Although Systems Engineering (SE) is becoming progressively the leading discipline in the development of engineered systems, most of the functions performed by a system are 'delivered' as services. This leads to what is referred to as Service Systems Engineering (SSE), which has its basis in the SE and, for specific types of services, represents the driving discipline to design successful systems capable of delivering effective services. In the defence market, the acquisition of defence and weapon systems is facing, in the last decades, a continuous budget contraction for both the acquisition and the support of the delivered systems. The main scope of support services, as an instance of SSE, is to define, deliver and asses the proper blend of services to ensure optimal system performance at an affordable life cycle cost. This paper introduces a model for the definition, implementation and continuous assessment of operational availability, in order to ensure that the required performance is met in a cost-effective way throughout the system lifecycle. The amount, distribution and continuous readiness of the logistic resources required to meet the target availability is the primary scope of what is referred to as performance based logistics, which this paper addresses by introducing a SSE framework to tailor, design, deliver and monitor logistic resources. © 2015 IEEE.