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Chapter 3

Satellite Subsystems

SPACECRAFT SUBSYSTEMS

Attitude and Orbital Control System (AOCS)Telemetry Tracking and Command (TT&C)Power SystemCommunications SystemAntennas

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More usually TTC&M - Telemetry, Tracking, Command, and Monitoring

AOCS

AOCS is needed to get the satellite into the correct orbit and keep it thereOrbit insertionOrbit maintenanceFine pointing

Major partsAttitude Control SystemOrbit Control System

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ORBIT INSERTION - GEO

High-Energy Apogee Kick Motor firingA few minutes, symmetrical about apogee

Low-Energy AOCS burnTens of minutes to > one hour burns,

symmetrical about apogeeUses Dual-Mode thrusters; i.e. thrusters

used for both orbit raising and attitude control

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TWO BASIC TYPES OF GEO INSERTION:

ORBIT MAINTENANCE - 1MUST CONTROL LOCATION IN GEO &

POSITION WITHIN CONSTELLATION

SATELLITES NEED IN-PLANE (E-W) & OUT-OF-PLANE (N-S) MANEUVERS TO MAINTAIN THE CORRECT ORBIT

LEO SYSTEMS LESS AFFECTED BY SUN AND MOON BUT MAY NEED MORE ORBIT-PHASING CONTROL

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ORBIT MAINTENANCE - 2

GEO STATION-KEEPING BURNS ABOUT EVERY 4 WEEKS FOR 0.05o

DO N-S AND E-W ALTERNATELY

N-S REQUIRES 10 E-W ENERGY

RECENT APPROACH USES DIFFERENT THRUSTERS FOR E-W AND N-S

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FINE POINTINGSATELLITE MUST BE STABILIZED TO

PREVENT NUTATION (WOBBLE)

THERE ARE TWO PRINCIPAL FORMS OF ATTITUDE STABILIZATIONBODY STABILIZED (SPINNERS, SUCH AS

INTELSAT VI)THREE-AXIS STABILIZED (SUCH AS THE

ACTS, GPS, ETC.)

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DEFINITION OF AXES - 1ROLL AXIS

Rotates around the axis tangent to the orbital plane (N-S on the earth)

PITCH AXISMoves around the axis perpendicular to the

orbital plane (E-W on the earth)YAW AXIS

Moves around the axis of the subsatellite point

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Roll Axis

Pitch Axis

Yaw Axis

Earth

Equator s

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TTC&M

MAJOR FUNCTIONSReporting spacecraft healthMonitoring command actions

Determining orbital elements

Launch sequence deploymentControl of thrustersControl of payload (communications, etc.)

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TTC&M is often a battle between Operations (who want every little thing monitored and Engineering who want to hold data channels to a minimum

TELEMETRY

MONITOR ALL IMPORTANTTEMPERATUREVOLTAGESCURRENTSSENSORS

TRANSMIT DATA TO EARTH

RECORD DATA AT TTC&M STATIONS

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TELEMETRY

TWO TELEMETRY PHASES OR MODESNon-earth pointing

During the launch phase During “Safe Mode” operations when the

spacecraft loses tracking dataEarth-pointing

During parts of the launch phase During routine operations

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TRACKINGMEASURE RANGE REPEATEDLY

CAN MEASURE BEACON DOPPLER OR THE COMMUNICATION CHANNEL

COMPUTE ORBITAL ELEMENTS

PLAN STATION-KEEPING MANEUVERS

COMMUNICATE WITH MAIN CONTROL STATION AND USERS

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COMMAND

DURING LAUNCH SEQUENCESWITCH ON POWERDEPLOY ANTENNAS AND SOLAR PANELSPOINT ANTENNAS TO DESIRED LOCATION

IN ORBITMAINTAIN SPACECRAFT THERMAL

BALANCECONTROL PAYLOAD, THRUSTERS, ETC.

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POWER SYSTEMS

SOLAR CELLS1.39kW/m2 available from sunCells 10 - 15% efficient (BOL=Beginning Of

Life)Cells 7 - 10% efficient (EOL=End of Life)

SOLAR CELL OUTPUT FALLS WHEN TEMPERATURE RISES2mV/degree C; Three-Axis hotter (less

efficient) than a spinner15

POWER SYSTEMS BATTERIES NEEDED

DURING LAUNCHDURING ECLIPSE (<70mins)

BATTERY LIMITSNiCd 50% (DOD=depth of discharge)NiH2 70% DOD

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NOTE: ISS uses 110V bus and will need 110 kW; 30 minute eclipses per day; 55 kW required from batteries

Solution: using Fuel Cells

POWER SYSTEMS

BATTERIES ARE “CONDITIONED” BEFORE EACH ECLIPSE SEASONBATTERIES DISCHARGED TO LIMITBATTERIES THEN RECHARGED

TYPICAL NiH2 BATTERY CAN WITHSTAND 30,000 CYCLES (AMPLE FOR GEO; WOULD BE 5 YEARS IN LEO)

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COMMUNICATIONS SUB-SYSTEMS

Primary function of a communications satellite (all other subsystems are to support this one)

Only source of revenue

Design to maximize traffic capacity

Downlink usually most critical (limited output power, limited antenna sizes).

Early satellites were power limited

Most satellites are now bandwidth limited. 18

SPACECRAFT ANTENNAS

SIMPLE: GLOBAL BEAM, ~17O WID LOW GAIN, LOW CAPACITY

REGIONAL: NARROW BEAM FROM REFLECTORANTENNA, TYPICALLY 3o 3o OR 3o

6o

ADVANCED: MULTIPLE NARROW BEAMSSTATIONARY, SCANNED, OR

“HOPPED”

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ANTENNA TYPESHORN

Efficient, Low Gain, Wide Beam

REFLECTOR High Gain, Narrow Beam, May have to

be deployed in space

PHASED ARRAYComplexElectronically steered

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