Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Confusion; Situational Awareness
Conflict NMAC; Deviation / Discrepancy - Procedural FAR
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance; General Police / Security Involved
Aircraft; Human Factors
Operating flight to SEA as First Officer and the Pilot Flying.  We had just been vectored to intercept the localizer for RW16R and were cleared ILS 16R Approach.  They had told us to slow our speed so we had our gear down and flaps 15 selected.  Just south of interstate; a relative motion of an object caught my eye.  My initial reaction for a couple seconds was 'Why there was an airplane passing below us going north bound when were on an Approach?'  But as we approached the object; I was able to discern the object as a drone.  I saw four fuzzy silver discs.  What I was seeing is what I believe to be white LED lights on the unit reflecting off of four spinning propellers making it look like fuzzy silver discs.  The drone appeared to be straight and level; not tilted in any direction as they would for forward flight.  I believe it was hovering.  The drone passed directly under our nose.  Because we were centered on the runway's localizer needle; this drone was also directly on extended runway centerline.  With the drone operating apparently already establishing their desired location; I imagine we weren't the first plane that came into close proximity to this drone on the localizer.  The Captain was busy with monitoring duties and did not see the drone so cannot speak to it. The Captain reported the drone sighting to SEA Tower ATC.  This is my first time seeing a drone from the flight deck of an aircraft.  It's hard to give a good comparison; but comparing this object to passing airplanes at 1;000 feet altitude separation; I estimate the drone to have passed just a couple or few hundred feet from our altitude.  We were descending thru about 3;500 feet MSL at the time I saw the drone.  After landing and referencing Google Maps.  I believe a good ground location we flew over at the time of the drone sighting to be in the area of Lake View Cemetery and Volunteer Park Conservatory. Require drone manufactures to implement technology that communicates with aircrafts TCAS traffic avoidance systems as drone operators; in my opinion; will always continue to break drone airspace rules and regulations.
First Officer reported near midair collision with a drone while centered on the localizer during approach.