Evolution of the Hubble Space Telescope Safing Systems

1
Evolution of the Hubble Space Telescope Safing
Systems

• Joyce A. K. Pepe
• Michael C. Myslinski
• June 24, 2006

2
Agenda

• Introduction
• Safing System Hardware
• Safemode Overview
• Safing Tests
• On-Orbit Safing Changes
• Summary

3
Introduction

• Safing system was designed to ensure autonomous
  HST survival following any single point failure,
  or any pointing control system failure
• Basic requirement is to autonomously protect HST
  from power depletion for up to any 72-hours
• Safing system was designed to detect both system
  and component level anomalies
• Safing System consists of a combination of fault
  detection tasks and vehicle control modes
• System is distributed between two separate
  on-board computers for system reliability

4
Hubble Space Telescope
V3
V1
V2
5
Safing Overview

• Safing system is active during all modes of HST
  operations
• Initial Launch of HST had three Safemodes
• Inertial Hold
• Software Sun Point
• Hardware Sun Point

6
Safing System Hardware
7
Safemode Overview

• Inertial Hold
• Interrupts Science timeline
• Vehicle pointing is unchanged
• Flight Computer stays in control
• 3 axis stabilized (gyro hold)
• Minimal impact to hardware
• Recovery to normal operations within hours
• HST has entered IH 15 times
• Typical cause
• Stored command problem

8
Safemode Overview

• Software Sun Point
• Interrupts Science timeline
• Flight Computer stays in control
• Vehicle pointing optimized for power and thermal
• SAs commanded perpendicular to either V3 or V1
• Recovery to normal operations 24 - 36 hours
• HST has entered SWSP 15 times
• Typical causes
• Power system problem
• Pointing control hardware anomaly

9
Safemode Overview

• Hardware Sun Point
• Interrupts Science timeline
• Flight Computer powered off
• Safemode Computer takes control
• Optimizes vehicle pointing for power and thermal
• 3 axis stabilized 3 gyros or backup safemode
  gyros
• Use hardware I/F for sensors and actuators
• Precludes data system failures
• Recovery to normal operations 24 to 72 hours
• HST has entered HWSP 4 times
• Typical cause
• Flight Computer anomalies

10
Safemode Overview
Science Mode
- Body rate errors - Stored Cmd list exhaustion

• - Loss of 486 Computer
• Bus Min Voltage
• Battery Pressure / Temp Limit
• DMU failure
• Gyro failure

• Power Balance SOC1
• PCS hardware failures

• - Power Balance SOC-2
• - Loss of 486 Computer
• Bus Min Voltage
• Battery Pressure / Temp Limit
• DMU failure
• Gyro failure

Software Sunpoint
11
Safing Tests

• The HST Safing software was designed to protect
  for any single failure in and maintain
  survivability until the vehicle can be return to
  normal operations
• Extra protection ensures survivability following
  two failures in the Pointing Control Subsystem

12
Safing Tests

• Original Launch Safing Tests Configuration

Pointing Control Subsystem
Data Management Subsystem
Electrical Power Subsystem
13
On-Orbit Safing Changes

• As the Hubble operations have matured, engineers
  have continuously evaluated the observatory
  performance
• Goal to improving efficiencies and minimizing
  vehicle risks
• Extensive constraint checking is performed in
  evaluating the feasibility of any changes

14
On-Orbit Safing ChangesEvolution

15
On-Orbit Safing Changes

• Safemode Changes
• Spin Stabilized Sun Point
• Zero Gyro Sun Point
• Kalman Filter Sun Point (expected implementation
  Spring 2007)

16
Safemode Changes

• Spin Stabilized Sun Point sub-mode to SWSP
• Interrupts Science timeline
• Flight Computer stays in control
• Aligns V1 vehicle axis with the sun, then spins
  the body at 0.8 deg/sec about the V1 axis
• SAs commanded perpendicular to V1
• HST has never entered this mode
• Typical causes
• Multiple reaction wheels failures
• Multiple magnetic torque rod failures

17
Safemode Changes

• Zero Gyro Sun Point sub-mode to SWSP
• Interrupts Science timeline
• Flight Computer stays in control
• Vehicle pointing with course sun sensor and
  magnetometers as sensors
• 2 Axis control on sun
• 3rd axis rate damped to the earths magnetic
  field
• SAs commanded perpendicular to either V3 or V1
• Recovery to normal operations 24 - 36 hours
• HST has entered ZGSP 8 times
• Typical cause
• Gyro anomalies

18
On-Orbit Safing Changes
19
Safemode Changes

• Kalman Filter Sun Point (expected implementation
  Spring 2007)
• Interrupts Science timeline
• Flight Computer stays in control
• Vehicle pointing 3 axis stabilized
• Vehicle pointing optimized for power and thermal
• Minimum sensor requirements
• Magnetometers and course sun sensors
• Reaction wheels and magnetic torque rods as
  actuators
• Pointing performance meets shuttle grapple
  requirements
• Will replace ZGSP

20
On-Orbit Safing Changes

• New safing tests were developed to enhance
  anomaly detection based on flight experience

Data Management Subsystem
Pointing Control Subsystem
Electrical Power Subsystem
21
Summary

• The Hubble Space Telescope was designed as a long
  duration mission
• Periodic Shuttle Servicing Missions
• Planned replacement of components
• Flight software enhancements monitoring vehicle
  safety
• Early mission changes were driven by hardware
  failures
• Later changes were driven by aging components and
  improving system robustness
• Evolution is continuing with the development of
  the Kalman Filter Sun Point mode