20a - 1 NASA’s Goddard Space Flight Center Attitude Control System (ACS) Eric Holmes, Code 591 Joe...

20a - 1NASA’s Goddard Space Flight Center

Attitude Control System (ACS)

Eric Holmes, Code 591Joe Garrick, Code 595

Jim Simpson, Code 596NASA/GSFC

August 16-17, 2005


Lunar Reconnaissance Orbiter (LRO)

ACS Subsystem Document Tree

•Level 2•431-RQMT-000004 LRO Mission Requirements Document•431-PLAN-000131 Lunar Reconnaissance Orbiter Spacecraft Performance Assurance Implementation Plan•431-OPS-000042 Lunar Reconnaissance Orbiter Concept and Operations Document•431-SPEC-000012 LRO Mechanical Systems Specification•431-SPEC-000091 Lunar Reconnaissance Orbiter General Thermal Subsystem Specification•431-ICD-000018 LRO 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-000063 Flight Dynamics Specification•431-RQMT-000113 LRO Pointing and Alignment Specifciation•431-PROP-000017 LRO Propulsion Subsystem SOW and Specification•431-ICD-000008 Lunar Reconnaissance Orbiter Electrical Systems Interface Control Document•431-RQMT-000045 Lunar Reconnaissance Orbiter Radiation Requirements•431-RQMT-000092 Lunar Reconnaissance Orbiter Thermal Math Model Requirements


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.


ACS Subsystem Level 2 Flow Down KeyRequirements


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-3.1.4.3 MRD-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





ACS will determine and track pointing commands and slewing profiles for HGA and Solar Array




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



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




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




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?


LRO Top Level Block Diagram



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



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

20a - 1 NASA’s Goddard Space Flight Center Attitude Control System (ACS) Eric Holmes, Code 591 Joe...

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