Categories of Satellites

1. Categories of Satellites Based on Functions • Passive Satellite – a communications satellite not equipped with electronic devices to retransmit communications. Such satellite acts only as a reflector of radio waves. • Active Satellite – a satellite equipped with electronic devices to receive, amplify, convert, and retransmit signals

2. Categories of Satellites Based on Axis Stabilization • Spinners – use angular momentum of its spinning body to provide roll and yaw stabilization • 3-axis Stabilizers – body remains fixed relative to earth’s surface while an internal subsystem provides roll and yaw stabilization

3. Categories of Satellites Based on Orbit • Non-Synchronous or Orbital – rotate around the earth in low altitude elliptical or circular pattern.

4. Categories of Satellites • Prograde Orbit – If the satellite is orbiting in the same direction as Earth’s rotation with an angular velocity greater than that of the Earth at an angle of inclination of less than 90°. Also called direct orbit. • Retrograde Orbit – If the satellite is orbiting in the opposite direction to the Earth’s rotation or in the same direction but at an angular velocity less than that of Earth

5. Categories of Satellites 2. Geostationary or Geosynchronous – orbit in a circular pattern with an angular velocity equivalent to that of earth with an orbital time of about 24 hours.

6. Categories of Satellites A satellite remains in orbit because of a balance between to factors: a. the satellite’s velocity; speed at which it would travel in a straight line b. the gravitational force between the satellite and the earth Satellite Attitude – refers to the satellite orientation with respect to earth.

7. Classifications of Satellite Orbital Patterns 1. Equatorial Orbit – rotation about the earth’s equator, usually for geosynchronous communications satellites. It permits fewer satellites and ground stations to be used.

8. Classifications of Satellite Orbital Patterns 2. Polar Orbit – rotation over the North and South Poles, used for non-communications mission such as weather and surveillance observation. Circular polar orbit is not used for communications but for navigation.

9. Classifications of Satellite Orbital Patterns 3. Inclined Orbit – rotation creating as angle between the equatorial polar plane. It is not suitable for global network. They are easily launched and have higher payload capacity but are difficult to track.

10. Orbital Terminology Orbit – the path of a celestial body or satellite in unpowered flight. Ascending Node – point where the orbit crosses the equatorial plane going from South to North Descending Node – point where the orbit crosses the equatorial plane going from North to South

11. Orbital Terminology Apogee – the highest point of the orbit or it is the point in an earth satellite orbit, which is the farthest away from the earth; lowest velocity Perigee – the point in an earth satellite orbit, which is the closest in the earth; fastest velocity

12. Orbital Terminology Eccentricity – a number between zero and one which describe the shape of an ellipse. A circle has an eccentricity of zero. defines how oval the satellite's orbit is. It is mathematically defined as the ratio of the orbit's focus distance (c) to the orbit's semi-major axis (a). e = c / a

13. Orbital Terminology Elliptical Orbits Major Axis – the longer axis of the ellipse, the line adjoining the perigee and apogee thru the center of the earth, called the line of apsides Minor Axis – the shorter axis of the ellipse, the line perpendicular to the major axis and halfway between the perigee and apogee

14. Orbital Terminology Mean Distance – the average distance from the center of the earth. The average distance to the centre of the Earth is 6,371 km or 3,959 miles.