Question
A geostationary satellite orbits the Earth with a period of 24 hours.
(a) State
(i) the direction of the orbit about the Earth,
………………………………………………………………………………………………………………………[1]
(ii) the position of the satellite relative to the Earth’s surface,
………………………………………………………………………………………………………………………[1]
(iii) a typical frequency for communication between the satellite and Earth.
frequency = …………………………………………….. Hz [1]
(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 [3]
(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]
Answer/Explanation
(a)(i) west to east
(a)(ii) above the Equator
(a)(iii) value in range \((1–300) × 10^9 ~Hz \)
(b)(i) \( gain/dB = 10 lg(P_2 /P_1)\)
\(–195= 10 lg(P/3000)\)
or
\(195= 10 lg(3000/P)\)
\(power= 9.5× 10^{–17}~W\)
(b)(ii) 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