While there are many sources to be collected under the general SIGINT umbrella, what are the overall advantages and challenges to Passive RF collection. Please provide an example of a collection target that would be a candidate for passive RF collectio

While there are many sources to be collected under the general SIGINT umbrella, what are the overall advantages and challenges to Passive RF collection. 

Please provide an example of a  collection target that would be a candidate for passive RF collection.  Please post by Friday, and respond to two classmates by the end of the week.