A method of simulating the response of a system to at least one simulated source, the at least one simulated source having at least a predetermined simulated source location and a predetermined simulated source activity level at a predetermined time, the system comprising: a plurality of nodes; a central processor programmed to determine whether a source is detected by applying a predetermined algorithm to data associated with the plurality of nodes, the central processor having stored within it the predetermined simulated source location and the predetermined simulated source activity level; a network linking each node and the central processor such that signals can be transmitted between each node and the central processor; and an output device; the method comprising: transmitting through the network from each node to the central processor a measured or inferred location for that node at the predetermined time; calculating, based on at least (a) the measured or inferred location of each node at the predetermined time, (b) the predetermined simulated source location at the predetermined time, and (c) the predetermined simulated source activity level at the predetermined time, simulated response data for each node associated with the predetermined time; in the central processor, applying the predetermined algorithm to the simulated response data for all the nodes, thereby determining whether a real source at the predetermined simulated source location having the predetermined simulated source activity level would have been detected at the predetermined time; and signaling with the output device whether a real source at the predetermined simulated source location having the predetermined simulated source activity level would have been detected at the predetermined time. a plurality of nodes; a central processor programmed to determine whether a source is detected by applying a predetermined algorithm to data associated with the plurality of nodes, the central processor having stored within it the predetermined simulated source location and the predetermined simulated source activity level; a network linking each node and the central processor such that signals can be transmitted between each node and the central processor; and an output device; transmitting through the network from each node to the central processor a measured or inferred location for that node at the predetermined time; calculating, based on at least (a) the measured or inferred location of each node at the predetermined time, (b) the predetermined simulated source location at the predetermined time, and (c) the predetermined simulated source activity level at the predetermined time, simulated response data for each node associated with the predetermined time; in the central processor, applying the predetermined algorithm to the simulated response data for all the nodes, thereby determining whether a real source at the predetermined simulated source location having the predetermined simulated source activity level would have been detected at the predetermined time; and signaling with the output device whether a real source at the predetermined simulated source location having the predetermined simulated source activity level would have been detected at the predetermined time.