ECE 5233 Satellite Communications

1. ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 6: Satellite sub-systems (Section 3.1-3.4) Spring 2014

2. Outline • Satellite subsystems • Communication subsystem • Satellite transponders • Examples Important note: Slides present summary of the results. Detailed derivations are given in notes.

3. Satellite subsystems • Major satellite subsystems • Altitude and Orbit Control Systems (AOCS) – maintain and stabilize satellite in the orbit • Telemetry, Tracking, Command and Monitoring (TTC&M) – take and process measurements on satellite health and position • Power subsystem – generate and distribute power to various components of the satellite • Communication subsystem – Receives, processes and re-transmits the signals • Satellite antenna – receive and transmit EM waves. • Superstructure – construction of the satellite that is used as a mount for all other components • Thermal subsystem – maintains the temperature of the satellite within prescribed range • Satellites have life expectancy 10-15 years • Many components are deployed in redundant configurations to minimize probability of satellite failure Major components of a Lockheed Martin remote sensing satellite BBC Documentary - How to build a satellite: https://www.youtube.com/watch?v=_Rp53U4mzZA

4. Altitude and Orbit Control • Two principle tasks • Stabilize the orientation of the satellite • Maintain the position of the satellite in orbit • Four ways of stabilizations • Spinning • Momentum wheels • Reaction wheels • Control moment gyro • Orbit is maintained using control thrusters • The amount of fuel available for thruster operation is a fundamental limit on the satellite life span Example of spin stabilized satellite Different methods for satellite stabilization Boeing 376 – one of the most popular GEO Comm. Satellites Operates in C, Ku bands Usually 24 transponders 50 satellites over five continents, used by more than 20 companies

5. Telemetry, Tracking, Command and Monitoring • TTC&M – distributed between satellite and Earth station • Satellite provides measurements • Position sensors • Environmental sensors • Alarms • Satellite may have few hundred of different sensors • Measured data sent over TTC&M link to Earth station • The TTC&M link is a narrowband link - allows for high sensitivity reception • At the Earth station measured data processed and commands are issued to the satellite • TTC&M may be operated by satellite owner or it may be outsourced • TTC&M systems are build with redundancy Block diagram of TTC&M system

6. Power systems • Used for generation and distribution of power throughout a satellite • Three types of power systems • Solar – the most frequently used in commercial satellites • Chemical – used for backup to power satellite during solar eclipses • Nuclear – used for satellites leaving the Earth orbit (deeper space exploration) • Source of power generation – Solar panels • Solar panels consist of many strings solar cells connected in parallel • Solar energy in Earth orbit has density of ~ 1390W/m2 • Three axis stabilized satellites use flat solar panels • Spinning satellites have solar panels on the cylindrical surface of the satellite • Efficiency of solar cells is about 20% (i.e. only 20% of the sunlight might be converted to energy) • The energy is used to charge satellite batteries and to power rest of the satellite • The power needed for a satellite may be in the range 0.5-10KW • Majority of the power is consumed by the communication equipment - RF amplifiers on the transponders Block diagram of solar power generation system

7. Power systems - examples Example 1. Consider a case where a spin-stabilized satellite has to generate 2000 watts of electrical power from the solar panels. Assuming that the solar flux falling normal to the solar cells in the worst case is 1250W/m2, the area of each solar cell is 4 cm2 and the conversion efficiency of the solar cells including the losses due to cabling, etc., is 15 %, determine the number of solar cells needed to generate the desired power. What would be the number of cells required if the sun rays fell obliquely, making an angle of 10◦ with the normal? Answers: Required number of cells: 83777 For 10% angle, required number of cells is 85070 Example 2. It is desired that the battery system on board the satellite is capable of meeting the full power requirement of 3600 watts for the worst case eclipse period of 72 minutes. If the satellite uses nickel–hydrogen cells of 1.3 volts, 90 A h capacity each with an allowable depth of discharge of 80 %, and discharge efficiency of 95 %, find • the number of cells required • (b) the total mass of the battery system. Given that the specific energy specification for the battery technology used is 60W h/kg. Answers: • Required number cells – 49 • Mass of the battery system – 94.74 kg

8. Communication systems • Most important (i.e. revenue generating part of the satellite) • Satellite – repeater in the sky • Bands for satellite operation: L(2GHz/1GHz), S(4GHz/2GHz), C(6GHz/4GHz), X(7/8 GHz) Ku(12-18GHz) and Ka(27-40GHz) • Early communication satellites – power limited, used narrowband transmission • Contemporary satellite – bandwidth limited, use wideband transmission and frequency reuse • Frequency allocation handled through ITU on the global basis • Management of the frequencies in the US are conducted by Federal Communications Committee (FCC) • A unit of satellite communication capacity - transponder Outline of satellite communication system

9. Satellite transponders • Two types of satellites • “Bent pipes” (transparent) • Regenerative (base band processing) • Smallest assignable recourse • Satellite transponder • Satellite usually hosts many transponders • Some of transponders may be spares • Typical active transponder count is 24 • Satellite usually operates in single band (although there are some multiband satellites) • Bandwidth of the satellite transponder is a compromise between power efficiency (favors larger bandwidth) and limitations on linearity of PA (favors smaller bandwidth) • Most common bandwidth of a transponder in 36MHz (with 40MHz channelization) • Some satellites adopt 54MHz or even 72MHz Basics of “bent pipe” architecture Satellite with onboard processing

10. Transponder arrangement – fixed frequency translation • Basic design – each transponder is individual chain with fixed frequency translation • Banks of transponders are arranged to achieve higher frequency separation (80MHz) • Minimizes intermodulation products Example of transponder arrangement for RCA’s SATCOM

11. Simplified single conversion transponder 1. Input signal 3. Output signal- translated by frequency of LO 2. After mixing stage