Attitude Control System

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Section 19 Attitude Control System
. . . Paul Sanneman EO-1 ACS Lead Systems
Engineer, Swales Aerospace, Inc.
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Contents

• Attitude Control Subsystem Block Diagram
• Changes since March 2000 Red Team Review
• Thermal/Vacuum II Evaluation
• Open Problem Reports
• Special Topics
• IRU Removal and Reinstallation
• On-orbit Testing of Safe Hold Mode
• Autonomous Star Tracker Issues (including IMAGE
  AST issue)

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ACS Architecture Block Diagram
4
As Run Verification Matrix
5
Projected Power On Time
6
Changes Since Red Team Review

• ACS Components
• AST ACDS DC/DC Converter lid stiffeners (see
  Special Topic)
• Removed IRU to prevent helium exposure (see
  Special Topic)
• ACE Software
• ACE RSN S/W update to Version 3.11 on 8-JUN-00
• PTR0927 Generic Code modified to have IRAM2
  checksum protection on had been missed since ACE
  is the only RSN with the extended RAM capability
• PTR0939 Prevent ACE 25 Hz Event Messages and use
  telemetry error counters instead
• Code changes tested on 8-JUN-00 416 failure
  free hours
• M5 ACS Software
• Minor parameter changes via M5 Table loads to RAM
• Related to improving operation issues - no S/C
  safety issues

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Summary of Open Items fromMarch Red Team Review
(1 of 2)

• Verification Matrix, no update necessary (page
  18-14)
• Updated operating hours included on Slide 7-5
  (page 18-15)
• PR 674-20-41 AST number of stars issue (page
  18-19)
• Confirmed this is a star field simulator issue
  (see Slide 19-9)
• GPS tensor warm start ground procedures (page
  18-20)
• Fully tested and available in MOC ASIST
• No updates to SHM from final CPT (page 18-34)
• Updates to remaining work
• AST/IRU alignmentdetailed in Slide 19-10
• Conclusion of AST DC/DC Converter concern (18-40)
• See Special Topic on AST DC/DC Converter

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Summary of Open Items fromMarch Red Team Review
(2 of 2)

• Updated summary of IRU PDV history (page 18-49)
• Details in Special Topic. No outstanding
  performance issues.
• SIRU return to spacecraft (page 18-57)
• Details in Special Topic. Reintegration
  successful. Degradation reversed.
• Open actions from ACS Phasing Review (page 18-63)
• CPT phasing tests following final MV S/W load
  -completed
• CSS phasing following final solar array
  attachment -completed
• Final magnetic phasing completed on 28-JUL-00 -
  no issues

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Thermal Vacuum II Test Results

• Attitude Control Subsystem performance validated
  as part of CPT segments and via other special
  tests
• All CPT associated tests run at hot and cold
  plateaus
• RWA Overspeed protection verified at hot and cold
  plateaus
• AST Thermal Electric Cooler operation modes
  verified
• AST TEC default state and Effective Focal Length
  changed/restored during T/V II as demonstration
  of planned on-orbit maintenance procedures
• IRU internal heater control of gyro platform
  validated

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Reaction Wheel Assemblies

• RWA performance has been verified in both air and
  vacuum
• Control torques issued by override and
  closed-loop software
• Overall run-up and coast down trends have been
  verified with vendor and are summarized on next
  page
• T/V II testing of overspeed protection provided
  below with wheel speeds in RPM

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RWA Run-up / Coast Down Trend Table
Trending data is consistent over time. Estimated
drag torque very consistent and near 50 of spec
level
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Critical PR Status

• PR-468-20-4 IRU Parametric Drive Voltage
• See Special Topic
• PR-674-20-41 AST Number of Stars 18
• Issue and problem report have been closed
• Similar operation noted on the MAP ASTs during
  Thermal/Vac testing at the component level
• Investigation of MAP occurrence with AST-GSE
  confirmed this is a Star Field Stimulator GSE
  issue
• Contamination on lens of GSE can cause double
  reflection that causes a match with a very dim
  star in the AST internal star catalog

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ACS Flight Readiness

• ACS components and hardware are ready
• ACS and ACE software are ready
• ACE Safehold Mode is ready
• Ops preparations/training are ready
• Remaining Work
• No outstanding problems or issues
• AST/IRU alignment measurements at launch site
• Red Book Item IRU sensitivity to helium exposure
• Under control via upgraded purge system on S/C
  and at launch site

Attitude Control Subsystem is READY
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Special Topic Star Tracker Issues
. . . Wayne Roher Litton Advanced Systems
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AST Interpoint DC/DC Converters Failure Mode

• MAP experienced failure with 15V DC/DC
  converter
• MAP2 - Open failure during ATP at ambient after
  Thermal/Vacuum
• DPA indicate inadequate stress relief on internal
  bond wire, insufficient solder, and solid
  adhesive attachment to lid caused bond wire to
  pull away from pad in vacuum.
• EO-1 AST X-ray investigation
• The two DC/DC converters on the EO-1 AST were
  X-rayed and found to have met the criteria
  determined by Code 562 as having sufficient
  relief distance from the transformer.
• Code 562 speculated that an additional failure
  mode exists
• Oil canning of converter package could cause
  adhesive on bottom of transformer to release.
  Transformer would move upward with package lid,
  causing transformer bond wire to pull away even
  if adequate stress relief existed for normal
  operation in a vacuum.

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AST Interpoint DC/DC ConvertersRework

• Lockheed Martin Palo Alto and Lockheed Martin
  Mission Success agreed that this failure mode
  potential justified corrective action.
• Stiffening the lid to prevent oil canning was
  determined to be the best solution to address all
  concerns of stress, unattached adhesives, and
  vibration.
• Lid stiffener of 20 mil Kovar and minimum 5 mil
  bond line of epoxy was added to both D/CDC
  converters. (See Converter Lid Stiffening
  Special Topic)
• Both converters tested to deflect less that 1 mil
  in vacuum

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AST Interpoint DC/DC ConverterRework Conclusion

• Retest
• AST workmanship vibration
• AST Functional on Spacecraft
• AST CPT on Spacecraft
• Spacecraft level T/V
• 272 hours on time since lid stiffener was added
• Conclusion
• EO-1 AST has successfully completed all testing
  post lid stiffening
• EO-1 AST is ready for flight

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Other Subsystem DC/DC Converter Rework and
Re-Qualification

• All MTRxxD type Interpoint DC/DC Converters were
  considered as potentially defective due to the
  failure mode detected on the MAP AST2
• Two other Subsystems contained Converters that
  required rework
• GPS PCU (1)
• ACDS box (2) CDH LVPC and ACE I/O cards

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GPS PCU DC/DC Converter

• Rework and Re -Qualification process of GPS PCU
• Removed from Spacecraft
• Added stiffener lids, 40 mil Kovar, 10 mil
  minimum bond line
• Vacuum test, deflection less than I mil
• One Axis workmanship vibration
• Functional on Spacecraft
• CPT on Spacecraft
• Spacecraft level Thermal Vacuum
• Conclusion
• No anomalies recorded
• GPS PCU is ready for flight

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ACDS DC/DC Converters

• Rework and Re -Qualification process of ACDS
• Box removed from Spacecraft, CDH LVPC and ACE
  I/O boards removed
• Added stiffener lids, 40 mil Kovar, 10 mil
  minimum bond line
• Vacuum test, deflection less than I mil
• Board level functional tests
• Boards reassembled in ACDS chassis
• Pre-vibe box level Functional Test run
• One Axis workmanship vibration
• Post-vibe box level Functional Test run
• Spacecraft Integration tests
• CPT on Spacecraft
• Spacecraft level Thermal Vacuum
• Conclusion
• No unexplained anomalies recorded
• ACDS is ready for flight

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Image AST Anomaly

• The IMAGE AST was unable to acquire between June
  9 and August August 3
• The AST remained in standby mode during this
  time power was not recycled
• On August 3, the AST re-acquired
• 1.4 degree angle further from the sun than on
  June 9
• Dropped out twice during that day, quickly
  recovered
• Has not dropped out since.

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Image AST Anomaly Investigation

• A Failure Review Board (FRB) was held to
  determine potential causes of the AST acquisition
  failure.
• Stray Light is the most probable root cause
• Housekeeping nominal (voltages, temperatures)
• Increased elevated background and reduction in
  guide star identification were observed during
  and leading up to the failure to acquire after a
  loss of track.
• Periodic trending occurred nearest the sun loss
  of track and re-acquisition. Symptoms got worse
  as sun angle decreased.
• Symptoms improved and re-acquisition was attained
  during eclipse portion of the the orbit.
• No increase in brightness or position error was
  found.
• All observation are symptomatic of stray light
  effects.

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Image AST Anomaly Investigation

• Straylight analysis indicates the AST should have
  been able operate under the current mission
  attitude.
• The IMAGE AST uses a lightshade (or baffle)
  designed to keep the sun or Earth out of the
  boresight (within 12.5 degrees) with no
  redirected light within 15.1 degrees of the
  boresight.
• Image minimum bore sight axis to the sun in July
  at loss of track was 22 -42 degrees (AST is 10
  degrees off spin axis)
• Most likely sources of Stray light are
• Contamination
• Glint from 10 Meter Boom
• Contributions from CCD Mask

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Differences Between Image EO-1

• Sun Angle
• Image AST approaches within 4 degrees of the Sun
• Lost track at 22-42 degrees
• EO-1 AST approaches within 70 degrees of the Sun
  for nominal nadir pointing operations
• Baffle differences
• Image
• 16 inches
• 12.5 degrees of protection from sun
• EO-1
• 12 inches
• 30 degrees of protection from sun

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Results of Image AST Anomaly and Investigation

• Lockheed believes the anomaly occurred due to
  unknown form of contamination
• Evidence indicates the AST did not experience a
  functional failure of the electronics
• Data indicates the AST is functioning nominally
  and the data is good
• Lockheeds preliminary conclusion of the anomaly
  event and investigation analysis is that this
  event is not a systemic failure of the ASTs and
  does not impact the expected reliability of the
  ASTs built for other missions.
• Formal conclusion of the FRB is expected late
  August
• Lockheed believes EO-1s AST is ready for flight.

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What Happens If the AST Fails?

• AST has been extensively tested on S/C and
  performance has been nominal. All issues have
  been aggressively pursued.
• Demonstrated reliability of product line is less
  than desirable.
• Error prone manufacturing and test efforts
• Similar AST on DSI failed 15 months into mission
• Flight dynamics ground based attitude
  determination could achieve 1 to 5 degree
  pointing accuracy
• Would allow Landsat 7 limited image comparison
  since we would be within its 15 degree swath
  width
• Ground attitude estimate developed using TAM and
  gyro data
• Current flight ACS S/W has capability to use
  daily uploaded attitude estimate, propagated
  using gyro data
• Would require some ground software to prepare and
  uplink daily attitude data would be developed if
  AST fails. Sun sensor safehold used until
  software developed.

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Special Topic IRU Removal and Re-installation
. . . Paul Sanneman EO-1 ACS Lead Systems
Engineer, Swales Aerospace, Inc.
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SIRU Nitrogen Purge - Background

• Litton Guidance Control Space Inertial
  Reference Unit
• Utilizes three Hemispherical Resonator Gyros
  (HRGs)
• Quartz crystal hemisphere oscillated at specific
  amplitude and frequency
• Input angular rate measured by nodal displacement
  on rim
• Quartz crystal becomes permeated with Helium
• Increases gaseous damping, requires larger drive
  voltage - Parametric Drive Voltage (defined as a
  negative voltage)
• Not an issue prior to arrival at GSFC Building 7
• Significant amounts of Helium released in
  Building 7 by environmental chamber testing
  processes

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Summary of PDV History
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Gyro A PDV Slope Estimate
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PDV Trends Since S/C Thermal/Vac
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SIRU Return To Spacecraft

• Re-integration performed 11-MAY-00
• Building 7 air sampling validated low helium
  concentration
• Building 7 air under constant He monitoring using
  leak detector
• Continue to have close coordination with Building
  7 operations personnel to further minimize
  exposure
• Re-integration included new aluminum-wrapped
  Teflon tube inside spacecraft and stainless steel
  tube from S/C to tank
• Gyro Parametric Drive Voltages indicate that both
  the storage at Swales and the upgraded purge
  system have reversed the degradation

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Outlook

• Mechanical and launch vehicle I/F engineers have
  focused activity for IRU protection throughout
  remaining S/C IT at GSFC, during transport and
  launch site processing
• The increased PDV is NOT a performance issue -
  just needs to stay within (-5V) capability
• About one volt of margin required for full 10
  deg/sec rate capability
• Desire PDV values of 3.5 V (_at_25C) at time of SC
  mount on LV
• PDV Voltages on 28-JULY-00 A-2.66, B-2.23, C
  -2.50 _at_23C
• Purge installation on Launch Vehicle (L/V) has
  been improved to the extent allowable by the L/V
  vendor/operator
• Expect Parametric Drive Voltage to remain stable
  or slight improvement through launch and will
  slowly recover over the course of the mission -
  residual risk has been minimized

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Special Topic On-Orbit Testing of Safe Hold Mode
. . . Paul Sanneman EO-1 ACS Lead Systems
Engineer, Swales Aerospace, Inc.
35
RFA 33.33

• Reviewer Ann Merwarth
• Specific Request
• Hold a technical peer review with operations
  experts from outside the project to identify the
  pros and cons of conducting the safemode test on
  orbit. Review the results with Center and Program
  management and ensure that all parties "buy into"
  the plan for this test. If the Project decides to
  do this test, we recommend that it be delayed as
  long as possible.
• EO-1 Response Peer review held on 20 APR 2000
• Chaired by Eric Holmes, GSFC Code 571 GNC
  Systems Engineering
• Board members Alan Reth, Sam Placanica, Landis
  Markley

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Summary of Peer Review

• Review Agenda
• Review Team Introductions and Purpose
• Safe Hold Mode Implementation
• Safe Hold Mode Reviews
• Safe Hold Mode Testing during S/C IT
• On-orbit SHM Test Plans for Two tests
• Benefit/Risk Summary for On-orbit SHM Test
• Review Conclusion
• The review panel unanimously agreed with the plan
  to proceed with the planned on-orbit test of SHM
  with the following actions to Swales Aerospace

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Peer Review Action Items

• Action 1 (by Sam Placanica)
• Verify that all instruments can survive dwelling
  on the Sun for at least 10 minutes.
• ACS, Thermal and Systems investigated and
  determined that there is enough attitude
  variation following SHM entry to prevent
  instrument thermal issues
• Action 2 (by Sam Placanica)
• The duration of the SHM shall be at least 1
  complete orbit.
• This is now included in the launch early orbit
  mission plan.
• Action 3 (by review panel)
• Augment test 1 as follows Have the solar array
  outside of the 5 degree "indexed" tolerance at
  the time SHM is initiated, so that SHM's indexing
  function via ECU2 and Pot 2 are demonstrated.
• STOL procedure ACS_SAD_10 developed and tested to
  perform this function prior to commanded SHM
  entry
• Second SHM test removed from mission plan as
  there was limited benefit and timing would be
  obtrusive to instrument activation/checkout

All action items closed. RFA 33.33 closed.