Independent Verification
Ron Graham
Independent verification, or independent verification and validation (IV&V), is a formal way of referring to the sanity check. It gives you increased confidence that something will work as advertised, for the cost of duplicating some sensitive aspect(s) of analysis and design.

Verification consists of

  • asking "what-if" questions based on the system's performance limits;
  • extending the design and analysis functions, when the primary designer(s) are constrained by time, funding, staff, or experience.
Validation refers to a check of actual design or analysis results, even down to the numbers. It's a check on methodology. IV&V generally involves some combination of both.

Case Study

Control dynamics IV&V, performed by NASA on behalf of a prime contractor (who's building a rocket or spacecraft), calls for the contractor to contribute

  • system dynamics (or, "plant model"): rigid and flexible effects, fluid motion, aerodynamics, mass properties, propulsion, articulation, and gravity and other external disturbances;
  • control system model;
  • starting assumptions;
  • input data (parameter values); and
  • contact(s).
NASA, in turn, supplies the following:
  • requirements and specifications;
  • environmental data;
  • system integration information; and
  • questions/concerns/comments.
The validation portion of the effort may consist of the IV&V team taking the math model(s) and control law(s) prepared by the contractor and actually running simulations on-site, with the goal of obtaining the same (or very nearly the same) numeric results as those obtained by the contractor. When this fails, the team takes steps to locate the cause of any discrepancy. When causes are isolated and corrected, the goal of validation is achieved.

Validation is sometimes nothing more than doing exactly what the contractor did on a different platform: a trivial case. At other times, validation can involve a detailed analysis of many subsystems of both the control system and plant. The confidence you have in the final design is directly proportional to the level of effort put into validation.

The verification portion of the effort may consist of the IV&V team developing a mathematical model of the system under consideration from scratch. The method used by the contractor in model development may not be necessary, since extra confidence and insight are often gained from obtaining the same results using two totally different methods.

Verification is necessary at times when a contractor considers portions of its model to be proprietary, or when the physical design of a system is changing frequently enough, or significantly enough, that a totally independent model may give the IV&V team results more quickly, or with greater confidence, than could the contractor's original model.

Having convinced themselves that both the contractor's model and methodology are sound, the IV&V team may extend the concept into potential problem areas which have not been examined by the contractor. This will happen when the IV&V team is actually on the "critical path" - that is, the contractor must commit to a basic design and move on toward a known launch, and any design changes must either be minor or possibly delay the launch. In that case, when concerns arise, the IV&V team must respond to them.

There are various ways the concept may be extended:

  • add features to the model, such as
    • nonlinear behavior,
    • discrete components, or
    • dispersions of uncertain parameters; or
  • simulate the model under extra conditions, such as
    • mechanism failures and partial failures,
    • center-of-gravity offsets,
    • actuation misalignments, or
    • previously unexpected disturbances.
These exercises can be used to define the limits of system stability and performance, and as tools to suggest minor design changes even late in a program. Some possible design changes might include
  • gain adjustments,
  • addition of baffles to a propellant tank, and
  • filtering.
Exercises described here may already have been done as part of the IV&V process, without any extension of the concept. This is negotiated as part of the original contract.
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