Assessing the Impacts of Uncertainty Propagation to System Requirements by Evaluating Requirement Connectivity
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This paper describes how materialization of risks that may affect fulfillment of a requirement may propagate to other requirements within a given level of abstraction due to requirement connectivity. DOI: http://dx.doi.org/10.1002/j.2334-5837.2013.tb03045.x
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Assessing the Impacts of Uncertainty Propagation to System Requirements by Evaluating Requirement Connectivity
- 1. Requirement Connectivity & Uncertainty Propagation Alejandro Salado Stevens Institute of Technology
- 2. Who USES requirements in their projects?
- 3. Who TRACES requirements in their projects?
- 5. How does it work? The customer wants to change a requirement, does it have any major impact? Well, it depends because E=mc² and f(x)=sqrt(i)… Mmmm, I see what you mean… Yes or no? Wait a sec, I’ll do a traceability analysis and check our margins!
- 6. Traditional traceability analysis Are we sure this type of analysis is comprehensive and most valuable? Req1 Req2 Req3 Req4 Req5 Req6 Req7 Req8 Req9 Req1 Req2 Req3 Req4 Req5 Req6 Req7 Req8 Req9 Req1 Req2 Req3 Req4 Req5 Req6 Req7 Req8 Req9 Req1 Req2 Req3 Req4 Req5 Req6 Req7 Req8 Req9 Req1 Req2 Req3 Req4 Req5 Req6 Req7 Req8 Req9 Req1 Req2 Req3 Req4 Req5 Req6 Req7 Req8 Req9 Req1 Req2 Req3 Req4 Req5 Req6 Req7 Req8 Req9 Level 0 Level 1 Level 2
- 7. A bit of theory… Lemma 9: Given a set of requirements, the set of systems that satisfy all requirements is the intersection of the sets of systems that satisfy each requirement independently. 𝑅𝑖 = 𝑟𝑖 , 𝑖 = 1, 2, … 𝑛 𝑅𝑗 = 𝑖=1 𝑛 𝑟𝑖 = {𝑟1, 𝑟2, … , 𝑟𝑛} 𝐶𝑇𝑅 𝑗 = { 𝑥 𝜖 𝑈𝑇 ∶ 𝑐 𝑥, 𝑟1 = 𝑡𝑟𝑢𝑒 ∧ 𝑐 𝑥, 𝑟2 = 𝑡𝑟𝑢𝑒 ∧ ⋯ ∧ 𝑐 𝑥, 𝑟𝑖 [Salado, Nilchiani, and Verma 2013]
- 8. What does it mean in practice? R1 R2 R3 ONE requirement is NOT easy or difficult. Difficulty lays on fulfilling TWO or MORE requierments SIMULTANEOUSLY So why do we do ACROSS analysis and not ALONG analysis?
- 9. This paper is about… Connectivity measured on requirements inside this box Level 0 Level 1 Level 2
- 10. Types of requirement dependency [Carlshamere et al. 2001] AND REQUIRES TEMPORAL CVALUE ICOST OR [Pohl 1996] Condition Content Documents Evolutionary Abstraction [Robinson, Pawloaski, and Volkov 1999)] Positive correlation Negative correlation Unspecified correlation No correlation Structure Resource Task Causality Temporal [Kulshreshtha, Boardman, and Verma 2012] Requires Requires (loop) Implementation sequence Value/cost Derive Structure Conflict
- 11. Types of representations Design Structure Matrix (DSM) Change Risk Plot Propagation Networks Propagation Tree
- 12. In this paper… Tailor [Robinson, Pawloaski, and Volkov 1999)] Positive correlation Negative correlation No correlation Resource Causality
- 13. What is uncertainty propagation? UncertaintyType1UncertaintyType2 UncertaintyType2
- 14. How do you structure uncertainties? [Salado, Nilchiani, and Efatmaneshnikh, 2012]
- 15. Uncertainty inter-dependencies Columns are triggered by rows Techn. Service performance Market Capability Legal Obsolescence Technologyreadiness Systemreadiness Reliability Availability Debris Radiation Weatherhazard Lifetime Performance Marketsize Discountrate Competitor Marketcapture Schedule Supplychain Cost Technicalcapability Keypeople V&V Design Requirements Customerinvolvement Export Frequencyallocation Mission-specific regulations Disposal Technology Obsolescence 11 12 21 41 100 110 79 Technology readiness 1 13 72 101 111 80 System readiness 2 73 102 112 81 Service performance Reliability 22 42 Availability 63 68 113 Debris 23 43 99 Radiation 24 44 Weather hazard 25 45 Lifetime 3 18 26 31 34 38 ? ? ? Performance ? 60 64 69 Market Market size 27 46 52 65 135 82 92 Discount rate Competitor 61 70 103 123 136 83 93 Market capture 28 47 53 66 84 94 Schedule 4 6 14 32 35 62 67 71 104 114 124 137 146 85 Capability Supply chain 74 115 150 127 147 Cost 139 148 Technical capability 7 15 54 75 128 132 140 149 Key people 119 V&V 19 29 48 56 Design 20 30 49 57 129 Requirements 9 17 36 37 39 ? 58 77 116 120 130 133 96 Customer involvement 78 105 117 121 125 131 134 141 Policy Export 5 10 40 55 107 106 118 122 126 142 Frequency allocation ? 108 143 Mission-specific regulations ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 144 ? 88 Disposal 33 ? ? ? 145 [Salado,Nilchiani,andEfatmaneshnikh,2012]
- 16. Case study: an EO space instrument Image the Earth in 4 spectral channels simultaneously. Image the Earth without obscuration between consecutive images. Provide image data at a maximum rate of 20 Mbps. Self-command and control. Performance (MTF, resolution or similar) better than 5 units. SSD lower than 2 nm. Power consumption lower than 200 W. Operate at 650 km altitude and 70° inclination. Mass lower than 950 kg. Fit inside an envelope of 1 m³.
- 17. Who deals with who? Simul t Obscu Data rate Self C&C Perf SSD Powe r Altitu Mass Envel 22 3 0 4 4 2 4 3 4 (eq. 7) (n) The amount of channels to be imaged influences the amount of data generated and therefore the data rate required to transfer all data. (r) The function requires power to operate. (n) Obscuration drives the amount of images to be taken per second, which influences the amount of data generated and therefore the data rate required to transfer all data. (p) (n) The satellite orbit speed depends on its altitude. Varying orbit speed results in different sizes of image taking, influencing therefore obscuration. For higher orbits effect is positive whereas for lower orbits effect is negative. (n) Higher resolution requires higher data rates.
- 18. We evaluate these ones Uncertainty Rationale or Example Market size New estimations on market size may result in adaptation of requirements. Competitor Introduction of competitors in the market may result in adaptation of requirements to be more competitive. Schedule The longer it takes to develop a system, the more probable stakeholders may change requirements. Cost Variation in cost may lead to modify (upgrade or waive) requirements. Technical capability Technical capability of the manufacturer may lead to modify (upgrade or waive) requirements. Customer involvement The more the customer is involved the more probable requirements will evolve. Export Export regulations may result in updating requirements.
- 19. Note on color code Requirements marked in red are the ones changing. Requirements marked in yellow are the ones being affected. Red lines and arrows represent probable negative impacts (more stringent requirement). Green lines and arrows represent additional margin to fulfill a requirement.
- 20. Market size Simul t Obscu Data rate Self C&C Perf SSD Powe r Altitu Mass Envel 22 3 0 4 4 2 4 3 4 (eq. 7) Less optimistic prediction than initially planned. Change satellite platform to reduce upfront investment. It results in lower resources for the instrument. Resources Functionality and performance.
- 21. Competitor Simult Obsc Data rate Self C&C Perf SSD Powe r Altitu Mass Envel 22 3 0 4 4 2 4 3 4 (eq. 7) Competitor same performance. Investors decide to upgrade. Increase performance. Performance Resources
- 22. Schedule Simult Obsc Data rate Self C&C Perf SSD Powe r Altitu Mass Envel 22 3 0 4 4 2 4 3 4 (eq. 7) New processing algorithm. Need overlapping images. Decrease obscuration level. Functionality Perfomance and resources
- 23. Cost Simult Obsc Data rate Self C&C Perf SSD Powe r Altit Mass Envel 22 3 0 4 4 2 4 3 4 (eq. 7) Severe cost overrun. Move capability to platform. Remove self C&C. Functionality Resources
- 24. Technical capability Simult Obscu Data rate Self C&C Perf SSD Powe r Altitu Mass Envel 22 3 0 4 4 2 4 3 4 (eq. 7) Cannot achieve SSD. Issue an RFD. Relax SSD requirement. No impacts
- 25. Customer involvement Simul Obscu Data rate Self C&C Perf SSD Powe r Altitu Mass Envel 22 3 0 4 4 2 4 3 4 (eq. 7) New HK 2 Customer veto at all levels. Decide to have more observability. Add a new HK requirement. Functionality Resources and performance
- 26. Export regulations Simult Obscu Data rate Self C&C Perf SSD Powe r Altitu Mass Envel 22 3 0 4 4 2 4 3 4 (eq. 7) Export control regulations change. Have to go with different rocket. Use different orbit. Interaction Functionality, resources and performance
- 27. No derived laws, yet… Case Causal requirement type Dependency impacts Affected requirement type C R Ch F P R I 1 Resource 9 5 9 X X 2 Performance 3 3 3 X 3 Function 2 2 2 X X 4 Function 3 3 3 X 5 Performance 0 0 0 6 Function 2 2 2 X X 7 Interaction 4 4 4 X X X
- 28. But some useful uses… Strategic compliance assessment Strategic verification approach Strategic deliverable definition
- 29. What’s next? Computational algorithms Formalize constructs Search for patterns
- 30. Open for questions Please, hold the tough ones…
- 31. Survey Please take the time to rate this presentation by submitting the web survey found at: www.incose.org/symp2013/survey
Notes de l'éditeur
- Power availability (Req. 7) constraints the amount of functions that can operate simultaneously (Req. 1, Req. 2 and Req. 4) and in some cases may limit performance (Req. 3). In optical instruments mass restrictions (Req. 10) constraint the size of optical elements, which influences optical performance (Req. 5). The dependency analysis shows that change in resources conditions fulfillment of functionality and performance. A joint connectivity of the resource requirements of 9 affects 5 requirements through 9 channels.
- During the course of system development a competitor has put into service a space system at the same resolution the instrument is being developed to. Therefore investors decide to upgrade the design and increase the required resolution (Req. 5). The effect of modifying such requirement is shown in Figure 4. Fulfillment of more stringent optical requirements (Req. 5) may require bigger and heavier components. Therefore mass (Req. 9) and envelop (Req. 10) requirements may not be fulfilled. The dependency analysis shows that change in performance conditions fulfillment of resources. A connectivity of 3 affects 3 requirements through 3 channels.
- During the course of system development scientists have come up with a new processing algorithm that brings different benefits to the analysis of Earth images. This algorithm requires satellites to take overlapping images during passes. In order to take benefit from this use investors have decided to change the obscuration requirement (Req. 2) and instead require the instrument to take consecutive images ensuring at least 60% overlap. The effect of modifying such requirement is shown in Figure 5. Increasing amount of images per second (Req. 2) may require higher power (Req. 7). Since significantly more data are produced, fulfillment of maximum data rate (Req. 3) is put into question. The dependency analysis shows that change in functionality conditions fulfillment of performance and resources. A connectivity of 2 affects 2 requirements through 2 channels.
- Due to severe cost overrun during development of the instrument system the principal contractor has decided to move command and control capabilities (Req. 4) back to the satellite platform in order to limit equipment investment at instrument side (at the risk of having a more complex verification and validation). The effect of modifying such requirement is shown in Figure 6. Removing functionality from the system (Req. 4) may result in removing system components, which frees the availability of power, mass and volume (Req. 7, Req. 9, and Req. 10). The dependency analysis shows that change in functionality conditions fulfillment of resources. A connectivity of 3 affects 3 requirements through 3 channels.
- During instrument development the instrument manufacturer realizes it cannot achieve the required SSD (Req. 6). As a consequence they issue a Request For Deviation (RFD) to the customer, which is approved. The effect of modifying such requirement is shown in Figure 7. Since SSD is associated to the physical properties of a material, its fulfillment does not have any impact on other requirements. A connectivity of 0 does not affect any requirement.
- In this particular project customer is significantly involved in all levels of the system development. After reviewing the operational concept of the instrument and evaluating test results of some lower level components, customer has decided to add a new requirement in order to acquire health status data of the instrument for post-processing when operational. The following requirement is added: Req. 11. The system shall provide the housekeeping data according to REF1. The effect of adding such requirement is shown in Figure 8. Adding housekeeping functionality to the system (Req. 11) may require additional power (Req. 7). Since additional data are generated, fulfillment of data rates may be questionable (Req. 3). The dependency analysis shows that change in functionality conditions fulfillment of resources and performance. A connectivity of 2 affects 2 requirements through 2 channels.
- During instrument development USA export control regulations changed, impacting the legality to use the pre-selected launcher. In order to avoid incrementing the complexity of in-orbit maneuvers investors have decided to use a different orbit to park their satellite (Req. 8). The effect of modifying such requirement is shown in Figure 9. Changing the orbit in which the instrument operates (Req. 8) directly influences optical performance (Req. 5), as the optical focus of the system changes. In addition, the different orbit position may risk the fulfillment of the obscuration requirement (Req. 2), as system may be unable to record images at certain rate. Furthermore, a different orbit may present different radiation environment, resulting in the need of additional mass and size (Req. 9 and Req. 10) to incorporate the appropriate level of radiation hardening. The dependency analysis shows that change in interaction conditions fulfillment of functionality, performance, and resources. A connectivity of 4 affects 4 requirements through 4 channels.