Narrative:

Aircraft X was on vectors for the ILS runway 13R approach at bfi. He had previously been advised of a VFR target not displaying a mode-C return while on descent into bfi. As I turned aircraft X to join the localizer I scanned and spotted the same target still heading northbound on what appeared to be a converging course. I allowed aircraft X's to track steady towards the localizer to verify that my concerns were accurate. I had not issued an approach clearance but I indicated to aircraft X that I was uncomfortable with the situation and gave the aircraft a left turn out away from the traffic. When the VFR target was about a mile and a half north of the localizer the mode-C began indicating an altitude of 2000 feet. As I brought aircraft X around to the left and back onto the localizer another VFR target was tracking the localizer outbound to the northwest at an indicated altitude of 1700 feet. I issued traffic multiple times but the target appeared to be remaining just slightly south of the localizer. Aircraft X reported the traffic in sight as it passed off their right side and subsequently landed without further incident.the VFR target that did not display a mode-C altitude could have been at any altitude as it crossed the runway 13R localizer. If it was at 2000 feet a mile and a half north of the localizer; it is not unreasonable to think that it could have been at 2200 feet as it crossed the localizer. Either way; it was in direct conflict with the bfi arrival and not in communication with ATC.something needs to change. The VFR aircraft are transiting a very narrow; busy corridor of airspace and are doing so without any communication with ATC. It is simply unsafe. The VFR aircraft in this area at the very least need to be in communication with ATC so that we can assign; as necessary; altitude restrictions ensuring the safety of all the aircraft involved. The solution(s) are not hard and while they are potentially more restrictive to VFR aircraft the bottom line is that what happens day in and day out in that airspace as it exists and operates now will eventually result in a very bad accident.

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Original NASA ASRS Text

Title: S46 TRACON Controller reported a non-mode C target tracked toward the localizer course while another aircraft was inbound.

Narrative: Aircraft X was on vectors for the ILS Runway 13R approach at BFI. He had previously been advised of a VFR target not displaying a Mode-C return while on descent into BFI. As I turned Aircraft X to join the localizer I scanned and spotted the same target still heading northbound on what appeared to be a converging course. I allowed Aircraft X's to track steady towards the localizer to verify that my concerns were accurate. I had not issued an approach clearance but I indicated to Aircraft X that I was uncomfortable with the situation and gave the aircraft a left turn out away from the traffic. When the VFR target was about a mile and a half north of the localizer the Mode-C began indicating an altitude of 2000 feet. As I brought Aircraft X around to the left and back onto the localizer another VFR target was tracking the localizer outbound to the northwest at an indicated altitude of 1700 feet. I issued traffic multiple times but the target appeared to be remaining just slightly south of the localizer. Aircraft X reported the traffic in sight as it passed off their right side and subsequently landed without further incident.The VFR target that did not display a Mode-C altitude could have been at any altitude as it crossed the Runway 13R localizer. If it was at 2000 feet a mile and a half north of the localizer; it is not unreasonable to think that it could have been at 2200 feet as it crossed the localizer. Either way; it was in direct conflict with the BFI arrival and not in communication with ATC.Something needs to change. The VFR aircraft are transiting a very narrow; busy corridor of airspace and are doing so without any communication with ATC. It is simply unsafe. The VFR aircraft in this area at the very least need to be in communication with ATC so that we can assign; as necessary; altitude restrictions ensuring the safety of all the aircraft involved. The solution(s) are not hard and while they are potentially more restrictive to VFR aircraft the bottom line is that what happens day in and day out in that airspace as it exists and operates now will eventually result in a very bad accident.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.