Estimating a state of at least one target using a plurality of sensors

The method includes receiving (202) a plurality of target observations (106) from a respective plurality of sensors and using (204) the target observations to compute target state estimates. Each of the target state estimates is assessed (206) so check if it suffers from one of a set of modelled possible fault types. The target state estimates are adjusted (208) to compensate for any modelled fault type if it is assessed to be suffering from that fault type. A reliability value is computed (210) for each of the target state estimates and the target state estimates are fused (212) together based on the computed reliability values to produce a fused target state estimate.