Derivation and allocation of requirements for the Crosstrack Infrared Sounder (CrIS)

For a complex remote sensor like the NPOESS Crosstrack Infrared Sounder (CrIS), the process of requirements flowdown is extremely important to the success of the project. When there is both an algorithm and a sensor, the task of allocating requirements between the sensor and the algorithm becomes a challenge. This is where the use of system models and simulations has been an invaluable tool. Complex requirements such as radiometric uncertainty and Instrument Line Shape (ILS) uncertainty have utilized system models and simulations for the allocation of requirements. For radiometric uncertainty the sensor model in conjunction with the algorithm which handles the calibration of the sensor was used to assess the contribution of parameters such as component and detector temperature stability on radiometric uncertainty. Variation of the parameter values within the sensor model allowed us to compute the impact on radiometric uncertainty and allocate requirements appropriately. Examples of how the model and simulations were used to develop requirements for the CrIS radiometric uncertainty will be presented. For the assessment of ILS uncertainty a model for predicting the ILS of a Michelson interferometer was employed. The model calculates the ILS and associated spectral shift based upon a set of input parameters. By varying the input parameters the sensitivity of the ILS to the specific parameters could be determined and used to allocate the requirements from a top level down to the module level. A description of the model, the input parameters and results for the CrIS requirements development will be presented.