Basic Satellite Communication (3) Components of Communications Satellite

1. Basic Satellite Communication (3)Components of Communications Satellite Dr. Joseph N. Pelton

2. Components of a Satellite Payload: Satellite Main Mission Antennas & Communications System Satellite Bus Components Satellite Tracking, Telemetry, Command & Monitoring (TTC&M) Methods to Cope with Rain Attenuation Satellite Power System Satellite “Bus”, Orientation and Stabilization System

3. Components of a Satellite Main Mission Antennas & Communications System Antennas are constantly pointed to the earth with larger antenna server the lower frequency and smaller antenna service the higher frequency The transponder in the communications system is responsible for receiving the signal, amplifying it and re-transmitting it in the lower frequency back to earth. Most transponders in the C- and Ku- bands now use solid state power amplifiers (SSPAs) to amplify the signal because of their weight, compactness and reliability. Satellites can have 12 to 96 transponders plus spares, depending on the size of the satellite.

4. Components of a Satellite Satellite Transponders & SSPA The transponder is the “active” part of the satellite communications system that provides the connection between the satellite’s receive and transmit antennas. A transponder bandwidth can frequently be 36 MHz, 54 MHz, or 72 MHz or it can be even wider. A transponders function is to receive the signal filter out noise, shift the frequency to a downlink frequency and then amplify it for retransmission to the ground. The main amplifier may be a Travelling Wave Tube (TWT) or Klystron tube (now usually used for higher frequencies above 20 GHz and at very high power levels (i.e. 100 to 200 watts) or it may be a Solid State Power Amplifier (SSPA) that would be used at lower L, C or Ku bands frequencies. If the transponder is a regenerative transponder then the signal will be converted to base band frequencies and processed there rather than handled at RF bands.

5. Components of a Satellite Antenna Gain & Path Loss Gain measures the performance of an antenna. The larger the size (or aperture) of an antenna the better it can point to or concentrate a signal towards the desired receiver location on earth. Thus the more the antenna acts like a spot light to illuminate the earth and not send signal uselessly out into space, the higher the gain of the antenna. An Omni antenna that sends signals in all directions equally has a gain of 1 and represents the lowest gain. Path loss comes from the spreading out of a signal from the antenna as it travels from the satellite to user antenna and vice versa. Since the signal spreads in a circle effective power (or flux density) is reduced by the square of the distance traveled.

6. Components of a Satellite Satellite Transmit Power (EIRP) EIRP = Effective Isotropic Radiate Power. The product of final amplifier power, filtering and coupling losses and antenna gain. Specified in watts or dB (relative to 1 watt) = dBw EIRP density, typically specified in terms of dBw/Hz is a key parameter in determining both uplink and downlink performance.

7. Components of a Satellite Satellite Transponder & SSPAs Transponder Traffic Loading is a Function of: Transponder Bandwidth Uplink and Downlink satellite and earth station parameters Number of Carriers per transponder. This determines “back-off” which is the reduction in amplifier power from the saturated or maximum output power. Modulation efficiency (i.e. bits/Hz)

9. Methods to Cope with Rain Attenuation Use lower frequencies where possible Provide link margin + additional link margin in beams where there is heavy rains On-board processing Site diversity Ability to increase dwell time and/or reduce information rate

10. Components of a Satellite: Satellite Tracking, Telemetry, Command & Monitoring (TTC&M) • Tracking is necessary to know exactly where a satellite is and thus be able to send it commands, receive telemetry and communication with it. • Omni antenna is used to be able to track and command satellite even if orbit is disturbed. • The TTC&M system is highly automated with computer alarms to sound in case of anomalies are detected. • Analogy: School bus represents: spacecraft bus, and passengers represents payload: telecommunications and TV traffic. TTC&M is needed to keep “bus” running and check on status of passengers.

11. Components of a Satellite: Satellite Power System Main source of power is solar cell panels that are constantly oriented towards the sun. New solar cells are increasingly efficient. The solar cell system is backed up by battery system that provides energy during solar eclipses and other periods of outages. Typical power levels of 2 to 5 KWs for Fixed Satellite Systems and 10 to 12 KWs for Mobile and Broadcast Satellite Systems in GEO. Levels are lower for LEO & MEO Systems.

12. Components of a Satellite: Satellite “Bus” & Stabilization System The “bus” is the key parts of the satellite that allows its operation in space. The early generation of satellites use gravity-gradient or spin stabilization but now 3-axis body stabilization using momentum wheels is most common because of lifetime & efficiency. The “bus” includes the antenna mast, the thermal control, the sun & earth sensors, the batteries, momentum wheels, solar array controls & on-board computers. Carbon-epoxy structures are used because of strength and light weight.

13. Components of a Satellite: Satellite “Bus” & Stabilization System

14. Assignment • Assignment 4: • Write short notes on the terms highlighted in yellow color (Slides 4 and 9 of lecture 4)