From UPSC perspective, the following things are important :
Prelims level : Tundra Satellite, Various types of Orbits
Mains level : Not Much
Russia has successfully placed into orbit a military satellite believed to be part of the Kremlin’s early warning anti-missile system. This launch could be delivering a Tundra satellite.
- The Tundra or EKS (Edinaya Kosmicheskaya Sistema) series of satellites is the next generation of Russian early-warning satellites.
- The development of the EKS started in 2000.
- These satellites carry a secure emergency communications payload to be used in case of a nuclear war.
- They are launched on Soyuz-2-1b Fregat boosters into Molniya-orbits, inclined highly elliptical 12 h orbits.
What are Tundra Orbits?
- A Tundra orbit is a highly elliptical geosynchronous orbit with a high inclination (approximately 63.4°), an orbital period of one sidereal day.
- A satellite placed in this orbit spends most of its time over a chosen area of the Earth, a phenomenon known as apogee dwell.
- It makes satellites particularly well suited for communications satellites serving high latitude regions.
- The ground track of a satellite in a Tundra orbit is a closed figure 8 with a smaller loop over either the northern or southern hemisphere.
- This differentiates them from Molniya orbits designed to service high-latitude regions, which have the same inclination but half the period and do not hover over a single region.
Back2Basics: Types of Orbits
 Geostationary orbit (GEO)
- Satellites in geostationary orbit (GEO) circle Earth above the equator from west to east following Earth’s rotation – taking 23 hours 56 minutes and 4 seconds – by travelling at exactly the same rate as Earth.
- This makes satellites in GEO appear to be ‘stationary’ over a fixed position.
- In order to perfectly match Earth’s rotation, the speed of GEO satellites should be about 3 km per second at an altitude of 35 786 km.
- This is much farther from Earth’s surface compared to many satellites.
- GEO is used by satellites that need to stay constantly above one particular place over Earth, such as telecommunication satellites.
- Satellites in GEO cover a large range of Earth so as few as three equally-spaced satellites can provide near-global coverage.
 Low Earth orbit (LEO)
- A low Earth orbit (LEO) is, as the name suggests, an orbit that is relatively close to Earth’s surface.
- It is normally at an altitude of less than 1000 km but could be as low as 160 km above Earth – which is low compared to other orbits, but still very far above Earth’s surface.
- Unlike satellites in GEO that must always orbit along Earth’s equator, LEO satellites do not always have to follow a particular path around Earth in the same way – their plane can be tilted.
- This means there are more available routes for satellites in LEO, which is one of the reasons why LEO is a very commonly used orbit.
- It is most commonly used for satellite imaging, as being near the surface allows it to take images of higher resolution.
- Satellites in this orbit travel at a speed of around 7.8 km per second; at this speed, a satellite takes approximately 90 minutes to circle Earth.
 Medium Earth orbit (MEO)
- Medium Earth orbit comprises a wide range of orbits anywhere between LEO and GEO.
- It is similar to LEO in that it also does not need to take specific paths around Earth, and it is used by a variety of satellites with many different applications.
- It is very commonly used by navigation satellites, like the European Galileo system of Europe.
- It uses a constellation of multiple satellites to provide coverage across large parts of the world all at once.
 Polar Orbit
- Satellites in polar orbits usually travel past Earth from north to south rather than from west to east, passing roughly over Earth’s poles.
- Satellites in a polar orbit do not have to pass the North and South Pole precisely; even a deviation within 20 to 30 degrees is still classed as a polar orbit.
- Polar orbits are a type of low Earth orbit, as they are at low altitudes between 200 to 1000 km.
 Sun-synchronous orbit (SSO)
- SSO is a particular kind of polar orbit. Satellites in SSO, travelling over the polar regions, are synchronous with the Sun.
- This means they are synchronised 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.
- Often, satellites in SSO are synchronised so that they are in constant dawn or dusk – this is because by constantly riding a sunset or sunrise, they will never have the Sun at an angle where the Earth shadows them.
- A satellite in a Sun-synchronous orbit would usually be at an altitude of between 600 to 800 km. At 800 km, it will be travelling at a speed of approximately 7.5 km per second.
 Transfer orbits and geostationary transfer orbit (GTO)
- Transfer orbits are a special kind of orbit used to get from one orbit to another.
- Often, the satellites are instead placed on a transfer orbit: an orbit where, by using relatively little energy from built-in motors, the satellite or spacecraft can move from one orbit to another.
- This allows a satellite to reach, for example, a high-altitude orbit like GEO without actually needing the launch vehicle.
- Reaching GEO in this way is an example of one of the most common transfer orbits, called the geostationary transfer orbit (GTO).
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