1. A method for designing a secondary sub-system for monitoring the condition of a platform having at least one primary sub-system, the method comprising: identifying stakeholders of the platform; capturing information requirements of the stakeholders; determining data requirements corresponding to the information requirements; defining a sensor array comprising sensors suitable for providing output signals relating to condition characteristics of the at least one primary sub-system; defining procedures for converting the output signals from the sensor array to the data requirements, thereby to fulfil the information requirements of the stakeholders; identifying new data requirements that could be fulfilled by the output signals of unused sensors of the sensor array; and identifying new information requirements that could be fulfilled by the new data requirements.