Chapter 14 - Satellite Comms

Question 1

List the advantages and disadvantages of geostationary orbiting satellites.

Answer 1

Advantages:
1. Wide Coverage: A single satellite can cover 40% of the Earth’s surface.
2. Simple Tracking: The satellite appears stationary in the sky, simplifying acquisition and tracking by earth stations. High gain directional antennas can be used since the satellite points at the same area.
3. No Doppler Shift: Transmissions do not experience Doppler shift because the satellite appears stationary.

Disadvantages:
1. Complex Launch: Achieving a precise, high-altitude orbit requires a complex launch.
2. Complex Telemetry: Maintaining the precise orbit and pointing requires a sophisticated telemetry system.
3. Time Delays: High orbital altitude introduces long time delays (0.25 seconds).
4. High Power Loss: Significant power loss (up to 200 dB) necessitates large antennas or high-power transmitters.

Question 2

List the advantages and disadvantages of sub-synchronous orbiting satellites.

Answer 2

Advantages:
1. Simplified Launch: Lower orbital height means less complex launches.
2. Simplified Telemetry: Precise orbit and pointing are unnecessary, reducing telemetry complexity.
3. Minimal Time Delays: Low altitude results in negligible time delays.
4. Strong Signal: Minimal signal attenuation and high return power levels due to low altitude.

Disadvantages:
1. Complex Tracking: Requires complex acquisition and tracking by earth stations, often needing double-headed systems.
2. Poor Coverage: Low altitude provides limited coverage, necessitating a larger number of satellites. For example, GPS uses 24 satellites at 19,000 km, while the IRIDIUM system uses 66 satellites at 780 km.
3. Doppler Shift: Doppler shift affects transmissions as the satellite rises and sets.

Question 3

WRT satellite communications briefly describe the four multiple access techniques

Answer 3

Demand Assigned Multiple Access (DAMA) - A technique where communication channels are dynamically allocated based on demand to optimize bandwidth usage.

Time Division Multiple Access (TDMA) - A method where multiple users share the same frequency channel by dividing the signal into different time slots.

Frequency Division Multiple Access (FDMA) - A technique where each user is assigned a unique frequency band within the overall bandwidth.

Code Division Multiple Access (CDMA) - A method where multiple users share the same frequency channel by using unique code sequences to modulate their signals.