Beach 99 captain reported flying through the final approach course due to GPS/vloc being set to GPS.

Attributes
ACN	1558384
Time
Date	201806
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	Airliner 99
Operating Under FAR Part	Part 135
Flight Phase	Final Approach
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	GPS & Other Satellite Navigation
Person 1
Function	Pilot Flying Captain Single Pilot
Qualification	Flight Crew Commercial Flight Crew Multiengine Flight Crew Instrument
Experience	Flight Crew Last 90 Days 132 Flight Crew Total 4000 Flight Crew Type 150
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

Narrative:

I was cleared for the ILS xx approach at ZZZ. While watching for the localizer needle to come alive; I realized that I had flown through the final approach course due to GPS/vloc being set to GPS. I was taking corrective action to re-intercept when ATC queried whether I needed a vector for re-intercept. I acknowledged and was issued a vector. Shortly thereafter; ATC cancelled the approach clearance and issued a new vector and climb-to instruction for a pre-emptive missed approach to take me back around to re-intercept the final approach course. During this process; I reset the approach on the garmin 530. I believe that due to my relative position the question on 'fly course reversal?' on the garmin; defaulted to 'yes' when entered and thereby caused the distance/DME function to be based in zzzzz. As I re-intercepted the final approach course; the distance/DME function counted up (increasing) as I flew down the final approach course. This caused confusion and with no glideslope indication; I attempted to fall back on localizer minimums as the localizer was fine. I was apparently mistaken in my assessment of actual position; as I received a 'low altitude alert' from the tower controller during the final approach phase. I acknowledged and arrested the descent. Almost simultaneously; the glideslope came alive and I completed the approach normally.

Original NASA ASRS Text

Title: Beach 99 Captain reported flying through the final approach course due to GPS/VLOC being set to GPS.

Narrative: I was cleared for the ILS XX approach at ZZZ. While watching for the localizer needle to come alive; I realized that I had flown through the final approach course due to GPS/VLOC being set to GPS. I was taking corrective action to re-intercept when ATC queried whether I needed a vector for re-intercept. I acknowledged and was issued a vector. Shortly thereafter; ATC cancelled the approach clearance and issued a new vector and climb-to instruction for a pre-emptive missed approach to take me back around to re-intercept the final approach course. During this process; I reset the approach on the Garmin 530. I believe that due to my relative position the question on 'fly course reversal?' on the Garmin; defaulted to 'yes' when entered and thereby caused the distance/DME function to be based in ZZZZZ. As I re-intercepted the final approach course; the distance/DME function counted up (increasing) as I flew down the final approach course. This caused confusion and with no glideslope indication; I attempted to fall back on localizer minimums as the localizer was fine. I was apparently mistaken in my assessment of actual position; as I received a 'low altitude alert' from the Tower Controller during the final approach phase. I acknowledged and arrested the descent. Almost simultaneously; the glideslope came alive and I completed the approach normally.

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.