Spatial and temporal variations in sound speed can have substantial effects on sound propagation and hence sonar performance. Operational oceanographic models can provide forecasts of oceanographic variables as temperature, salinity and sound speed up to several days ahead. These four-dimensional forecasts can be used as input for a sonar performance prediction model. This paper presents results of such coupled ocean and sonar performance modelling, carried out during the Battlespace Preparation 2007 sea trial. The new aspect of the work is that the complete modelling chain from ocean forecast to probability of detection plots was carried out in near real-time at sea, aboard HNLMS Snellius (Figure 1).