Key Communication Technologies: From Early Devices to Satellites

Communication Systems: An Overview

A communication system fundamentally establishes a connection between a sender and a receiver. Such systems typically consist of several key components:

• The Satellite or Payload: The core component that facilitates communication.
• Control Center: Monitors the satellite's functionality and ensures proper operation.
• Ground Stations: Equipped with suitable antennas for transmitting and receiving signals from the satellite.

Satellite Communication Systems

Types of Satellites by Orbit

Satellites are classified based on their orbital characteristics:

• Low Earth Orbit (LEO):

  LEO satellites orbit at approximately 1,000 km above Earth and complete a full revolution in about 2 hours. They are primarily used for providing data on movement and for applications requiring low latency.

• Medium Earth Orbit (MEO):

  MEO satellites move in orbits around 10,000 km. They are commonly used for telephony and television communications, as well as navigation systems like GPS.

• Highly Elliptical Orbit (HEO):

  HEO satellites have non-circular, elliptical orbits. These are often utilized for mapping, reconnaissance, and providing coverage to high-latitude regions.

• Geostationary Earth Orbit (GEO) Satellites:

  GEO satellites have a velocity equal to the Earth's rotational speed, making them appear stationary from the ground. Their orbits are at approximately 36,000 km. They are extensively used for television and telephony broadcasts, and for long-distance data transmission due to their wide coverage area.

Types of Satellites by Function

Satellites serve diverse purposes based on their design and mission:

• Telecommunications Satellites:

  Used to transmit information between different points on Earth.

• Earth Observation Satellites:

  Designed to observe terrestrial land and monitor meteorological conditions.

• Space Observation Satellites:

  Utilized for scientific observation of outer space.

• Location Satellites:

  Determine the precise position of objects on the Earth's surface, such as those used in Global Positioning Systems (GPS).

• Space Stations:

  Inhabited by humans, these serve as laboratories for scientific research and as staging points for space missions.

• Space Probes:

  Unmanned spacecraft designed to explore and gather data from other celestial bodies.

The Telegraph

The telegraph is an electronic communication mechanism connecting two stations via a single cable. It operates by allowing the action of a button at one end to transmit a signal, which is received at the other end and associated with a code and a bell.

Morse Code: Each letter is represented by a unique sequence of dots and dashes. These are processed as short-duration electrical impulses (dots) and long-duration electrical impulses (dashes). To separate symbols, a silence equivalent to one dot's length is used, while the spacing between words is equivalent to five dots.

The Telephone

The telephone incorporates two primary elements into its design, building upon the principles of the telegraph:

• Microphone:

  The microphone in the handset contains a resistance whose value varies with the sound vibrations of our voice. This change in resistance causes a corresponding change in the intensity of the current in the circuit.

• Earpiece (Receiver):

  This variation in current is then collected by the earpiece, which reproduces the original sound that was captured by the microphone.

Telephone Central Switches

When a user picks up the telephone, a dial tone is sent, indicating that the system is ready to receive input. The user then dials a series of digits. To connect the call, the central telephone exchange either sends ringing tones to the recipient's phone or sends a request to another central station if the call is long-distance.

If the recipient picks up the phone, a connection can be established. However, if the recipient's phone is busy, the central switch sends a busy signal to inform the caller.