ISRO’s Record and Understanding Orbits – Monica Ekal

Unless you have been living under a rock for the past few months, you have heard of the feat accomplished by the Indian Space Research Organisation(ISRO). ISRO launched 104 satellites in space with its Polar Satellite Launch Vehicle (PSLV) on 15th of February this year. The total payload was 1378 kg, where ISRO’s Cartosat-2 alone weighed 714 kg. The remaining 103 were nano-satellites, weighing between 1 kg and 10 kg, out of which two were ISRO’s INS-1A and INS-1B.

Now we at Sapioplasm focus not only on presenting news, but on breaking down the science behind it. Let’s try to demystify some of the terms used for describing launch. To quote from the post “The unique triumph of PSLV-C37” on ISRO’s home page:

“The satellites were inserted into a Sun-Synchronous orbit at 506 km above the earth, with an inclination of 97.46°.”

What is the sun-synchronous orbit?

So, a sun-synchronous orbit is a special case of the polar orbit, which is a type of Low Earth Orbit (LEO). Too much? Lets take it one step at a time.

The low earth orbit is where all manned space missions except the Apollo program have taken place. The International Space Station operates in the LEO. Objects in orbit between 160 to 2,000 km above the earth’s surface are said to be in LEO. Satellites in this orbit are still within the upper layers of the earth’s atmosphere, which is why they still face atmospheric drag, but it decreases with altitude. The benefits of using this orbit are numerous as far as navigation, weather and some communication satellites are concerned: Low orbital periods (between 88 to 127 mins to orbit the earth) for observing regions multiple times, low latency and high bandwidth for communication and ease of getting a closer look at clouds and weather patterns from above. Most of all, less fuel is needed, i.e lesser costs are incurred to launch satellites in this orbit than others like the Geosynchronous orbit (35,786 kms above the earth’s surface).

Of course, too much of anything is never good and frequent launches have led to accumulation of space debris in the LEO.

credits: Wikimedia Commons

It is easy to guess what a polar or near-polar orbit is – the satellite tracks an orbit above the poles while the earth rotates. The inclination of this orbit with respect to the equator is then 90° (for strictly polar orbits). The earth’s gravity and rotation cause the areas the satellite covers on earth to be shifted to the west after each orbital period. This short video helps visualize how satellites in polar orbits map the earth, one swathe at a time.

Now, the earth is not a perfect sphere. Bulges around the equator cause the satellite’s polar orbit itself to rotate around the earth. This is called nodal regression and it depends on the orbit’s inclination and height. To quote an example from chap.4 of Visual satellite observing(http://www.satobs.org/faq/Chapter-04.txt), “ For 555 km (300 nm) altitude, 130 degrees inclination, the nodal regression is 4.7 degrees per day eastward.”   Sun-synchronous satellites make use of nodel regression in order to cancel out the changes in the position of the sun relative to any point on earth, which is a result of the earth’s path around the sun. So what does this mean? Everday, when the satellite goes over a region on the earth, the position of the sun with respect to the satellite and the earth would stay the same. 

The orbit is chosen at such an inclination that the satellite crosses regions on the earth at the same local solar time each day. 

NASA’s Earth Observatory page about Three Classes of Orbit (http://earthobservatory.nasa.gov/Features/OrbitsCatalog/page2.php) clarifies this with an example:  ” For the Terra satellite for example, it’s always about 10:30 in the morning when the satellite crosses the equator in Brazil. When the satellite comes around the Earth in its next overpass about 99 minutes later, it crosses over the equator in Ecuador or Colombia at about 10:30 local time.”

This is an advantage because every time that the satellite passes over the region, it will have the same illumination conditions, i.e the sun will shine on the earth from the same angle at least for the duration of that season. So satellites that take pictures would need bright conditions while long wave radiation measuring satellites would need complete darkness.

However, the downside to all these kinds of orbits is that constant monitoring of one spot on the earth is not possible, which is where the geosynchronous orbit used by many communication satellites comes in.

The sun-synchronous orbit has about about 95 to 100 degrees of inclination depending upon its altitude and gives a nodal

regression of 0.98 degrees eastwards.

So now we know exactly what scientists at ISRO mean when they say “Sun-Synchronous orbit at 506 km above the earth, with an inclination of 97.46°.”

Returning to the achievement in question. Apart from this being a record for the most number of satellites inserted in orbit till date, this move also required an immense amount of calculations as far as safe separation of these satellites is concerned. Inserting such a large number of satellites in one orbit meant that the angle of separation and timing was key to avoid collisions, as Dr. K. Sivan, Director of the Vikram Sarabhai Space Centre (VSSC) has said.

More details in this link: http://www.thehindu.com/sci-tech/science/How-ISRO-plans-to-launch-103-satellites-on-a-single-rocket/article17075073.ece

India’s PSLV has proven to be a reliable launch vehicle for the past few years, also launching the lunar probe, Chandrayaan in 2008 as well as the Mars Orbiter Mission, Mangalyaan in 2013. 

ISRO will now focus on perfecting PSLV’s more powerful counterpart, the Geosynchronous launch vehicle(GSLV), which will be uses indigenously developed cryogenic technology…but more on that later.

*Russia held the record for most satellites launched so far after launching 37 satellites in June 2014 with its Dnepr rocket

 External Links:

http://isro.gov.in/unique-triumph-of-pslv-c37

https://www.nasaspaceflight.com/2014/06/russian-dnepr-rocket-record-launch-37-satellites/

http://www.universetoday.com/85322/what-is-low-earth-orbit/

http://satellites.spacesim.org/english/anatomy/orbit/low.html

http://earthobservatory.nasa.gov/Features/OrbitsCatalog/page2.php

http://www.satobs.org/faq/Chapter-04.txt

https://marine.rutgers.edu/cool/education/class/paul/orbits2.html

http://tornado.sfsu.edu/geosciences/classes/m407_707/Monteverdi/Satellite/PolarOrbiter/Polar_Orbits.html

http://www.thehindu.com/sci-tech/science/How-ISRO-plans-to-launch-103-satellites-on-a-single-rocket/article17075073.ece

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