ECE 422 Wireless Communications - Satellite Comm

ECE 422 Wireless Communications - Lesson 7 - Satellite Communications

• Course: ECE 422, Wireless Communications
• Lesson: 7, Satellite Communications
• Instructor: Engr. Victor Solido, Dr. Isaac
• Objectives: Define Kepler's Law and satellite orbits; Describe geosynchronous satellites and antenna look angle.

Introduction to Satellites

• Definition (Astronomical): A satellite is a celestial body orbiting a planet.
• Definition (Aerospace): A satellite is a human-launched space vehicle orbiting Earth or another celestial body.
• Types of Satellites:
  • Natural Satellites: Celestial bodies orbiting planets (e.g., the Moon).
  • Artificial Satellites: Human-made space vehicles orbiting Earth or other celestial bodies.

Brief History of Satellites

• 1954: The US Navy transmitted the first Earth-to-moon-to-Earth message, making the Moon a passive satellite.
• 1957: Sputnik 1, the first active satellite (capable of receiving, amplifying, and retransmitting information), was launched by Russia.
• 1957: Explorer 1, launched by the US, was the first spacecraft to detect the Van Allen radiation belt.
• 1958: SCORE, a delayed repeater, was launched by NASA, rebroadcasting President Eisenhower's 1958 Christmas message.
• 1960:
  • Courier, the first transponder type satellite, was launched by the Department of Defense.
  • ECHO, launched by NASA and Bell Telephone Laboratories, was a passive satellite used for signal propagation testing.

Satellite Systems

• Transponder: A satellite radio repeater.
• System Components:
  • Satellite space vehicles
  • Ground-based station
  • User network of earth stations
• Categories:
  • Bus: Control mechanisms supporting payload operation.
  • Payload: Actual user information transmitted through the system.

Kepler's Law

• Originator: Johannes Kepler (1571-1630)

• Description: Kepler's Laws govern satellite motion, defining orbit shape, velocity, and distance from the primary body (e.g. the Sun).

• Application: Can be used for any two bodies in space interacting gravitationally.

• Primary/Secondary: Larger body is the primary, smaller is the secondary (or satellite).

• Kepler's First Law: Planets move in ellipses with the sun at one focus.

• Kepler's Second Law: A line joining the sun and a planet sweeps out equal areas in equal intervals of time.

• Kepler's Third Law: The square of the time of revolution of a planet divided by the cube of its mean distance from the sun is a constant value for all planets. This relates the orbital period (time) to the mean distance from the primary body.

Satellite Orbital Properties

• Eccentricity (ε): Measure of how elliptical an orbit is.

• Semimajor Axis (a): Measure of the orbit's average width.

• Semiminor axis (b): Measure of the orbit's average height.

• Equation to calculate eccentricity: (ε = √(α² - β²)/α)

• Second law (Law of Areas): A line joining a satellite and the primary body sweeps out equal areas in equal time intervals.

  • Satellite moves faster when closer to the Earth, and slower when farther away.

• Third Law: The square of the period ( time it takes for a cycle) of revolution of a satellite is proportional to the cube of its semimajor axis (average distance from the primary).

Geosynchronous Earth Orbits (GEO)

• Altitude: 35,786 km (22,300 miles).
• Revolution Time: 24 hours (synchronises with Earth's rotation)
• Orbital Characteristics: Circular, same direction of Earth's rotation.
• Orbital velocity: ~ 6,840 mph
• Uses: Broadcast, communication, and monitoring weather.
• Issues:
  • Propagation Delays: Increasing distance increases the time it takes for the satellite to transmit/receive signals to/from earth stations.
  • Station Keeping: Requires periodic adjustment to counteract forces from the sun, moon, and other factors.

Limits of Visibility, Satellite Orbital Patterns

• Visibility Limits: The Earth's curvature determines the farthest point a satellite can be seen/observed from a given location.
• Minimum Usable Elevation Angle: 5° or greater. The signal from a satellite will fade rapidly if its elevation angle is too close to the horizon, due to atmospheric absorption and signal strength weakening.
• Apogee: Farthest point from Earth in an orbit.
• Perigee: Closest point to Earth in an orbit.
• Major Axis: The line joining the perigee and apogee through the center of the Earth.
• Minor Axis: Perpendicular to the major axis, midway between the perigee and apogee.
• Line of Apsides: The line joining perigee and apogee
• Equatorial orbit: Orbits located above the equator.
• Polar Orbit: Orbits over the Earth's geographic poles.
• Inclined Orbits: Orbits at any angle relative to Earth's equatorial plane
• Angle of Inclination: Angle between the orbital plane of the satellite and Earth's equatorial plane.
• Sub-satellite Point (SSP): The point on the Earth's surface directly below the satellite.

Antenna Look Angle

• Boresight: Direction of maximum antenna gain.
• Azimuth Angle: Horizontal angular distance from a reference direction (typically true north or south).
• Elevation Angle: Vertical angular distance from the horizontal plane.
• Sub-Satellite Point (SSP): The point on the Earth’s surface directly below the satellite used to specify satellite locations.