A major problem affecting an observing system is the definition of a suitable observational network. In practice, the design of an observing system is generally constrained by logistic and economic reasons and an optimisation is hard to achieve. OSSEs can be used to assess and compare the usefulness of the various sampling strategies and data combinations which can be adopted in the observing system.
In the framework of MFSPP, OSSEs were performed for the first time in the Mediterranean Sea to assess and compare the usefulness of temperature sampling strategies in a Mediterranean operational observing system.

The OSSE approach consists of:
a) simulating the observing system using a model that is supposed to represent the ‘truth’;
b) reconstructing the ocean state from these simulated data using either a mapping or a data assimilation technique;
c) assessing the contribution of the simulated observing system through the comparison of the reconstructed fields with the model reference fields.