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Gravitational Fields

Geostationary satellites

  • 3 things to know
  • Unit A2 2

Overview

Geostationary satellite, three conditions a geostationary orbit must satisfy and one disadvantage of a geostationary orbit for communications.

What you need to know

Answer each one before you reveal it.

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Key term

What is a geostationary satellite?

Key term

State the three conditions a geostationary orbit must satisfy.

Key term

Why must a geostationary satellite orbit from west to east?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is a geostationary satellite?

    Model answer

    A satellite that stays above the same point on the Earth's equator, appearing stationary to an observer on the ground.

  2. 2

    State the three conditions a geostationary orbit must satisfy.

    Model answer

    The period must equal the Earth's rotation period of 24 hours, the orbit must lie in the plane of the equator, and the satellite must travel in the same direction as the Earth's rotation, from west to east.

  3. 3

    Why must the period be exactly 24 hours?

    Model answer

    Only then does the satellite complete one orbit in the time the Earth takes to turn once, so it stays above the same point.

  4. 4

    Why must a geostationary orbit lie over the equator?

    Model answer

    An orbit in any other plane would carry the satellite north and south of the equator during each revolution, so it would not stay above one point.

  5. 5

    Why must the satellite orbit from west to east?

    Model answer

    The Earth rotates from west to east, so a satellite going the other way would move across the sky rather than keeping pace with the ground.

  6. 6

    Calculate the radius of a geostationary orbit, taking the Earth's mass as 6.0 × 10²⁴ kg and the period as 8.64 × 10⁴ s.

    Model answer

    r³ = GMT²/(4π²) = 4.00 × 10¹⁴ × 7.46 × 10⁹ ÷ 39.48 = 7.56 × 10²², so r is the cube root of that, 4.2 × 10⁷ m.

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