Why IRNSS is put in Geosynchronous Orbit?

singha

Hi guys,

May be you can provide some insight into this:

Conventionally navigation satellite systems like GPS, GLONASS, COMPASS etc. have been put in Medium Earth Orbit (MEO). However, our IRNSS system is to have satellites in Geosynchronous orbit (GO).

What could be advantages/disadvantages of the two orbits in this case?

One thing which I can think of is, GO is at a higher altitude => can cover larger surface area => perhaps less no. of satellites required for the system.

Other inputs are appreciated.

TIA.

earlybird

Hi,

I was also looking up Geosynchronous Orbits too. If I’m not mistaken, Geosynchronous Orbit Satellites last longer than MEO Satelilites. Because of atmospheric drag in lower atmosphere levels?

I’ve got a query as well. Why are Polar Satellite Launch Vehicles called that? I mean, they can launch satellites into Geosynchronous Orbits too, is that correct? Is it a kind of misnomer?

Would appreciate any reply on this. Thanks.

Siddharth

@singha
Other than the reason of having lesser no. of satellites (Both GPS and GLONASS have 24 satellites), the low spatial resolution of data from geosynchronous orbits (at 35k km) was also an impediment in 20th century. With advancement of technology, this is no longer an issue.

@earlybird
PSLV are so called because they are mainly for launching Remote Sensing satellites (IRS/RESOURCESATs) into 620 km sun-synchronous polar orbit. However, they have also been occasionally used to launch satellites upto 1 tonne in geo-synchronous orbits (e.g. Kalpana-I, a METSAT). But to carry 2- 2.5 tonne satellite to 36,000 km range, GSLV vehicle is required (which unfortunately hasn't delivered two consecutive successful launches). India is also simultaneously working for GSLV Mk III to make ISRO fully self reliant in launching heavier communication satellites of INSAT-4 class, which weigh 4500 to 5000 kg.

faltugs

Geosynchronous are not meant for covering the entire sweep area of Planet Earth. Hence they rotate with same angular speed of the earth, such that they seems to be stationery if looked from earth's surface.

Now IRNSS is meant for regional navigation, and for regional application satellite should be focused or pointing towards particular region. So Geosynchronous is the best for its application.

Meanwhile PSLV satellites orbits from North Pole to South Pole which are meant to cover the entire Earth Area. So it is application driven.

I hope I have answered your query.

GaneshM

The IRNSS consists of three geosynchronous satellites each over the equator, but in longitude terms one over allahabad, one over eastern africa and one east of china. These are sort of three reference points more or less in a straight line. For a good location fix you need three satelites located at the vertices of a triangle. The GPS uses MEO, which means that any any one time a satellite would be out of sight. This entails having a very large number of satellites in space so that at least three are visible to any user. Since the IRNSS is meant just for India, you need satellites that are all the time visible over the subcontinent. The third vertex of the triangle therefore is obtained by four satellites in geosynchronous orbit. Two of these are over Gujarat and two over Assam (roughly speaking, and in terms of longitude alone). In effect, after taking into account the earths rotation, they describe longish figures of eight with the centre point over the equator, and the loops extending north and south. When any one of the two satellites concerned is over the equator (no longer available as the third vertex of a triangle, the second one is located at the extreme end of a loop, and serving as this third vertex.) At any one time, therefore, a user in any part of the country sees a minimumof three satellites. A well conceived design for our localized need