Launching, Orbital Effects & Satellite Subsystems Joe Montana IT 488 - Fall 2003

Agenda Satellite Subsystems

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

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

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

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

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

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.)

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

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

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

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.

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

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

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.

POWER SYSTEMS - 1 SOLAR CELLS 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 - 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

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)

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.

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”

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