2011 GS Paper I, all questions

UPSC Prelims 2011 · GS Paper I

Question 3: Science and Technology

Satellites used for telecommunication relay are kept in a geostationary orbit. A satellite is said to be in such an orbit when :

  1. 1.The orbit is geosynchronous.
  2. 2.The orbit is circular.
  3. 3.The orbit lies in the plane of the Earth's equator.
  4. 4.The orbit is at an altitude of 22,236 km.

Select the correct answer using the codes given below :

a1, 2 and 3 only
b1, 3 and 4 only
c2 and 4 only
d1, 2, 3 and 4

AnswerThe answer is (a).

A geostationary satellite appears fixed over one spot on the equator, so its orbit must be circular, lie in the plane of the Earth's equator, and have a period equal to the Earth's rotation (geosynchronous), about 24 hours. That fixes the height at roughly 35,786 km above the surface. The figure of 22,236 in statement 4 is the height in miles, so it is wrong when read in km. Remember: geostationary means geosynchronous, circular and equatorial.

  • 1. The orbital period matches the Earth's rotation.
  • 2. A circular orbit keeps the satellite at a constant height and speed.
  • 3. Only an equatorial orbit lets the satellite stay over one point.
  • 4. The height is about 35,786 km; 22,236 is the figure in miles.

How you could have got this

  1. If you were sure statement 4 is not correct, (b), (c) and (d) go: each of them includes 4.

    What you needed to know: The height is about 35,786 km; 22,236 is the figure in miles.

Only (a) is left. You didn’t need statements 1, 2 or 3.

This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.

Which options go is worked out from UPSC’s key and the options, not by a model.

Why each option is right or wrong
  • (a) Right: statements 1, 2 and 3 describe a geostationary orbit.
  • (b) Adds statement 4, whose 22,236 figure is in miles, not km, and drops statement 2.
  • (c) Drops statements 1 and 3, which are essential conditions, and keeps the wrongly stated height.
  • (d) Includes statement 4, which gives the height in the wrong unit.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2011, General Studies Paper I, Series A. The paper on upsc.gov.in.