Technical Performance Measures Module Space Systems Engineering, version 1.0

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Technical Performance Measures Module Space
Systems Engineering, version 1.0

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Module Purpose Technical Performance Measures

• To define Technical Performance Measure (TPM).
• To show how TPM trends are used to predict
  delivered system performance.
• To describe how TPMs are used to monitor project
  progress and, when compared with standard
  resource contingency values, highlight when
  corrective action should be considered.
• To provide example TPMs from current NASA
  development projects.

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Technical Performance Measures

• TPMs are measures of the system technical
  performance that have been chosen because they
  are indicators of system success. They are based
  on the driving requirements or technical
  parameters of high risk or significance - e.g.,
  mass, power or data rate.
• TPMs are analogous to the programmatic measures
  of expected total cost or estimated
  time-to-completion. There is a required
  performance, a current best estimate, and a trend
  line.
• Actual versus planned progress of TPMs are
  tracked so the systems engineer or project
  manager can assess progress and the risk
  associated with each TPM.
• The final, delivered system value can be
  estimated by extending the TPM trend line and
  using the recommended contingency values for each
  project phase.
• The project life trend-to-date, current value,
  and forecast of all TPMs are reviewed
  periodically (typically monthly) and at all major
  milestone reviews.

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Tracking Technical Performance Measures

• Tracking TPMs and comparing them against typical
  resource growth provides an early warning system
  designed to detect deficiencies or excesses.
• Contingency allocations narrow as the design
  matures.
• TPMs that violate their contingency allocations
  or have trends that do not meet the final
  performance should trigger action by the systems
  engineer.

Mass Allocation
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2
15
20
35
Mass
Contingency violated, decisions are needed! Is
the trend dependable and no action is needed? Act
now to avoid more drastic action in the future?

Today
Time
Concept
CDR
PDR
Test
Launch
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Chandra Mass TPMSystem Requirements Review to
Launch
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Design Contingencies

• Design contingencies are largest during concept
  exploration and uniformly shrink as the project
  matures. For example, mass contingencies are
  typically 35 at SRR, 20 at PDR, 15 at CDR and
  2 at the launch readiness review.
• Why? Contingencies are used to account for
  development risks, interface uncertainties, and
  less than perfect design fidelity. As the design
  becomes more established and the team has greater
  confidence in their estimates for resource use or
  system performance, less contingency is needed.
• The trends of past, successful projects have been
  used to create guidelines for new projects.
• Why not carry even more contingency? Say 50 mass
  contingency at PDR to cover an even greater range
  of possible risks against system mass. With
  greater contingencies there is less allocation
  for the design - greater contingencies make the
  design problem harder. So there is a balance
  between contingency for risk management and
  allocation for design flexibility.

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Contingency Guidelines for Common TPMs For
Different Project Phases
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JWST Key Technical Performance Measures

• Observatory Mass Margin
• Observatory Power Margin
• Observing Efficiency
• OTE Wave-front Error
• Wave-front Error Stability
• Strehl Ratio
• Sensitivity
• Image Motion
• Stray Light Levels
• Cryogenic Thermal Margins
• Commissioning Duration
• Data Volume / Link Margin
• Momentum Acceleration

James Webb Space Telescope (JWST)
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JWST TPM - Mass
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JWST TPM Mass Reserve
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JWST TPM Power
6 year Power System Capability 1826
Watts Spacecraft OTE Allocation (882 50)
932 Watts ISIM Cryocooler Allocation (310430)
740 Watts Power Margin (Estimate vs. Allocated)
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Notes 5/05 ISIM allocation changed to 740 W
12/06 Power Margin being carried as
Load Margin not Solar Array Margin (Golden Rules
Compliance) 4/07 Solar Array
Capability decrease due to 1 wing baseline
8/07 Cryocooler separated from ISIM, Solar
Array Capability increased
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Module Summary Technical Performance Measures

• TPMs are measures of the system technical
  performance that have been chosen because they
  are indicators of system success.
• The trends of past, successful projects have been
  used to create contingency guidelines for new
  projects.
• Tracking TPMs and comparing them against typical
  resource growth provides an early warning system
  designed to detect deficiencies or excesses.
• TPMs that violate their contingency allocations
  or have trends that do not meet the final
  performance should trigger action by the systems
  engineer.
• The final, delivered system value can be estimate
  by extending the TPM trend line and using the
  recommended contingency values for each project
  phase.
• There is a balance between contingency for risk
  management and allocation for design flexibility.
  This balance is apparent since contingency
  allocations shrink as designs mature.

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Backup Slidesfor Technical Performance Measures
Module
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Technical Performance Measures

• TPM Basics
• Parameter for meeting key requirements and
  constraints.
• Sound engineering parameter that is always
  tracked regardless of mission, such as mass
  margin or milestone achievements.
• TPMs are usually tracked over the development
  life cycle of a project.
• TPM trends over time usually compare a planned
  profile with the actual profileplanning is very
  important in order to meet specified targets.
• TPMs are usually reported monthly or quarterly in
  management/engineering status meetings.
• TPM Sources
• Responsible NASA Center guidance (e.g., GSFC
  STD-1000 The Golden Rules)
• Industry Practices
• Mission-specific risk assessments

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JWST TPM - Mass
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JWST TPM Strehl Ratio
Science Requirement L1-14 The Observatory, over
the field of view (FOV) of the Near-Infrared
Camera (NIRCam) shall be diffraction limited at 2
micrometers defined as having a Strehl Ratio
greater than or equal to 0.8.
Definition The modern definition of the Strehl
ratio is the ratio of the observed peak intensity
at the detection plane of a telescope or other
imaging system from a point source compared to
the theoretical maximum peak intensity of a
perfect imaging system working at the diffraction
limit. This is closely related to the sharpness
criteria for optics defined by Karl Streh. Unless
stated otherwise, the Strehl Ratio is usually
defined at the best focus of the imaging system
under study.
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JWST TPM Wavefront Error