The method includes receiving a plurality of target observations from a respective plurality of sensors and using the target observations to compute target state estimates. Each of the target state estimates is assessed to check if it suffers from one of a set of modelled possible fault types. The target state estimates are adjusted to compensate for any modelled fault type when it is assessed to be suffering from that fault type. A reliability value is computed for each of the target state estimates and the target state estimates are fused together based on the computed reliability values to produce a fused target state estimate.