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Launching, Orbital Effects & Satellite Subsystems

Joe MontanaIT 488 - Fall 2003

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Agenda

• Satellite Subsystems

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

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

We will look at each in turn

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AOCSAOCS is needed to get the satellite into the correct orbit and keep it there

Orbit insertionOrbit maintenanceFine pointing

Major partsAttitude Control SystemOrbit Control System

Look at these next

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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

TWO BASIC TYPES OF GEO INSERTION:

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ORBIT MAINTENANCE - 1

MUST CONTROL LOCATION IN GEO & POSITION WITHIN CONSTELLATIONSATELLITES NEED IN-PLANE (E-W) & OUT-OF-PLANE (N-S) MANEUVERS TO MAINTAIN THE CORRECT ORBITLEO SYSTEMS LESS AFFECTED BY SUN AND MOON BUT MAY NEED MORE ORBIT-PHASING CONTROL

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ORBIT MAINTENANCE - 2GEO STATION-KEEPING BURNS ABOUT EVERY 4 WEEKS FOR 0.05o

DO N-S AND E-W ALTERNATELYN-S REQUIRES 10 E-W ENERGYRECENT APPROACH USES DIFFERENT THRUSTERS FOR E-W AND N-S

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FINE POINTING

SATELLITE MUST BE STABILIZED TO PREVENT NUTATION (WOBBLE)THERE ARE TWO PRINCIPAL FORMS OF ATTITUDE STABILIZATION

BODY STABILIZED (SPINNERS, SUCH AS INTELSAT VI)THREE-AXIS STABILIZED (SUCH AS THE ACTS, GPS, ETC.)

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DEFINITION OF AXES - 1

ROLL AXISRotates 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 AXISMoves around the axis of the subsatellite point

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DEFINITION OF AXES - 2

Roll Axis

Pitch Axis

Yaw Axis

Earth

Equator s

o

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TTC&M

MAJOR FUNCTIONSReporting spacecraft healthMonitoring command actions

Determining orbital elements

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

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

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TELEMETRY - 1

MONITOR ALL IMPORTANTTEMPERATUREVOLTAGESCURRENTSSENSORS

TRANSMIT DATA TO EARTHRECORD DATA AT TTC&M STATIONS

NOTE: Data are usually multiplexed with a priority rating. There are usually two telemetry modes.

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TELEMETRY - 2

TWO TELEMETRY PHASES OR MODES

Non-earth pointing• During the launch phase• During “Safe Mode” operations when the

spacecraft loses tracking data

Earth-pointing• During parts of the launch phase • During routine operations

NOTE: for critical telemetry channels

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TRACKINGMEASURE RANGE REPEATEDLYCAN MEASURE BEACON DOPPLER OR THE COMMUNICATION CHANNEL COMPUTE ORBITAL ELEMENTSPLAN STATION-KEEPING MANEUVERSCOMMUNICATE WITH MAIN CONTROL STATION AND USERS

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COMMANDDURING LAUNCH SEQUENCE

SWITCH ON POWERDEPLOY ANTENNAS AND SOLAR PANELSPOINT ANTENNAS TO DESIRED LOCATION

IN ORBITMAINTAIN SPACECRAFT THERMAL BALANCECONTROL PAYLOAD, THRUSTERS, ETC.

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POWER SYSTEMS - 1

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 RISES

2mV/degree C; Three-Axis hotter (less efficient) than a spinner

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BATTERIES NEEDEDDURING LAUNCHDURING ECLIPSE (<70mins)

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

POWER SYSTEMS - 2

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

Solution: using Fuel Cells

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POWER SYSTEMS - 3BATTERIES ARE “CONDITIONED” BEFORE EACH ECLIPSE SEASON

BATTERIES 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 revenueDesign to maximize traffic capacityDownlink usually most critical (limited output power, limited antenna sizes).Early satellites were power limitedMost satellites are now bandwidth limited.

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SPACECRAFT ANTENNASSIMPLE: 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 TYPES

HORNEfficient, Low Gain, Wide Beam

REFLECTORHigh Gain, Narrow Beam, May

have to be deployed in space

PHASED ARRAYComplexElectronically steered