Chapter 3 Satellite Subsystems 1

Chapter 3

Satellite Subsystems

SPACECRAFT SUBSYSTEMS

 Attitude and Orbital Control Control System System (AOCS) (AOCS) Telemetry elemetry Tracking Tracking and an d Command Command (TT&C) ( TT&C) Power System Communications System More usually TTC&M  Antennas Telemetry, Tracking,

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Command, and Monitoring

AOCS 

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

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Orbit insertion Orbit maintenance Fine pointing

Major parts 

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 Attitude Control System Orbit Control System

ORBIT INSERTION - GEO TWO BASIC TYPES OF GEO INSERTION: 

High-Energy Apogee Kick Motor firing 

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 A few minutes, symmetrical about apogee

Low-Energy AOCS burn 

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Tens of minutes to > one hour burns, symmetrical about apogee Uses Dual-Mode thrusters; i.e. thrusters used for both orbit raising and attitude control

ORBIT MAINTENANCE - 1 

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

ORBIT MAINTENANCE - 2 

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

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DO N-S AND E-W ALTERNATELY 

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N-S REQUIRES  10  E-W ENERGY 

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

FINE POINTING 

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

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

DEFINITION OF AXES - 1 

ROLL AXIS 

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PITCH AXIS 

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Rotates around the axis tangent to the orbital plane ( N-S on the earth) Moves around the axis perpendicular to the orbital plane (E-W on the earth)

 YAW AXIS 

Moves around the axis of the subsatellite point

DEFINITION OF AXES

Earth

o

Equator

s Yaw Axis

Roll Axis

Pitch Axis

TTC&M 

MAJOR FUNCTIONS 

Reporting spacecraft health Monitoring command actions

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Determining orbital elements

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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

Launch sequence deployment Control of thrusters Control of payload (communications, etc.)

TELEMETRY 

MONITOR ALL IMPORTANT  

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TEMPERATURE  VOLTAGES CURRENTS SENSORS

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TRANSMIT DATA TO EARTH

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RECORD DATA AT TTC&M STATIONS

TELEMETRY 

TWO TELEMETRY PHASES OR MODES  

Non-earth pointing 

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During the launch phase During “Safe Mode” operations when the spacecraft loses tracking data

Earth-pointing 

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During parts of the launch phase During routine operations

TRACKING 

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MEASURE RANGE REPEATEDLY  CAN MEASURE BEACON DOPPLER OR THE COMMUNICATION CHANNEL

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COMPUTE ORBITAL ELEMENTS

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PLAN STATION-KEEPING MANEUVERS

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COMMUNICATE WITH MAIN CONTROL STATION  AND USERS

COMMAND 

DURING LAUNCH SEQUENCE  

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SWITCH ON POWER  DEPLOY ANTENNAS AND SOLAR PANELS POINT ANTENNAS TO DESIRED LOCATION

IN ORBIT 

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MAINTAIN SPACECRAFT THERMAL BALANCE CONTROL PAYLOAD, THRUSTERS, ETC.

POWER SYSTEMS 

SOLAR CELLS 

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1.39kW/m2 available from sun Cells 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

POWER SYSTEMS 

BATTERIES NEEDED  

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DURING LAUNCH DURING ECLIPSE (<70mins)

BATTERY LIMITS  

NiCd 50% (DOD=depth of discharge) NiH2 70% DOD

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

Solution: using Fuel Cells

POWER SYSTEMS

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BATTERIES ARE “CONDITIONED” BEFORE EACH ECLIPSE SEASON BATTERIES DISCHARGED TO LIMIT BATTERIES THEN RECHARGED 

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

COMMUNICATIONS SUB-SYSTEMS 

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

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Only source of revenue

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Design to maximize traffic capacity 

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

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Early satellites were power limited

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Most satellites are now bandwidth limited.

SPACECRAFT ANTENNAS

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SIMPLE:

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REGIONAL:

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

 ADVANCED:

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NARROW BEAM FROM REFLECTOR   ANTENNA, TYPICALLY 3o 3o OR 3o 6o

MULTIPLE NARROW BEAMS STATIONARY, SCANNED, OR “HOPPED”

ANTENNA TYPES 

HORN 

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REFLECTOR  

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Efficient, Low Gain, Wide Beam

High Gain, Narrow Beam, May have to be deployed in space

PHASED ARRAY   

Complex Electronically steered