1. Launching, Orbital Effects & Satellite Subsystems
Joe Montana
IT Fall 2003

2. Agenda
Satellite Subsystems

3. SPACECRAFT SUBSYSTEMS
Attitude and Orbital Control System (AOCS)
Telemetry Tracking and Command (TT&C)
Power System
Communications System
Antennas
More usually TTC&M - Telemetry, Tracking, Command, and Monitoring
We will look at each in turn

4. AOCS
AOCS is needed to get the satellite into the correct orbit and keep it there
Orbit insertion
Orbit maintenance
Fine pointing
Major parts
Attitude Control System
Orbit Control System
Look at these next

5. ORBIT INSERTION - GEO TWO BASIC TYPES OF GEO INSERTION:
High-Energy Apogee Kick Motor firing
A few minutes, symmetrical about apogee
Low-Energy AOCS burn
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

6. ORBIT MAINTENANCE - 1
MUST 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

7. 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

8. 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.)

9. DEFINITION OF AXES - 1 YAW AXIS ROLL AXIS PITCH AXIS
Rotates around the axis tangent to the orbital plane (N-S on the earth)
PITCH AXIS
Moves around the axis perpendicular to the orbital plane (E-W on the earth)
YAW AXIS
Moves around the axis of the subsatellite point

10. DEFINITION OF AXES - 2
Earth o
Equator s
Yaw Axis
Roll Axis
Pitch Axis

11. TTC&M MAJOR FUNCTIONS Reporting spacecraft health
Monitoring command actions
Determining orbital elements
Launch sequence deployment
Control of thrusters
Control 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

12. TELEMETRY - 1 MONITOR ALL IMPORTANT TRANSMIT DATA TO EARTH
TEMPERATURE
VOLTAGES
CURRENTS
SENSORS
TRANSMIT DATA TO EARTH
RECORD DATA AT TTC&M STATIONS
NOTE: Data are usually multiplexed with a priority rating. There are usually two telemetry modes.

13. 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

14. TRACKING MEASURE RANGE REPEATEDLY
CAN MEASURE BEACON DOPPLER OR THE COMMUNICATION CHANNEL
COMPUTE ORBITAL ELEMENTS
PLAN STATION-KEEPING MANEUVERS
COMMUNICATE WITH MAIN CONTROL STATION AND USERS

15. COMMAND DURING LAUNCH SEQUENCE IN ORBIT SWITCH ON POWER
DEPLOY ANTENNAS AND SOLAR PANELS
POINT ANTENNAS TO DESIRED LOCATION
IN ORBIT
MAINTAIN SPACECRAFT THERMAL BALANCE
CONTROL PAYLOAD, THRUSTERS, ETC.

16. POWER SYSTEMS - 1 SOLAR CELLS
1.39kW/m2 available from sun
Cells % efficient (BOL=Beginning Of Life)
Cells % efficient (EOL=End of Life)
SOLAR CELL OUTPUT FALLS WHEN TEMPERATURE RISES
2mV/degree C; Three-Axis hotter (less efficient) than a spinner

17. POWER SYSTEMS - 2 BATTERIES NEEDED BATTERY LIMITS DURING LAUNCH
DURING ECLIPSE (<70mins)
BATTERY LIMITS
NiCd 50% (DOD=depth of discharge)
NiH2 70% DOD
NOTE: ISS uses 110V bus and will need 110 kW; 30 minute eclipses per day; 55 kW required from batteries
Solution: using Fuel Cells

18. POWER SYSTEMS - 3
BATTERIES ARE “CONDITIONED” BEFORE EACH ECLIPSE SEASON
BATTERIES DISCHARGED TO LIMIT
BATTERIES THEN RECHARGED
TYPICAL NiH2 BATTERY CAN WITHSTAND 30,000 CYCLES (AMPLE FOR GEO; WOULD BE 5 YEARS IN LEO)

19. 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.

20. SPACECRAFT ANTENNAS
SIMPLE: GLOBAL BEAM, ~17O WID LOW GAIN, LOW CAPACITY
REGIONAL: NARROW BEAM FROM REFLECTOR ANTENNA, TYPICALLY 3o  3o OR 3o  6o
ADVANCED: MULTIPLE NARROW BEAMS STATIONARY, SCANNED, OR “HOPPED”

21. ANTENNA TYPES HORN REFLECTOR PHASED ARRAY
Efficient, Low Gain, Wide Beam
REFLECTOR
High Gain, Narrow Beam, May have to be deployed in space
PHASED ARRAY
Complex
Electronically steered