Narrative:

I was working sector 40/41 combined with no d-side. There was IFR weather throughout the sector with an approaching front. The weather at santa rosa was low IFR with several aircraft going missed. Indefinite ceiling of 200. Traffic was routine for the time of day and weather. Equipment was working normally. Aircraft X was performing an IFR flight to sts attempted to execute the GPS 32 approach at sts. The approach was normal; until I noticed the aircraft approximately 4 miles from the airport at 500 feet. The airport elevation is 128 feet. I know from experience that that altitude is way too low to be at that position. I called sts tower and told them to issue a low altitude alert. Shortly thereafter the tower told the pilot to start the missed approach because the aircraft continued to descend; below the minimums for the approach. The pilot came back on my frequency and I told her that she was 4 miles away and 500 feet. She said that she never saw the ground; and added that she wasn't receiving vertical guidance on the approach. The lowest the aircraft went on the approach was 300 feet. I'm not sure how accurate that is; though. I know that on a non-precision approach; after the FAF; the pilot can descend rapidly to the missed approach altitude in order to attempt to see the runway. I feel like this event was unsafe though and that's why I'm submitting the report. On eram [en-route automation modernization]; the MSAW [minimum safe altitude warning] has parameters that inhibits MSAW alerts when the aircraft is a certain distance from the destination airport. The only suggestion that I can make right now is to adapt the technology that terminal facilities/stars has and I believe that is if the aircraft exceeds a vertical rate of decent on the approach; the MSAW alerts as well. I feel like that would be a useful tool.

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Original NASA ASRS Text

Title: Oakland Center Controller reported an aircraft descended below the minimums on the approach to STS; resulting in a Low Altitude Alert.

Narrative: I was working Sector 40/41 combined with no D-side. There was IFR weather throughout the sector with an approaching front. The weather at Santa Rosa was low IFR with several aircraft going missed. Indefinite ceiling of 200. Traffic was routine for the time of day and weather. Equipment was working normally. Aircraft X was performing an IFR flight to STS attempted to execute the GPS 32 Approach at STS. The approach was normal; until I noticed the aircraft approximately 4 miles from the airport at 500 feet. The airport elevation is 128 feet. I know from experience that that altitude is way too low to be at that position. I called STS Tower and told them to issue a low altitude alert. Shortly thereafter the Tower told the pilot to start the missed approach because the aircraft continued to descend; below the minimums for the approach. The pilot came back on my frequency and I told her that she was 4 miles away and 500 feet. She said that she never saw the ground; and added that she wasn't receiving vertical guidance on the approach. The lowest the aircraft went on the approach was 300 feet. I'm not sure how accurate that is; though. I know that on a non-precision approach; after the FAF; the pilot can descend rapidly to the missed approach altitude in order to attempt to see the runway. I feel like this event was unsafe though and that's why I'm submitting the report. On ERAM [En-route Automation Modernization]; the MSAW [Minimum Safe Altitude Warning] has parameters that inhibits MSAW alerts when the aircraft is a certain distance from the destination airport. The only suggestion that I can make right now is to adapt the technology that terminal facilities/STARS has and I believe that is if the aircraft exceeds a vertical rate of decent on the approach; the MSAW alerts as well. I feel like that would be a useful tool.

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.