Know that communication with artificial satellites is mainly by microwaves: (a) some satellite phones use low orbit artificial satellites (b) some satellite phones and direct broadcast satellite television use geostationary satellites

Published by Patrick Mutisya · 14 days ago

Cambridge IGCSE Physics 0625 – 3.3 Electromagnetic Spectrum

3.3 Electromagnetic Spectrum

Objective

Know that communication with artificial satellites is mainly by microwaves:

  • Some satellite phones use low‑orbit artificial satellites.

  • Some satellite phones and direct‑broadcast satellite television use geostationary satellites.

1. Position of Microwaves in the Electromagnetic Spectrum

Microwaves occupy the part of the spectrum between radio waves and infrared radiation.

Typical frequency and wavelength ranges are:

  • Frequency: \$300\ \text{MHz} \;-\; 300\ \text{GHz}\$

  • Wavelength: \$1\ \text{m} \;-\; 1\ \text{mm}\$

These ranges are ideal for satellite communication because:

  1. Atmospheric attenuation is relatively low.

  2. Antenna sizes are practical for both ground stations and satellites.

  3. High data‑rate transmission is possible.

2. Why Microwaves Are Used for Satellite Links

Microwaves can penetrate the Earth's atmosphere with minimal loss, especially in the so‑called “microwave windows” around 2 GHz, 8 GHz, and 12 GHz. This makes them suitable for:

  • Two‑way voice communication (satellite phones).

  • High‑definition television broadcasting.

  • Data transmission for navigation and remote sensing.

3. Low‑Orbit (LEO) Satellite Phones

LEO satellites orbit at altitudes of roughly 500 km to 2 000 km. Because they are close to the Earth, the path length for the microwave signal is short, reducing signal delay.

Key characteristics:

  • Orbital period: about 90–120 minutes.

  • Typical operating frequency: \$1.5\text{–}2.5\ \text{GHz}\$ (L‑band) or \$2.4\text{–}2.5\ \text{GHz}\$ (S‑band).

  • Latency: \$< 100\ \text{ms}\$, which is perceptible as a slight delay but acceptable for voice.

  • Coverage: Requires a constellation of many satellites to provide continuous global service.

Suggested diagram: Sketch of a low‑orbit satellite communicating with a handheld phone, showing the short signal path.

4. Geostationary (GEO) Satellites for T \cdot and Phone Services

Geostationary satellites remain fixed relative to a point on the equator at an altitude of about 35 786 km. They are used for direct‑broadcast satellite (DBS) television and some satellite phone services.

Key characteristics:

  • Orbital period matches Earth’s rotation (24 h).

  • Typical operating frequencies: \$12\text{–}14\ \text{GHz}\$ (Ku‑band) for TV, \$1.5\text{–}2.5\ \text{GHz}\$ (L‑band) for some phone services.

  • Latency: about \$250\text{–}300\ \text{ms}\$ due to the long round‑trip distance.

  • Coverage: One satellite can serve roughly one third of the Earth's surface; three satellites provide near‑global coverage.

Suggested diagram: Geostationary satellite above the equator with a large footprint covering a continent.

5. Comparison of LEO and GEO Satellite Systems

FeatureLow‑Orbit (LEO)Geostationary (GEO)
Altitude (km)500 – 2 000≈ 35 786
Orbital period≈ 90–120 min24 h (synchronous)
Typical microwave bandL‑band (1.5–2.5 GHz) or S‑band (2.4–2.5 GHz)Ku‑band (12–14 GHz) for TV, L‑band for phones
Signal latency (one‑way)≈ 5–10 ms≈ 125–150 ms
Coverage per satelliteSmall footprint; many satellites neededLarge footprint; few satellites needed
Typical applicationsSatellite phones, IoT, Earth observationDirect‑broadcast TV, some satellite phones, broadband internet

6. Summary

Microwaves are the preferred part of the electromagnetic spectrum for satellite communication because they travel through the atmosphere with relatively low loss and can be efficiently generated and received with reasonably sized antennas. Low‑orbit satellites provide low latency and are ideal for mobile voice services, while geostationary satellites offer wide‑area coverage suitable for broadcast television and certain satellite phone services, despite higher latency.