P5e: Satellite communication

P5e: Satellite communication

1. The highest frequency radio waves are microwaves. They are next to each other in the EM spectrum.

2. High frequency waves are used to send signals to Geostationary satellites as they can penetrate the Earth’s atmosphere. Low frequency can be used for low orbit satellites.

3. Microwave signals are sent from a transmitter, then received and amplified by the satellite. Then the signal is retransmitted to Earth and picked up by a receiver.

4. The ionosphere is a layer of charged particles, high in the atmosphere.

5. Microwaves between frequencies of 3GHz and 30GHz are used for satellite communication.

6. Below 3GHz, the waves are reflected from the ionosphere

7. Above 30GHz the waves are scattered and absorbed by the atmosphere.

8. Long wavelength radiowaves diffract around hills, so have a longer range.

9. Microwaves pass straight through the atmosphere and are suitable for communicating with distant geostationary satellites, while radio waves are suitable for communicating with satellites in low orbit

10. Radio waves and microwaves have different properties

    • Radio waves have longer wavelengths and are reflected by the ionosphere (part of the Earth’s atmosphere).
    • Microwaves have shorter wavelengths and pass through the Earth’s atmosphere.

11. Different frequencies are used to communicate with these satellites:

    • relatively high frequencies are used to communicate with geostationary satellites
    • relatively low frequencies are used to communicate with low orbit satellites

12. Digital signals are used to communicate with satellites. Digital signals are affected by noise less than analogue signals, and they do not attenuate (lose energy) as quickly.

13. The wavelength of the microwaves emitted by satellites is usually between 1 and 10 cm. The dishes that emit them from satellites usually have a diameter much larger than this. This produces very little diffraction, giving a narrow beam that does not spread out.

    • As a result, the transmitting and receiving dishes must be aligned exactly

14. Diffraction is the spreading of waves when they meet the edge of an obstacle or pass through a gap

15. Radio waves have a very long wavelength and can ‘spread’ around large objects such as buildings and hills. This gives radio waves with long wavelengths a very long range.

16. Waves are diffracted when they pass through a gap. The smaller the gap compared to the wavelength, the more the waves are diffracted.

17. Maximum diffraction happens when the gap is the same size as the wavelength, producing semi-circular waves.