A very unique USAF surveillance aircraft has been flying highly defined circles over Seattle and its various suburbs for nine days now. Nobody at the DoD seems to know who the aircraft belongs to or what exactly it is doing flying so many missions over the Seattle area. But based on its visibly exotic configuration, and information collected by open source flight tracking websites, we can get a good idea of its capabilities and guess as to what it's up to.
The aircraft, which goes by the callsign "SPUD21" and wears a nondescript flat gray paint job with the only visible markings being a USAF serial on its tail, is a CASA CN-235-300 transport aircraft that has been extensively modified for the surveillance mission. You can see more pictures of the aircraft here.
It is covered in a dizzying array of blisters, protrusions, humps and bumps. These include missile approach warning detectors and large fairings on its empennage for buckets of forward-firing decoy flares, as well as both microwave—the dome antenna behind the wing and flat antenna modification in front of the wing—and ultra high-frequency satellite communications—the platter-like antenna behind the dome antenna. A communications intelligence suite also appears to be installed on the aircraft, with the antenna farm on the bottom of its fuselage being a clear indication of such a capability.