see the surface of the earth more clearly
  • Communication satellites
    • Especially the satellite phones
  • International Space Station is at a height of 400 km.

SUN SYNCHRONOUS ORBIT (CIRCULAR OR ALMOST CIRCULAR) (POLAR ORBIT)

• Satellites in Polar Orbit usually travel past Earth from north to south rather than from west to east, passing roughly over Earth’s pole. They don’t have to pass the north pole or south pole precisely. Even a deviation within 20 to 30 degrees is still classed as polar orbit.
• Sun Synchronous Orbit is a kind of Polar Orbit. In this orbit, satellites are synchronized to always be in the same fixed position relative to the Sun. This means that the satellite always visits the same spot at the same local time. In this orbit, whenever and wherever the satellite crosses the equator, the local solar time on the ground is always the same.

• • A sun synchronous combines altitude and inclination in such a way that an object on that orbit will appear to orbit in the same position, from the perspective of the sun, during its orbit around the earth. In other words, it orbits in such a way that it precesses once a year. The surface illumination angle will nearly be same every time.
  • This is achieved by having the osculating orbital plane precess (rotate) approximately one degree each day with respect to celestial sphere, eastward, to keep pace with earth’s movement around the sun.
  • Typical sun-synchronous orbits are about 600-800 Km in altitude, with periods in the 96-100-minute range, and inclination of around 98 degrees.
  • Possible only around oblate planets like Earth, Mars etc. The extra mass around the equator makes the precess possible. But Venus is too spherical to have a Sun Synchronous Satellite orbit.
• Significance of Sun-Synchronous Orbit
  • The Sun-Synchronous orbit is necessary for science because it keeps the angle of sunlight on the surface of the earth as consistent as possible, though the angle will change from season to season.
  • This consistency means that scientists can compare images from the same season over several years without worrying about too much extreme changes in shadows and lightening, which can create illusions of image.
• Kinds of satellite put in Sun-Synchronous orbit
  • Sun-synchronous orbit can place a satellite in constant sunlight and is useful for imaging, spy and weather satellites.
    E.g., Cartosat-2 series

B) MEDIUM EARTH ORBIT

• Height: 2000 km to 3,5786 kms
• Satellite speed is lower (compared to LEO)
• Orbital Period range from 2 to 24 hours
• Most common use of satellite in this orbit is for navigation, communication, and geodetic/space environment science.
• Most common altitude is approximately 20,200 km, which yields an orbital period of 12 hours as used for examples by GPS.
• E.g:
  • GPS Satellites Fly in Medium earth orbit at an altitude of approximately 20,200 km.
  • Galileo (the satnav system of Europe) is also located in MEO

C) GEOSYNCHRONOUS ORBIT AND GEOSTATIONARY ORBIT

GEO-SYNCHRONOUS ORBIT

• It is a satellite orbit around the earth with an orbital period that matches Earth’s rotation period on its axis (i.e., orbital period is 23 hours 56 minutes and 4 seconds), irrespective of inclination.
• A person on a point on Earth, will see a satellite in this orbit in the same place in the sky at the same time of the day,
  every day.
• Over the course of a day, the object’s position in the sky traces out a path, typically in a figure-8 form, whose
  precise characteristics depend on the orbit’s inclination and eccentricity.
• Requirements:
  • Circular Orbit of Height 35786 km. At this height an orbital period of satellite is equal to earth’s rotation period.
  • Direction of revolution of satellite should be same as direction of rotation of earth.

GEO-STATIONARY – A SPECIAL CASE OF GEOSYNCHRONOUS

• A Geostationary Orbit is a particular type of Geosynchronous orbit, the distinction being that while an object in Geosynchronous orbit returns to the same point in the sky at the same time each day, an object in geostationary orbit never leaves that position.
• Requirements for a satellite to be geostationary?
  • Geosynchronous requirements
  • The equatorial plane of earth must be coplanar with the orbital plane of the satellite revolution (i.e., angle of inclination of orbit to equator is 0 degrees)
  • Communication satellites and weather satellites are often placed in Geostationary orbits, so that satellite antenna which communicate with them don’t have to rotate to track them but can be pointed permanently at the position in the sky where they stay.
• Advantages
  • Geo systems have significantly greater available bandwidth that the Low Earth Orbit – LEO and Medium Earth Orbit
  • Covers 1/3rd of Earth’s surface.
  • Less expenses on tracking activities
  • Higher life span of satellites
• Limitations
  • Would require line of sight communication paths between terrestrial antenna and the satellites.
  • Long path length, and hence losses when compared to LEO, or MEO.
    • Long path length introduces delays.
    • Satellite costlier to install in GEO in view of the greater altitude
    • Geostationary Orbit (GEO) can only be above equator and therefore poles can’t be covered.

3) HIGHLY ELLIPTICAL SATELLITE ORBITS

• Elliptical Orbits are often called Highly Elliptical orbits or HEO.
• Key Features
  • Follows the curve of an ellipse.
    • Moves much faster when it is near earth and slower when it is away from earth.
  • There are two focal points and one of the these is the geo-centre of the earth.
  • Apogee: Point where the satellite is furthest from Earth –gravitation pull is lowest – satellite moves the slowest
  • Perigee: Point where the satellite is nearest from earth – gravitation pull is highest – satellite moves the fastest
  • How permanent coverage can be achieved?
• Applications
  • Provide coverage at any point on the globe
    • It may provide high latitude and polar coverage.
      • Countries such as Russia which needs coverage over polar and near polar areas make significant use of highly elliptical orbits.

4) TRANSFER ORBITS

• These are special kind of orbits used to transfer satellites/spaceships from one orbit to another. These orbits are elliptical, with its perigee closer to earth. Satellites are taken to Perigee with the help of a rocket. After reaching this orbit, satellites by using relatively little energy from built in motors, can move to another larger orbit.
• This allows a satellite to reach a very high orbit, without needing the rocket to go to that height. Geostationary Transfer Orbit (GTO) is the most common type of transfer orbit.

A) GEOSTATIONARY TRANSFER ORBIT

• It is a Hohmann transfer orbit used to reach, geosynchronous or geostationary orbit. It is highly elliptical earth orbit with an apogee of 42,164 km, or 35786 km above sea level. Perigee can be anywhere above atmosphere, but it is generally restricted to few hundred Kms above the earth’s surface.
• Hohmann transfer orbit: It is an elliptical orbit used to transfer between two circular orbits of different radii in the same plane.

5) SOME OTHER BASICS (CLASS DISCUSSION)

Why are all satellites launched from east coast?

Why are satellites launched from near the equator?

2. TIMELINE: INDIA IN SPACE, THROUGH THE YEARS

1. 1962: The Indian National Committee for Space Research is formed under the leadership of Vikram Sarabhai and physicist Kalpathi Ramakrishna Ramanathan
2. 21 Nov 1963: India’s space program takes off with launch of a sounding rocket from Thumba Equatorial Rocket Launching Station in Kerala. It was for probing upper atmosphere.
3. Aug 15, 1969: ISRO is formed.
4. Aug 19, 1975: Aryabhata – India’s first satellite is launched from a Soviet Kosmos-3M rocket from Kapustin Yar in the Soviet Union. It was designed and built in India.
5. 1979: Bhaskara-1, the first experimental remote sensing satellite built in India, is launched. Images taken by its camera were used in hydrology, forestry and oceanography.
6. 1980: Satellite Launch Vehicle (SLV)-3, India’s first experimental satellite launch vehicle, takes off with Rohini Satellite RS-D2. Camera had the ability to use data for classifying ground features like water, vegetation, bare land, clouds and snow.
7. 1982: INSAT 1-A is launched. Abandoned in 1983 where its altitude control propellant was exhausted.
8. 1984: Rakesh Sharma, former IAF pilot, becomes the first Indian in space. In a joint India-Soviet Union Mission, Sharma boards the Soyuz T-11 spacecraft to the Salyut 7 orbital station.
9. 2008: Launch of Chandrayaan-1. It orbits the Moon but doesn’t land. It performs high resolution remote sensing aiming, among various missions, to prepare a 3D atlas of both the near and far sides of the moon.
10. 2013: Launch of Mangalyaan, the Mars Orbiter Mission. Orbiting and studying Mars since Sep 24, 2014.
11. 2016: All 7 satellites of IRNSS system placed in Orbit
12. 2019: Chandrayaan-2 launched using GSLV MK-III
13. 2022: Plans to take humans to space with Gaganyaan
14. 2023: Chandrayaan-3 succeeded in landing on the surface of the moon.

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