1. Recovering NEOSSat following major attitude control system failures
Feb 14, 2018
Satellite Operations and Infrastructure

2. Acknowledgments

3. Near Earth Objects Surveillance Satellite
NEOSSat
NESS (CSA)
HEOSS (DND-DRDC)
Near Earth Space Surveillance  NESS
High Earth Orbit Space Surveillance  HEOSS
Launched 25 February 2013
PSLV C-20

4. NEOSSAT Missions HEOSS (High Earth Orbit Space Surveillance):
Demonstrate military utility of microsatellite platform
Gather metric observations of resident space objects in support of space situational awareness research & development
NESS (Near-Earth Space Surveillance):
Discovery, tracking and study of asteroids, especially interior to Earth orbit, comets and other astronomical objects

5. NEOSSat Architecture Architecture: Team: 730 km Sun-Synchronous Orbit
75kg micro-satellite based on MOST space telescope
15-cm aperture Maksutov telescope, with 0.8⁰ field-of-view
Sensors: 1024 x 1024 E2V CCDs (star tracker and science)
3-axis stabilization w/ pointing accuracy ~2 arcseconds
Coarse Sun Sensor (CSS)
Magnetometer (MAG)
Custom Star Tracker along payload boresight
3 rate sensors
4 reaction wheels
3 torque rods for momentum dumping
2 GPS receivers for orbit determination
Team:
Funded by CSA and DRDC
Built by Microsat Systems Canada Inc (MSCI)
Operated by CSA and SED in multi-mission control centre

6. NEOSSAT Modes Key Operational Modes: Support Modes:
“Fine Point”: aka “Star Stare” mode
characterize satellite/asteroid motion through streaks in images
good for imaging astronomical targets
“Fine Slew”: aka “Track Rate” mode
track moving space objects by matching their speed (stars appear as streaks)
Support Modes:
“Coarse Point” with CSS and MAG to provide initial pointing for star tracker acquisition
enables transition to “fine” modes
“Rate Slew” to quickly move target-to-target

7. Activities pre-failure
NESS (University of Calgary)
Delivered astrometry of known NEOs to Minor Planet Centre (MPC)
Obtained observatory code (C53)
Comet observations at ~34⁰ solar elongation during eclipse period
Other experiments/studies
Limiting magnitude ~19 in Fine Point for 100s exposures, after post-processing
HEOSS (DRDC)
Routine tasking of resident space objects
Metric observations demonstrating residuals within 3” spec

8. Magnetometer Failure Failure Signature Impact Severity: Critical
Magnetometer became very noisy and unreliable
Impact
Satellite could not achieve/maintain Coarse Point, let alone Fine states
All mission activities suspended due to lack of satellite controllability
Severity: Critical
Investigation:
Attempts to recalibrate / reset magnetometer and/or control parameters were unsuccessful
Ultimate conclusion was that failure was permanent
Recommendation:
Find an alternate coarse attitude determination strategy for NEOSSat

9. Health and Safety Satellite Status Maintaining thermal profile
Unable to perform closed-loop control, NEOSSat was tumbling in space
Ability to communicate with satellite nominal (two TTC receivers)
Command and telemetry capabilities fully operational
Shutter closed to prevent damage to payload CCD
Battery temperatures are key health parameters monitored in telemetry
Maintaining thermal profile
When tumble is such that +Z face is constantly seeing the Sun, battery temperatures rise above safety limits
In that situation, operators would command reaction wheels directly to “flip” the satellite to create a more favorable tumble
Strategy successfully maintained health and safety as software patches were planned for a return to nominal operations

10. Software Update #1: “Sun Point” mode
With no realistic option for hardware on-orbit servicing, software updates were the only option to recover flight operations
OPS-8 Flight Software
New “Sun Point” control mode, to control the Sun vector in body frame
Satellite still free to rotate around Sun vector (not fully constrained)
Rate sensors used to minimize satellite rotation / tumbling
Augmented GPS logs, providing details on contributing GPS space vehicles
Satellite Status
“Sun Point” parking allowed thermal control without operator intervention
Allowed opening of shutter to take image exposures and get attitude truth
Post-processed attitude truth and new GPS logs allowed evaluation of new strategies to recover coarse attitude determination with available sensors

11. Software Update #2: GPS Attitude Sensor
Team evaluated various strategies to replace failed magnetometer
Strategies attempted to get more information from existing equipment, such as rate sensors, sun sensors, star tracker, GPS, torque rods, etc.
Ultimately, GPS-based attitude sensor was selected for implementation
Proposed by Magellan Aerospace, based on Alexrad et al. literature
Augmented GPS logs and Flatsat facility allowed offline validation of strategy
Ref: “Single GPS Antenna Attitude Vector Pair - NEOSSat Recovery”, Eagleson et al.
OPS-9 Flight Software
MSCI incorporates GPS attitude sensor into attitude control flight software
Satellite Status
After improvements to the coarse sun sensor and a few other software patches, CSS + GPS produced sufficient accuracy to re-achieve “Fine Point”

12. But before Fine Point was achieved…

13. Failure #2: Torque Rod Controller
Failure Signature
All commands to torque rod controller would time-out
Impact
Unable to command torque rods for perform desaturation
Increase in overall satellite momentum during “Sun Point” control
Eventual wheel saturation
Severity: Critical
Investigation:
Attempts to power cycle or rewrite controller were unsuccessful
Ultimate conclusion was that failure was permanent
Recommendation:
Find a new desaturation strategy for NEOSSat

14. Health and Safety Satellite Status Maintaining thermal profile
Closed-loop control could only be performed for short intervals before wheel saturation would occur
GPS attitude sensor not yet available, so “Sun Point” was still highest operational mode achievable
However, it could not be used constantly due to momentum build-up while counteracting disturbance torques, leading to eventual wheel saturation
Command and telemetry capabilities fully operational
Maintaining thermal profile
“Sun Point” used intermittently for thermal control; tumbling otherwise
Momentum build-up would be gradually dissipated during tumbling periods
In low-momentum situations, “Sun Point” to maintain battery temperatures
In high-momentum situations, “Flip” was used instead

15. Software Update #3: DESAT
Team evaluated strategies to recover satellite controllability in parallel to implementing the GPS attitude sensor for attitude determination
Reaction wheels could be used for slews as long as momentum was available
Conservation of momentum: wheels can only transfer momentum, not dump
Ultimately, the satellite’s own residual dipole was key to the solution
Earlier, body-fixed residual dipole had been discovered as a disturbance force
Selected concept: new control mode orienting the satellite to optimize the desaturation effect of the residual dipole  never before proposed or demonstrated
Ref: “A Novel Reaction Wheel Desaturation Method Using Satellite Residual Dipole”, Sekhavat et al.
OPS-10 Flight Software
MSCI successfully implements “DESAT” mode
Enables return to routine “Fine” operations, with desaturation between tasks
OPS-10 (flight)
Momentum dumping

16. NEOSSat Activities Post-Recovery
HEOSS Satellite Tracking (DRDC)
Failed GEOs
Maneuvering
Special events
Close Approach
SSA Experiments …
HEOSS Target: Mar 31

17. NEOSSat Activities Post-Recovery
Photometry Investigations
Tabby’s Star
Unexplained brightness dimming events, first discovered by Kepler space telescope in Sep 2015
Beta Pictoris / Hill’s sphere transit
Proposed by Bishop’s/J. Rowe, timely opportunity to observe exoplanet Hill’s sphere influence using photometry
KIC

18. NEOSSat Activities Post-Recovery
Astronomy Images
Trifid Nebula: Jun 24, 2017
Lagoon Nebula: Jun 30, 2017
Eagle Nebula: July 14, 2017

19. NEOSSat Activities Post-Recovery
Near-Earth Objects
Asteroid 2012 TC4
Oct 11, 2017
Asteroid 1989VB, July 29, 2017
Asteroid 2012 TC4, Oct 12, 2017

20. 2012 TC4 Close Approach Observation Timeline
NASA-led global observation campaign, to practice strategies in the event of a real asteroid collision threat + characterize asteroid’s orbit for future passes
Note: all dates are year 2017
2012 TC4 Direction of Motion
Oct-11 05:46
First NEOSSat Detection
Oct-12 03:00
TC4 peak brightness
Lunar Orbit
2017-Oct-12 04:54
Last confirmed NEOSSat observation
Oct-12 05:41
TC4 closest approach (~44,000 km altitude)
Oct-12 06:18
-NEOSSat attempts observations on following orbit
-Sun safety shutter closes
-No observations after this time
2012-TC4 on sunward of Earth. Undetectable to NEOSSat
NEOSSat can track at spefific event times (Sapphire can’t).
Above shows Skynet 5A inserting itself next to NSS-6, SES-12.
Notice the short positional shift of the objects between the two images relative to NSS-6 and SES-12.
Sun Direction

21. Conclusions Recovery Effort Outcomes: Mission Outcomes:
Innovative software techniques to save an operational mission
Improved resiliency for space assets
New applications for GPS
New desaturation technique
Mission Outcomes:
Demonstrated utility and potential for microsatellite platforms
NEOSSat contributing to space situational awareness for Canada
New space-based science applications

22. Thank you!