1. A computer system for determining a survivability aspect control signal for an aircraft, said computer system comprising: a processor; and a memory including software instructions adapted to cause the computer system to perform the steps of: (a) providing a plurality of signature exposure models, each signature exposure model corresponding to a threat sensor and including a threat sensor characteristic and a threat operational characteristic; (b) selecting a mission portion; (c) selecting, based on the selected mission portion, a plurality of the provided signature exposure models, each selected signature model corresponding to a threat estimated to have current or potential visibility of the aircraft at the selected mission portion; (d) calculating a signature exposure index based on the plurality of the provided signature exposure models and the selected mission portion, the signature exposure index being calculated by determining an individual estimate of a signature of the aircraft for each threat sensor using, the corresponding model and combining the individual estimates into the signature exposure index representing the signature exposure of the aircraft at the selected mission portion throughout a range of an aspect by representing values of the combined individual estimates; and (e) providing a survivability aspect control signal based on the signature exposure index.