A-LevelPhysicsAstronomy and cosmology9702/41 · May/June 2018 · Q58 Marks
5 A geostationary satellite orbits the Earth with a period of 24 hours. (a) State (i) the direction of the orbit about the Earth, (ii) the position of the satellite relative to the Earth's surface, (iii) a typical frequency for communication between the satellite and Earth. frequency = .............................................. Hz (b) A signal transmitted from Earth to a satellite has an initial power of 3.0 kW. The signal power received by the satellite is attenuated by 195 dB. (i) Calculate the signal power received by the satellite. power = .............................................. W (ii) By reference to your answer in (i), explain why different frequencies are used for the up-link and the down-link in communication with the satellite.
- (a)(i)the direction of the orbit about the Earth,[1]
- (a)(ii)the position of the satellite relative to the Earth's surface,[1]
- (a)(iii)a typical frequency for communication between the satellite and Earth. frequency = .............................................. Hz[1]
- (b)(i)Calculate the signal power received by the satellite. power = .............................................. W[3]
- (b)(ii)By reference to your answer in (i), explain why different frequencies are used for the up-link and the down-link in communication with the satellite.[2]
Mark scheme
Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.
- [B1] west to east
- [B1] above the Equator
- [A1] value in range (1–300) × 10⁸ Hz
- [C1] gain/dB = 10 lg (P₂/P₁)
- [C1] -195 = 10 lg (P/3000) OR 195 = 10 lg (3000/P)
- [A1] power = 9.5 × 10⁻¹⁷ W
- [B1] up-link has been (greatly) attenuated (before reaching satellite) OR down-link signal must be (greatly) amplified (before transmission back to Earth) OR up-link has (much) smaller intensity/power than down-link
- [B1] (different frequency) prevents down-link (signal) swamping up-link (signal)
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