Medium Large Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Confusion; Distraction
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Procedure; Human Factors
Aircraft X was cleared for an ILS approach via the SABAT transition. The transition altitude all the way to the localizer is 11;500 feet. The pilot descended to 9;500 feet. I checked the approach plate to see if there was a descending altitude on the transition that I missed; but I couldn't see one; so I queried the pilot about the procedure. He said I was correct; that the transition altitude remained at 11;500 feet. I gave the pilot a low altitude alert; then climbed him to the MIA [Minimum IFR Altitude] of 10;000 [feet] and vectored the aircraft onto the localizer.
Salt Lake ARTCC Controller reported an aircraft descending below a transition altitude by 2;000 feet.