1. 20a - 1 NASA’s Goddard Space Flight Center Attitude Control System (ACS) Eric Holmes, Code 591 Joe Garrick, Code 595 Jim Simpson, Code 596 NASA/GSFC August 16-17, 2005

2. 20a - 2 NASA’s Goddard Space Flight Center Lunar Reconnaissance Orbiter (LRO) ACS Subsystem Document Tree Level 2 431-RQMT-000004LRO Mission Requirements Document 431-PLAN-000131Lunar Reconnaissance Orbiter Spacecraft Performance Assurance Implementation Plan 431-OPS-000042Lunar Reconnaissance Orbiter Concept and Operations Document 431-SPEC-000012LRO Mechanical Systems Specification 431-SPEC-000091Lunar Reconnaissance Orbiter General Thermal Subsystem Specification 431-ICD-000018LRO Power Subsystem Electronics Electrical Interface Control Document Level 3 431-SPEC-000162 Lunar Reconnaissance Orbiter Guidance, Navigation and Control Attitude Control System Specifications 431-SPEC-000063Flight Dynamics Specification 431-RQMT-000113LRO Pointing and Alignment Specifciation 431-PROP-000017LRO Propulsion Subsystem SOW and Specification 431-ICD-000008Lunar Reconnaissance Orbiter Electrical Systems Interface Control Document 431-RQMT-000045Lunar Reconnaissance Orbiter Radiation Requirements 431-RQMT-000092Lunar Reconnaissance Orbiter Thermal Math Model Requirements

3. 20a - 3 NASA’s Goddard Space Flight Center This list reflects the ACS hardware complement needed to meet requirements and with selective redundancy ensure minimal risk The LRO ACS will utilize the hardware needed to implement the general and derived requirements stated in this ACS Specifications document. - IRU shall provide 3-axis angular rate and delta angle information to the ACS. - 8 CSS total to allow for as much spherical coverage as possible and for redundancy. - 2 STs to ensure the ACS onboard knowledge is maintained during occultations. - 4 reaction wheels for momentum storage and attitude control. - 8, 5 lbf (TBD) for attitude/orbit/momentum unloading - 2, 20 lbf (TBD) for orbit insertion/maintenance - Engine Valve Driver (EVD) electronics for thruster control - The ACS will interface with other spacecraft hardware electronics ACS-3.2.12.1 ACS-3.2.13.7 ACS-3.2.14.2 ACS-3.2.15.1 ACS-3.2.16.1 MRD-084 MRD-085 MRD-087 Control modes reflect pointing and functional requirements for the different phases of the mission The LRO ACS will implement the necessary control modes to meet all pointing requirements during all phases of the mission. - Sun Acquisition: coarse sun pointing for anomalous conditions - Observing: science taking mode - Delta-H: momentum unloading - Delta-V: orbit insertion, orbit adjusts and station keeping maneuvers. ACS-3.2.4.1 ACS-3.2.5.1 ACS-3.2.6.1 ACS-3.2.7.1 MRD-088 MRD-089 MRD-091 ACS needs to ensure proper pointing throughout lifetime LRO ACS will be responsible for maintaining the proper spacecraft attitude throughout the mission lifetime MRD-084 RequirementParagraph Concept/ComplianceLevel 3: RequirementsLevel 2 Req. Lunar Reconnaissance Orbiter (LRO) ACS Subsystem Level 2 Flow Down Key Requirements

4. 20a - 4 NASA’s Goddard Space Flight Center Accuracies needed to maintain velocity vector that will meet orbit maintenance requirements during burns The LRO ACS shall meet the pointing, knowledge and stability requirements associated with all thruster modes. Knowledge (Resolution) ± 5.0 deg, per axis, (3  ) Accuracy (Control) ± 0.1 deg, per axis, (3  ) Stability N/A ACS-3.1.5.2 MRD-049 MRD-087 MRD-092 Represents needed ACS and Mechanical allocations for pointing accuracies The LRO ACS shall meet the pointing, knowledge and stability requirements at the instrument interface associated with the Observing Mode. Knowledge (Resolution) ± 60.0 arcsec, per axis, (3s) Accuracy (Control) ± 30.0 arcsec, per axis, (3s) Stability 5 arcsec, per axis, over 1 msec. 10 arcsec, per axis, over 100 msec. 20 arcsec, per axis, over 4 sec. ACS-3.1.4.3MRD-049 The LRO ACS will compute and provide HGA and Solar Array gimbal commands. -The ACS shall ensure commanding of the HGA to an accuracy of TBD degrees. -The ACS shall be responsible for commanding the Solar Array to an index position -The ACS shall ensure that the Solar Array tracks the sun to an accuracy of 5 degrees. ACS-3.1.13.3 ACS-3.1.14.4 ACS-3.2.17.1 ACS-3.2.18.1 MRD-015 MRD-016 MRD-049 MRD-084 RequirementParagraph Concept/ComplianceLevel 3: RequirementsLevel 2 Req. Lunar Reconnaissance Orbiter (LRO) ACS Subsystem Level 2 Flow Down Key Requirements ACS will determine and track pointing commands and slewing profiles for HGA and Solar Array

5. 20a - 5 NASA’s Goddard Space Flight Center Lunar Reconnaissance Orbiter (LRO) ACS Subsystem Level 2 Flow Down Key Requirements Need to keep frequency of science interruptions to a manageable level The ACS shall implement hardware and software capable of managing momentum for periods of no less than 2 weeks over the life of the mission. ACS-3.1.10.4 MRD-017 Control system design practices that have long been used by GN&C will be employed here All ACS control modes shall meet stability margins in accordance with established GN&C design practices with a single set of gains for the entire range of inertias over the mission lifetime, from Beginning of Life (BOL) to End of Life (EOL). ACS-3.1.3.1 Requirement to keep spacecraft in a power positive and thermally protected pointing The LRO ACS shall meet the pointing, knowledge and stability requirements associated with the Sun Acquisition Mode. Knowledge (Resolution) N/A Accuracy (Control) ± 15 deg from targeted sun line, RSS (3  ) Stability N/A ACS-3.1.7.1 MRD-090 RequirementParagraph Concept/ComplianceLevel 3: RequirementsLevel 2 Req. Prolonged sun exposure can damage science instruments The ACS shall ensure sun avoidance of science instruments to within 30 degrees of nadir (+Z axis ), or shall ensure the sun passes through their boresights with a rate of no less than 0.1 deg/sec. ACS-3.1.12.1 MRD-052

6. 20a - 6 NASA’s Goddard Space Flight Center Lunar Reconnaissance Orbiter (LRO) ACS Subsystem Level 2 Flow Down Key Requirements ACS will monitor status and react appropriately to anomalies The ACS subsystem shall provide an onboard means of failure detection and correction (FDC) for anomalous conditions. ACS-3.2.11.1 Use method that will ensure capability to null residual rates to allow transition to initial pointing. Nulling of tip-off rates and residual despin rates will be initiated autonomously after separation, using thrusters or wheels. Wheels shall be able to handle up to 2.0 deg/sec, per axis. ACS-3.1.10.1 ACS-3.2.3.1 MRD-005 MRD-006 MRD-026 MRD-027 Yaw maneuver to keep sun on warm side of spacecraft and minimize science interruption The LRO ACS shall provide a 180° yaw maneuver during the Observing Mode and shall take no more than 20 minutes. ACS-3.1.15.6 ACS-3.1.15.7 MRD-018 RequirementParagraph Concept/ComplianceLevel 3: RequirementsLevel 2 Req.

7. 20a - 7 NASA’s Goddard Space Flight Center Separation CSS Sun Acquire (with and w/o gyros) Nadir Pointing (Primary Mission) Inertial Pointing Offset Pointing Delta-H Delta-V Thruster Modes Observing Mode Power On / Reset LRO GN&C Nominal Control Mode Flow Command Only Transition Autonomous Transition Autonomous (Post Separation Sequence, Thrusters) No transitions between Thruster Modes Any Observing Sub-Mode can be commanded to any other Observing Sub-Mode = Command-Only Transition = Autonomous (Post Separation) Transition = Autonomous (Nominal) Transition Autonomous (Post Separation Sequence, wheels) All Modes (FDC Corrective Action) Note that autonomous transition paths can also be commanded, but command-only paths cannot occur autonomously Momentum Level?

8. 20a - 8 NASA’s Goddard Space Flight Center LRO Top Level Block Diagram

9. 20a - 9 NASA’s Goddard Space Flight Center Lunar Reconnaissance Orbiter (LRO) ACS Subsystem Preliminary Verification Approach Components –All sensors and actuators, are expected to have been previously qualified –Acceptance testing required of all components (mechanical, thermal, and electrical) –Subsystem –Subsystem verification at the spacecraft level will be performed using a combination of functional and performance tests augmented by analysis and simulation –Dynamic simulator to perform hardware in the loop testing

10. 20a - 10 NASA’s Goddard Space Flight Center Lunar Reconnaissance Orbiter (LRO) ACS Summary LRO ACS driving requirements are defined Pointing performance and ACS functional requirements have been defined Preliminary architecture has been identified which will satisfy driving requirements ACS control modes have been defined which will satisfy driving requirements Interfaces between the propulsion system and the following subsystems have been defined at Level 3 and are being developed at Level 4: Mechanical Thermal C&DH Propulsion Electrical Power FSW ACS subsystem ready to proceed PDR Performance and Functional Requirements Understood Sufficient to Size Actuators Sufficient to Proceed with Hardware Procurements Sufficient to Develop Controller Algorithms