Technical Performance Measures Module Space Systems Engineering, version 1.0

1. Technical Performance Measures ModuleSpaceSystems Engineering, version 1.0

2. 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.

3. 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.

4. Mass Contingency Plan Current Best Estimate Trend 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 5% 2% 15% 20% 35% Contingency violated, decisions are needed! Is the trend dependable and no action is needed? Act now to avoid more drastic action in the future? Mass Today Time Concept PDR CDR Test Launch

5. Chandra Mass TPMSystem Requirements Review to Launch

6. 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.

7. Contingency Guidelines for Common TPMs For Different Project Phases

8. 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)

9. JWST TPM - Mass

10. JWST TPM – Mass Reserve

11. JWST TPM – Power 6 year Power System Capability = 1826 Watts Spacecraft + OTE Allocation (882 + 50) = 932 Watts ISIM + Cryocooler Allocation (310+430) = 740 Watts Power Margin (Estimate vs. Allocated) = 25 % 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

12. 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.

13. Backup Slidesfor Technical Performance Measures Module

14. 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 profile…planning 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

15. JWST TPM - Mass

16. 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.

17. JWST TPM – Wavefront Error