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Questions and Answers
Which advantage of geosynchronous satellites allows them to be accessible to all earth stations within their coverage?
Which advantage of geosynchronous satellites allows them to be accessible to all earth stations within their coverage?
What is one of the main disadvantages of using geosynchronous satellites?
What is one of the main disadvantages of using geosynchronous satellites?
What two angles must be determined to optimize the performance of an earth station antenna aimed at a satellite?
What two angles must be determined to optimize the performance of an earth station antenna aimed at a satellite?
How does Doppler shift affect geosynchronous satellites?
How does Doppler shift affect geosynchronous satellites?
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What does the sub satellite point (SSP) refer to in the context of geosynchronous satellites?
What does the sub satellite point (SSP) refer to in the context of geosynchronous satellites?
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What standard angle conventions are used to specify the location of satellites?
What standard angle conventions are used to specify the location of satellites?
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Which of the following would NOT be considered a disadvantage of geosynchronous satellites?
Which of the following would NOT be considered a disadvantage of geosynchronous satellites?
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What effect does the altitude of a geosynchronous satellite on the equator have?
What effect does the altitude of a geosynchronous satellite on the equator have?
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What is the main advantage of geosynchronous satellites?
What is the main advantage of geosynchronous satellites?
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Which of the following is a disadvantage of geosynchronous satellites?
Which of the following is a disadvantage of geosynchronous satellites?
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How should an antenna be aligned to effectively communicate with a geosynchronous satellite?
How should an antenna be aligned to effectively communicate with a geosynchronous satellite?
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What effect does Doppler shift have on signals from geosynchronous satellites?
What effect does Doppler shift have on signals from geosynchronous satellites?
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What is one significant advantage of geosynchronous satellites?
What is one significant advantage of geosynchronous satellites?
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What is the Sub Satellite Point (SSP) for a geosynchronous satellite positioned at 30˚E longitude?
What is the Sub Satellite Point (SSP) for a geosynchronous satellite positioned at 30˚E longitude?
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Why is a minimum angle of elevation of 5° recommended for satellite communications?
Why is a minimum angle of elevation of 5° recommended for satellite communications?
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Why is proper antenna alignment crucial for satellite communication?
Why is proper antenna alignment crucial for satellite communication?
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What does the azimuth angle represent in satellite communication?
What does the azimuth angle represent in satellite communication?
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What angle is typically associated with a significant increase in signal power loss due to atmospheric conditions?
What angle is typically associated with a significant increase in signal power loss due to atmospheric conditions?
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What does the term 'look angle' refer to in satellite communications?
What does the term 'look angle' refer to in satellite communications?
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How does Doppler shift affect communication signals from satellites?
How does Doppler shift affect communication signals from satellites?
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What is one characteristic of geostationary satellites?
What is one characteristic of geostationary satellites?
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Which concept best explains the distance relationship between a satellite and the Earth's surface?
Which concept best explains the distance relationship between a satellite and the Earth's surface?
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What is the Sub Satellite Point (SSP)?
What is the Sub Satellite Point (SSP)?
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Study Notes
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.
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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.
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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.
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System Components:
- Satellite space vehicles
- Ground-based station
- User network of earth stations
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Categories:
- Bus: Control mechanisms supporting payload operation.
- Payload: Actual user information transmitted through the system.
Kepler's Law
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Originator: Johannes Kepler (1571-1630)
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Description: Kepler's Laws govern satellite motion, defining orbit shape, velocity, and distance from the primary body (e.g. the Sun).
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Application: Can be used for any two bodies in space interacting gravitationally.
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Primary/Secondary: Larger body is the primary, smaller is the secondary (or satellite).
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Kepler's First Law: Planets move in ellipses with the sun at one focus.
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Kepler's Second Law: A line joining the sun and a planet sweeps out equal areas in equal intervals of time.
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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
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Eccentricity (ε): Measure of how elliptical an orbit is.
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Semimajor Axis (a): Measure of the orbit's average width.
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Semiminor axis (b): Measure of the orbit's average height.
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Equation to calculate eccentricity: (ε = √(α² - β²)/α)
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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.
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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.
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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.
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Description
In this quiz, you will explore key concepts from Lesson 7 of ECE 422 on Satellite Communications. Topics will include an overview of satellite definitions, types, and the history of satellite technology, along with important principles like Kepler's Law and geosynchronous satellites.
