Since the early nineties, performance modelling of active sonars has been developed at TNO in the ALMOST model, including propagation and sonar processing, based on point targets of given Target Strength. Recently, the modelling was extended with a computation module for target echo structure, resulting in realistic Target Strength figures, fully integrated in the existing model. The model includes a tool for making extended reflecting targets from clouds or surfaces of coherently scattering pixels with associated local normal vectors. For each scattering pixel, the impulse response function is computed including multi-path propagation. The final output is suitable for testing performance of sophisticated signal processing algorithms. Also the background of mainly bottom reverberation is modelled as a realistic time series, applying spatial roughness spectra from literature in the same echo structure modelling method. In this study a realistic (Kilo class) submarine was made as a target for a low frequency active sonar, including the most pronounced construction details. Modelled Target Strength values are shown for different aspect angles, using wideband transmitted signals. Sensitivity to variation in operational parameters like range, bandwidth of pulse, and centre frequency, are analysed.