1. A collision avoidance system comprising: means for determining own aircraft data including position, altitude, velocity, and bearing; means for interrogating other aircraft to determine other aircraft data including position, altitude, velocity, and bearing; means for determining a threat of collision with other aircraft in accordance with the own aircraft data and with the other aircraft data; means for displaying at least a portion of the own aircraft data and at least a portion of the other aircraft data; means for alerting a crew member of a threat of collision with other aircraft; wherein: for at least one of the means for determining own aircraft data, means for interrogating, means for determining a threat of collision with other aircraft, means for displaying own aircraft data, means for determining own aircraft position, means for displaying a terrain map, means for determining a threat of collision with terrain, and the means for alerting a crew member: the at least one means is packaged in a replaceable unit for installation on the own aircraft at an interface, the interface having a first means for storing first content and a plurality of signature values, each signature value for a respective portion of the first content; the at least one means further comprises: second means for storing second content; means for validating the second content with reference to at least one signature value of the first content; means for updating the second content in accordance with the first content; and means for operating the subsystem in accordance with the updated second content.