1. A method for designing a secondary sub-system for monitoring the condition of a platform, the platform comprising 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; and defining procedures for converting the output signals from the sensor array to the data requirements, thereby to fulfil the information requirements of the stakeholders.