ECE 5233 Satellite Communications Prepared by:

Dr. Ivica Kostanic

Lecture 6: Satellite sub-systems(Section 3.1-3.4)

Spring 2014

Florida Institute of technologies

Page 2

Satellite subsystemsCommunication subsystemSatellite transpondersExamples

Outline

Important note: Slides present summary of the results. Detailed derivations are given in notes.

Florida Institute of technologies

Satellite subsystems

Major satellite subsystemso Altitude and Orbit Control Systems (AOCS)

– maintain and stabilize satellite in the orbito Telemetry, Tracking, Command and

Monitoring (TTC&M) – take and process measurements on satellite health and position

o Power subsystem – generate and distribute power to various components of the satellite

o Communication subsystem – Receives, processes and re-transmits the signals

o Satellite antenna – receive and transmit EM waves.

o Superstructure – construction of the satellite that is used as a mount for all other components

o 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

Page 3

Major components of a Lockheed Martin remote sensing satellite

BBC Documentary - How to build a satellite: https://www.youtube.com/watch?v=_Rp53U4mzZA

Florida Institute of technologies

Altitude and Orbit Control Two principle tasks

o Stabilize the orientation of the satelliteo Maintain the position of the satellite in orbit

Four ways of stabilizationso Spinning o Momentum wheelso Reaction wheelso 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

Page 4

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

Example of spin stabilized satellite

Florida Institute of technologies

Telemetry, Tracking, Command and Monitoring

TTC&M – distributed between satellite and Earth station

Satellite provides measurementso Position sensorso Environmental sensorso 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

Page 5

Block diagram of TTC&M system

Florida Institute of technologies

Power systems Used for generation and distribution of power

throughout a satellite Three types of power systems

o Solar – the most frequently used in commercial satellites

o Chemical – used for backup to power satellite during solar eclipses

o 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

Page 6

Block diagram of solar power generation system

Florida Institute of technologies

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

(a) the number of cells required

(b) (b) the total mass of the battery system. Given that the specific energy specification for the battery technology used is 60W h/kg.

Answers:

a) Required number cells – 49

b) Mass of the battery system – 94.74 kg

Page 7

Florida Institute of technologies

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

Page 8

Outline of satellite communication system

Florida Institute of technologies

Satellite transponders Two types of satellites

o “Bent pipes” (transparent) o Regenerative (base band processing)

Smallest assignable recourseo Satellite transpondero Satellite usually hosts many transponderso Some of transponders may be spareso Typical active transponder count is 24 o Satellite usually operates in single band

(although there are some multiband satellites)

o Bandwidth of the satellite transponder is a compromise between power efficiency (favors larger bandwidth) and limitations on linearity of PA (favors smaller bandwidth)

o Most common bandwidth of a transponder in 36MHz (with 40MHz channelization)

o Some satellites adopt 54MHz or even 72MHz

Page 9

Basics of “bent pipe” architecture

Satellite with onboard processing

Florida Institute of technologies

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)

o Minimizes intermodulation products

Page 10

Example of transponder arrangement for RCA’s SATCOM

Florida Institute of technologies

Simplified single conversion transponder

Page 11

1. Input signal

tfAtx G61 2cos

2. After mixing stage

tffKAtffKA

tfXtfAKtx

LOGLOG

LOLOG

66

62

2cos212cos

21

2cos2cos

3. Output signal- translated by frequency of LO

tffGKAtx LOG 63 2cos