On the Problem of Jitter in Spacecraft

1. On the Problem of Jitter in Spacecraft Richard W. Longman Columbia University, New York City, USA Edwin S. Ahn Air Force Research Laboratory, Albuquerque, NM

2. Columbia is and Important University • First Spiderman movie starts on campus. • Spider gets loose in lab and bites him. • If scientist more careful – no Spiderman movies

3. Columbia Continues to be important (Filming latest Spiderman movie 4/5/2011 outside my bedroom window)

4. Spacecraft Often Have Requirements for High Pointing Accuracy Design spec: Maintain pointing accuracy to 5 milli arc seconds !

5. The Jitter Problem in Spacecraft • Attitude control system actuators • Reaction Wheels • Control Moment Gyros (CMG’s) • Momentum Wheel • Cryo pump • Disturb fine pointing equipment • Slight imbalance produces vibrations • With periods that can be known • Various control methods make use of knowledge of a periodic disturbance to learn to cancel – e.g. Repetitive Control

11. There are other examples of systems with periodic disturbances • Jefferson National Accelerator Facility • Computer Disk Drives • Xerox Copy Machines

12. Examples of Periodic Disturbance Problems • Jefferson National Accelerator Facility, 8 Giga Electron Volt continuous beam accelerator – eliminate 60 Hz and harmonics from beam position

13. Examples (cont.) • Computer disk storage • Improved track following allows closer tracks and more storage

14. Example: Constant Velocity Timing Belt Drive at Xerox

16. Velocity error frequency content before

17. Velocity error frequency content after (too good)

18. Another Application: Laser Communication Relay Satellite

19. Laser Communication Relay (LCR) • Laser Communication (LaserCom) VS Radio Frequency (RF) Communication • 1) RF Communication: • High power is required for sending signal: Mars Reconnaissance Orbiter (MRO) requires 100 W for transmitting about 6 Mbps • Small accuracy required in pointing • 2) LaserCom: • Less power required (< 1W) within space • Data transmission rate up to 100 Mbps for HD video • Large accuracy required in pointing

20. Challenge in LCR • Precision pointing is required for interplanetary laser communication link • Acquisition, Tracking, and Pointing (ATP) needed • Optical jitter induced by vibration in spacecraft hinders pointing accuracy

21. News Alert • Lunar Laser Communication Demonstration Breaks Records. • The Los Angeles Times (10/23, Hubbard) “Science Now” blog continues coverage of NASA’s groundbreaking Lunar Laser Communication Demonstration aboard the LADEE spacecraft, as it just accomplished a “record-shattering” data download rate of 622 megabits per second. • In 2017, the Laser Communications Relay Demonstration “is going to test the ability to relay data from one ground station at White Sands NM to another at NASA JPL through a laser communications terminal in geostationary orbit.” • The International Business Times (10/24, Poladian) reports the ISS will also test its own laser communication system with the Optical Payload for Lasercomm Science (OPALS), which will send “video data” to the Jet Propulsion Laboratory.

22. Hardware Setup – Spacecraft #1 • Three-axis simulator (TAS2) testebed: Representing spacecraft #1 Mars orbiter Optical setup on spacecraft platform TAS2 testbed

23. Hardware Setup • Hardware representation of Mars communication tower and Earth orbiter – Spacecraft #2 Communication tower: Generates main transmitting beam • Spacecraft #2: • Records target error • Transmits laser beacon beam Source and Target Located 29 ft from TAS2

24. Hardware Setup LCD SCREEN WITH STAR IMAGE • Communication link Star Tracker Camera PSD SOURCE FSM WIRELESS ROUTER CMG CMG TARGET PSD Laser communication link setup

25. Optical Jitter Characteristics • Measuring error from the center of the onboard PSD : Voltage output readings • Primary peak within frequency spectrum (35.5 Hz) is correlated to rotor speed of CMG

26. Optical Jitter Characteristics • Modal surveying analysis Identified structural modes are compared (Top plot) with averaged DFT magnitude data (Bottom plot) in order to identify reoccurring modes within optical jitter

27. Optical Scheme for Jitter Correction • Source path jitter correction loop (3-12) corrects jitter within beam at 13

28. Jitter Control Algorithms • Multiple-Period Repetitive Control (MPRC) • Repetitive Control (RC) : A control method that utilizes past data history specifically for periodic signals. Ex: Tracking periodic reference or rejecting periodic disturbances

29. Jitter Control Algorithms • Multiple-Period Repetitive Control (MPRC) : Repetitive controller for addressing period : FIR filter RC compensator : FIR zero-phase low-pass filter : Plant : Model of plant multiplied by RC compensator

30. Jitter Control Algorithms • Filtered X-LMS Algorithm (XLMS) – feedback method • Stochastic gradient approach - Cost function: J = “mean-square-error” Cross-correlation vector: Auto-correlation matrix:

31. Target Track Loop • The effectiveness of adding the target track loop • ABM (source) and XLMS (target)

32. Conclusions • Many control systems are subject to periodic disturbances • Often the period of these disturbances can be known, e.g. base on the inputs of three phase motors • Control systems can be made smarter, to make use of knowledge of the period of the disturbance • In theory, such control laws can converge to zero tracking error • A challenging application is to Laser Communication. A series of methods are tested, evaluated • Each method has its own advantages and disadvantages, and can be preferred on some class of problems