5.1. Model Organization The architectural approach for the IAAM is a modular approach that integrates a number of key modules. These include the SSP Architecture, Competing Supply, Markets, and Financial Environment which are related by an Integration module. The primary output of the model is provided by the Key Discriminator module. All parameters used in the model are defined in the data dictionary, which is described in a subsequent section. This approach is illustrated in Figure 5-4. The IAAM was developed in Microsoft Excel and is configured as a workbook. Several different spreadsheets are linked together. Variable parameters are identified by color highlighting and can be modified by the user to generate alternate scenarios, and to conduct sensitivities. Key parameters of most interest (e.g. SSTO launch rate, SPS power output, SPS refurbishment requirements) are in the performance and cost summary pages. All other parameters are included in the individual element sheets. Uncertainty is introduced into the model by the addition of an Excel add-in call @Risk. This allows the user to specify a probability distribution for a selected parameter, rather than a point estimate. The @Risk add-in then conducts a Monte-Carlo simulation of the overall model, generating the values for those parameters which have been assigned probability distributions to incorporate uncertainty. Overall model results can then be represented by a probability distribution. 'igure 5-4 Organization of the Integrated Architecture Assessment Model
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