Transcript OMNI Mission - Center for Astrophysics and Space Astronomy

Josephine San
Dave Olney
18 August, 1999
 Appears to be Feasible
 Requirements
 Coarse Pointing baselined on NGST
 Future technology
 Control modes
 Area of concerns
15 July 1999
NASA/GSFC/IMDC
2
 Pointing Requirements on Optical Spacecraft ACS

Accuracy (1 s) :
 Course (with tracker) Pitch/Yaw ± 2.0 arcsec; Roll 20 arcsec
 Fine (long term drift) pitch/yaw ± 0.5 marcsec

Jitter
 Coarse
 Fine
± 0.1 arcsec
± 0.1 marcsec
 Pointing Requirements on Detector Spacecraft ACS


Stability (1 s) :
Lateral Stability (1 s) :
15 July 1999
Pitch/Yaw ± 20 arcsec; Roll 20 arcsec
3 mm
NASA/GSFC/IMDC
3
 Rate null/Sun Acquisition: null rates, point arrays normal to sun


Sensors : Coarse Sun Sensor, Gyro
Actuator: options: large wheel/small wheel + good PAF/thruster
 Acquisition: acquire stars to establish reference attitude


Sensors: HD301 Star tracker(ST), Fine Sun Sensor (FSS), gyro (ring laser)
Actuator: same as rate null/sun acquisition
 Science: inertial pointing


Sensors: coarse pointing - ST, FSS, gyro; Fine pointing - instrument
Actuator: options:/large wheel with thruster/PPT/small wheel with PPT
 Slew: eigenaxis rotation capability


Sensor: gyro
Actuator:
options: wheel/PPT
 Safehold: independent safe mode, same as sun acquisition
15 July 1999
NASA/GSFC/IMDC
4
Detector ACS mode scenarios
 Rate null/Sun Acquisition: null rates, point arrays normal to sun

Sensor same as Optics spacecraft; actuator can be wheel or thruster
 Initial Acquisition: acquire stars to establish reference attitude

Sensors same ad Optics spacecraft’s acquisition; actuator same as sun acquisiton
 Science: inertial pointing


Pointing: same as optics spacecraft coarse pointing
Lateral control: laser and PPT
 Slew: Acquire optics spacecraft, acquire new target


Sensor: gyro
Actuator: wheel or thruster
 Delta V - Re-acquire new target position
 Safehold: independent safe mode, same as sun acquisition
15 July 1999
NASA/GSFC/IMDC
5
Actuator Selection Criteria
 Quantization - Science requirement
 Disturbance torque - Science requirement
 Torque Capability - slew and solar torque
 Momentum capability
Tip off rate
 Solar pressure at drift orbit

 Solar force is about 0.2 m N
 Assuming 0.1 meter cp offset for optics spacecraft, Solar torque is 20 micro Nm
 In one day the momentum build up is about 1.8 Nms
15 July 1999
NASA/GSFC/IMDC
6
Optics - sun acq/rate null
 Large wheel (80 Nms)
Tip off rate less than 0.05 deg/sec
 Imbalance torque disturbance
 Weight and Power

 Small wheel (40 Nms)+ PAF
Assuming with good PAF, tip off rate less than 0.01 deg/sec
 40 Nms wheel
 Weight and Power

 Thruster
PPT is not sufficient to null the rate (0.01 deg/sec) and acquire the sun
 hydrazine - Sloshing problem
 Cold gas - only choice

15 July 1999
NASA/GSFC/IMDC
7
Optics - SCIENCE
 PPT only









Better quantization*
No need to unload momentum*
No heritage yet, EO1 will have
one axis PPT control as test
Need 12 PPT with no redundancy*
Mass, power, cost
Limited number (10 million) of
firing (fire every 3 s for 1 year)
Plume impingement
Electro-magnetic contamination
Need to further investigate
items without ‘*’
15 July 1999
 Wheel with isolation;
thruster for momentum
unloading








NASA/GSFC/IMDC
Quantization
Need to unload momentum*
Has heritage*
Longer life time*
With four wheel provides
redundancy*
Mass, power, cost
Imbalance torque disturbance
Same as PPT last item
8
Optics - Science (con’t)
 Small wheel with isolation /PPT

Wheel for pointing, PPT for momentum unloading
 Finer quantization of wheel
 Extend PPT life time

Wheel for coarse pointing, PPT for fine pointing and momentum
unloading
 Better quantization for fine pointing
 Extend PPT lifetime

Depend on the actuator induced disturbance and other studies
15 July 1999
NASA/GSFC/IMDC
9
Optics - Slew
 Wheel
Less than 6 hours to slew 45 degree
 Remain a zero-momentum system

 PPT


12 hours to slew 45 degree
After the slew, the system momentum may not be zero
15 July 1999
NASA/GSFC/IMDC
10
Technology
 New Generation Integrated Wheel
Wheel and electronic all integrated
 Low noise, low imbalance torque, low power
 Spartan 400 series and Triana heritage

 New Generation Star Tracker
NGST heritage
 Accuracy 1.35 arcsec accuracy per star

 Pulse Plasma Thruster

As three axis fine control actuator
15 July 1999
NASA/GSFC/IMDC
11
 PPT concerns
Plume impingement, EMI, Life time
 Maintain a zero momentum system

 A better solar torque estimation
 Tracking strategy of optics and detector
spacecraft
 Fine pointing Strategies
 Operation scenarios post separation
Null rate before separate two spacecraft
 Rate after two spacecraft separation
 What is the rate after two spacecraft separation

15 July 1999
NASA/GSFC/IMDC
12
Component (wheel option)
Power
Power Power
Cost
Mass
Orbit
Peak Standb
Qty
($K)
(Kg)
Avg (W)
(W)
y (W)
Component
Model
Coarse Sun Sensors
Adcole 11866
8
48
0.037
0
0
0
Digital Sun Sensors
Adcole 17061
2
400
0.644
0.13
1.4
0
2
2000
16
22
26
22
Inertial Reference Unit Litton SIRU (4 axis)1
1500
5.44
22
40
0
Reaction Wheels
Integrated Wheel 4
1200
64
72
320
40
12
3600
108
2160
2160
2
1200
7.264
18
18
Attitude Control Electronics
(MAP ACE)
Pulse Plasma Thruster
Star Trackers
HDOS HD-301
Totals = $9,948 201.4
15 July 1999
NASA/GSFC/IMDC
2294.13 2565.4
0
62
13