Satellite systems and typical transmission frequencies.

The idea

Satellite communication uses microwaves because they pass through the ionosphere and atmosphere with little absorption or reflection, unlike longer radio waves that may be refracted by the ionosphere. A signal is sent from an Earth station to a satellite, amplified and retransmitted on a different frequency to avoid interference between uplink and downlink. Geostationary satellites orbit above the equator with a period of 24 hours, so a dish can be fixed; they are used for television and telephony but have a long path and noticeable delay. Low-Earth-orbit satellites have shorter delays but move relative to the ground, so tracking or constellations are needed. Typical frequencies are in the microwave region, roughly 1–30 GHz, with specific bands such as C, Ku and Ka used for different services.

What earns the marks

• Satellite links use microwaves because these frequencies pass through the ionosphere and atmosphere with relatively little attenuation.
• An uplink transmits from a ground station to the satellite; the satellite amplifies and retransmits the signal as a downlink, often using a different frequency to prevent interference.
• Geostationary satellites have a 24-hour orbital period above the equator, so ground dishes can remain fixed; this suits broadcast and continuous communication.
• Low-Earth-orbit satellites travel faster and closer, giving shorter signal delay but requiring tracking or a constellation of satellites for continuous coverage.
• Typical satellite transmission frequencies lie in the microwave region, approximately 1–30 GHz, with bands such as C, Ku and Ka allocated to different services.
• Higher microwave frequencies allow greater bandwidth but suffer more atmospheric attenuation, especially in heavy rain.

Common mistakes and how to put them right

• Stating that satellites use sound waves or visible light.

  Put it right by identifying microwaves as the typical transmission medium.

• Confusing uplink and downlink.

  Put it right by defining uplink as Earth-to-satellite and downlink as satellite-to-Earth.

• Claiming all satellites are geostationary.

  Put it right by distinguishing geostationary orbits from low-Earth orbits and their different delay and tracking requirements.

Exam tips

• Quote typical frequencies in the microwave region, such as gigahertz values, and link the choice to low atmospheric absorption.
• Compare geostationary and low-Earth-orbit systems in terms of delay, coverage and tracking rather than listing only one type.
• Explain why uplink and downlink frequencies differ: to avoid the strong transmitted signal interfering with the much weaker received signal.