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The value of either of these methods is that they

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Unformatted text preview: surement of TPMs should become increasingly more accurate because of the availability of more "actual" data about the system. Lastly, the system engineer should select those TPMs that must fall within well-defined (quantitative) limits for reasons of system effectiveness or mission feasibility. Usually these limits represent either a firm upper or lower bound constraint. A typical example of such a TPM for a spacecraft is its injected mass, which must not exceed the capability of the selected launch vehicle. Tracking injected mass as a high-level TPM is meant to ensure that this does not happen. Assessment Methods. The traditional method of assessing a TPM is to establish a time-phased planned profile for it, and then to compare the demonstrated value against that profile. The planned profile represents a nominal "trajectory" for that TPM taking into account a number of factors. These factors include the technological maturity of the system, the planned schedule of tests and demonstrations, and any historical experience with similar or related systems. As an example, spacecraft dry mass tends to grow during Phases C and D by as much as 25 to 30 percent. A planned profile for spacecraft dry mass may try to compensate for this growth with a lower initial value. The final value in the planned profile usually either intersects or is asymptotic to an allocated requirement (or specification). The planned profile method is the technical performance measurement counterpart to the Earned Value method for cost and schedule control described earlier. A closely related method of assessing a TPM relies on establishing a time-phased margin requirement for it, and comparing the actual margin against that requirement. The margin is generally defined as the difference between a TPM's demonstrated value and its allocated requirement. The margin requirement may be expressed as a percentage of the allocated requirement. The margin requirement generally declines through Phases C and D, reaching or approaching zero at their completion. Depending on which method is chosen, the system engineer's...
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