Target System
The dingus. Not to be confused with the system of Engineering practices used to develop the Target System, or the system of manufacturing facilities, equipment, and practices that are used to produce it. All three systems are, however, the proper province of a good System Engineer.
Taxonomy
A classification scheme identifying the kinds of things that exist and the similarity between them, usually presented in the form of a dendrogram. The classes are mutually exclusive: any given thing can belong to at most one class1. The concept does not admit to the notion of multiple inheritance. Contrast with ontology. Footnotes In a... read more
Technical Characteristic
A Technical Feature designated as being characteristic of the dingus1. Footnotes Which might, or might not, be abstract.[↩]
Technical Feature
A feature for which a data structure similar to that of Table 1 can be populated. If we put enough technical features together for a single dingus we can start building models. As noted in the table comments, at least two general forms exist1, both of which might be in simultaneous use on the same project. This... read more
Technical Performance Measurement/Monitoring (TPM)
TPM is a sometimes elaborate process used to identify important quantitative technical characteristics and features of the system, and report them to management on a regular basis. For each characteristic, a plan is usually developed to show how its predicted (or estimated) value will improve over the course of development, and “trigger” values will be... read more
Technical Requirement
A requirement with regard to what an Engineered item does for the End User. Contrast with Operational Requirement, which is what the End User accomplishes through the use of the item. read more.
Technical Reviews and Audits
Any review or audit having technical material (as opposed to cost or schedule) as its subject. For any given developmental project, these reviews are organized according to the hierarchy of CI’s. Legacy practices in this regard for DoD development were governed by MIL-STD-1521B, which is worth a read just to understand what they had in... read more
Technology
The most basic meaning is “a practical application of knowledge” (often “…scientific knowledge…”), but that definition can be used to mean pretty much anything that anybody wants it to mean. I find it more useful to define the concept as “an abstract use of knowledge outside the context of any specific application”. This definition admits... read more
Tensor-valued
A tensor is an array of mathematical expressions that expresses dependency of the array’s output parameters on the input parameters. In the context of System Engineering, tensor-valued parameters admit to explicit relationships between numbers and circumstances. A “value” can (therefore) then be a recipe1 for generating a number based on circumstances, rather than just some... read more
Terminological Inexactitude
A downright lie (Churchill, speaking to Parliament in 1906). No, no, no…I didn’t mean to say he was the one telling lies, although that might have been the case. I meant to say that he coined the phrase. Either way, not an appropriate System Engineering approach (see also The Four Rules).
Terms and Conditions (T&C)
Of the three things that provide direction to developers, the T&C have the highest legal precedence, and can over-ride all other considerations. The T&C are found directly in the contract. The term is sometimes completed with “…of the contract” in order to be explicit and precise. It may be sad, but after all is said... read more
Test
The indirect observation of the item using intrusive instrumentation, fixtures or external excitation that may alter the behavior of the item. Applicable IFF the characteristic specified is quantifiable and the test method has a known effect on the item’s behavior. See also demonstration for a discussion about selecting levels of aggregation for any given test... read more
Test Article
A now mostly-obsolete synonym for UUT.
The Point of Implementation
An abstract concept denoting the lowest layer of design of general interest to System Engineering during development. At this point in the product structure, no further abstract decomposition (e.g., Functional) is necessary to achieve the intended developmental purpose, so no more Development Requirements remain to be allocated to a design. Any requirements below this layer are written directly... read more
Thermal Circumstances
Pre-berth temperatures of exterior spacecraft components are a critical issue1. They’re strongly driven by several factors 2: Footnotes MIL-A-83577B “Assemblies, Moving Mechanical, For Space and Launch Vehicles, General Specification for” (1978)[↩]Conley, Peter L. (ed), “Space Vehicle Mechanisms: Elements of Successful Design”, John. Wiley & Sons (1998), Chapter 20 “Thermal Design”[↩]
Things I Think I Know
This is the foundation page for Things I Think I Know The definitions and explanations contained herein are intended exactly as the title suggests: they are things I think I know. They should, therefore, not be taken as authoritative: I am no Granddaddy, and neither is this web site. Many are gleaned from my collection... read more
Threshold Requirement
A requirement considered mandatory for successful completion of development. “Mandatory” could apply seperably to the topic or the measure of the requirement.
To Be Determined (TBD)
A value (often numeric) that is recognized as necessary, but is not currently known. TBD exists in at least the following forms: TBD: To Be Determined TBS: To Be Supplied TBR: To Be Resolved See TB or Not TB for a discussion of how these concepts should be used during procurement and specification development.
Tolerance
A measure of willingness to permit variation of a feature, including performance. Tolerance is a measure of the capacity to accommodate imperfection. Contrast with uncertainty, which carries no connotation of acceptability.
Tool (Programming)
Software, which may be Application Software from the perspective of the original programmer that assists in the development of other software without being part of said other software. Examples include programming environments and compilers. A collection of programming tools used in sequence is sometimes referred to as a “tool chain”.
Top-down
A developmental method in which a single set of overall objectives are decomposed into smaller objectives until individual designs can be accomplished with theoretically perfect confidence of subsequent integration. One of two competing System Engineering theories, both of which are wrong. Contrast with Bottom-up.
Topic (Requirement)
Every requirement has to be about something. This is it. I used to call this the “subject”, but people interpreted everything that followed as “grammar”. I found that changing the name made them pay attention longer1. The concept generalizes to apply to any characterization of a system. Footnotes If they paid attention at all.[↩]
Trade Study
An analytical (and, sometimes experimental) process used to select a “best” or “preferred” option or course of action from among several candidates by ranking them with respect to application-specific criteria. A good trade study should be organized as an optimization problem (which may be either linear or non-linear) seeking to maximize (or minimize) an Objective... read more
Uncertainty
Uncertainty expresses the extents of the range within which a value can exist, quoted as a distribution about some nominal value. The magnitude(s) of the uncertainty in any given situation can be influenced by (for example) the calibration accuracy of an instrument, or the numerical precision of a calculation. Unlike tolerance, the concept of uncertainty... read more
Unilateral ICD
An ICD that establishes one side of an interface, where the other sides have no input (but must necessarily comply). Contrast with multi-lateral ICD.
Unintended Function
An alteration (or maintenance) of the state of a system as mere happenstance of the design given a set of the circumstances in which it might be found. An unintended function is never proactively driven by Development Requirements. Requirements can (however) be backfilled if the function is important enough once discovered (see Verification by Surprise). Backfill can make... read more
Unit Under Test (UUT)
The article being tested1. Synonymous with Test Article. Footnotes or demonstrated[↩]
Units
A standard for expressing the amount of a property. Units serve as the basic technical discriminator between property values in preference to dimensions for two reasons: 1) Dimensional concepts established by most international conventions address little beyond the physical sciences. 2) The established conventions do not uniformly distinguish between affine concepts and positional concepts1. Don’t... read more
Use As-Is
An MRB disposition where the non-conformant article is determine acceptable for use even though it is not in compliance with the identifying drawing. This disposition literally appends the NC number to the P/N, such that the article is uniquely identified. During rate production, this disposition can be horrendously expensive, because none of the support documentation... read more
Utility (Software)
A type of software (which may, or may not, be Application Software from the perspective of the original author) performing general purpose tasks (e.g., coordinate system conversion) considered standardized to the point of not requiring End Use-specific validation.
Validate
Unfortunately, this is one of those “overloaded” terms having more than one meaning, depending on context. In the context of Requirements Management, the concept of validation generally stems from Software Systems Engineering usage1. In that context, “technical requirements” were more or less hypothetical until Footnotes It was not normally used in legacy System Engineering practices... read more
Value
(1) In the technical vernacular, a value is the exact amount of some particular property, usually represented as a number. (2) As generally used herein, a value is a non-empty set of points in a space associated with a topical parameter. This notion arises because exact amounts are rarely encountered outside of theoretical constructs1. ... read more
Vector
Formally, an element of a vector space. An array of order 1. Mostly, that doesn’t help very much. As a practical matter, a simple list of scalar-valued data (usually numeric) in which their position in the list denotes some particular meaning of relevance to the user. Different combinations of values (in the various positions in... read more
Vendor Item Control Drawing (VICD)
A type of PCD that establishes the acceptability and interchangeability of one or more vendor part numbers for some specific usage. This drawing type permits Procurement and Manufacturing to purchase and install any suitable part. We only (and always) use a VICD when all of the IP (including any qualification data and identifying drawings) are... read more
Verification Analysis Cycle (VAC)
The project-specific sequence of analyses repeated during development as a leading indicator of successful compliance with requirements. Ideally, the VAC should use analysis methods and tools that are different than those used in support of design in order to minimize susceptibility to systemic errors, but that can really drive a project’s cost. Contrast with DAC... read more
Verification Cross-Reference Matrix/Index (VCRM, VCRI)
Legacy development specifications typically organized their verification sections according to activity. In that case, it was necessary to indicate which verification requirements were relevant to each of the design-to requirements, each of which was considered to have been fully verified when all verification requirements indicated in the VCRI were successfully completed. This specification practice resulted... read more
Verification Measure
A measure for which the following are true: (1) Any combination of the algorithm, meta-arguments, or input arguments are distinct from and independent of those used to evaluate the Technical Requirements or support the design process for the dingus. (2) Those algorithm elements and input arguments that quantify features of the design are traceable to... read more
Verification Method
Major classes of verification based on the type of logical inference employed. read more.
Verification Requirement
A type of technical requirement mandating some characteristic of a formal verification activity. A verification requirement can apply in either the development or production phases.
Verify
Determination, through the use of objective evidence generated by a repeatable process, knowledge regarding conformity of an item to a technical claim. read more.
Vicissitudes of Developmental Fortune
Issues will crop up that were not foreseen by the SEMP/SMP/MVP or Recovery Plan of the Week. “It seemed like a good idea when we wrote the requirements. It just didn’t work very well.” New technologies and new uses of existing technologies will turn out to need restrictive or integrating requirements that were not understood... read more