Originally Posted by
Torquetalk
The FMS (inc this UNS model) will update its position very quickly once it has sight of satellites (out of the hangar) and in receipt of signals from VORs and DME equipment (the aircraft was flying in well-served airspace). It is very unlikely that the UNS did not know exactly where it was. That is usually a pilot problem.
Let me first say that I acknowledge that I am out of my area of expertise, and so this is my amateur speculation. I believe the Reference Position in the NTSB report (l34.25080n 118.76038w) recorded at 17:37:43 does not accurately reflect the actual position of the aircraft at that time. I am wondering if the UNS incorrectly determines position in the following scenario:
1) UNS Powered off in Santa Catalina
2) UNS Powered on at LGB
3) UNS Never receives DME.
From the report:
- "DME Stations for shortrange navigation: No DMEs Received"
- "Scanned DME Stations: Fifteen DMEs being scanned. None responded."
4) UNS records Reference Position (according to the report "Reference Position is Blended GPS/DME position propagated with GPS velocities")
This is last reported position information as detailed in the report:
- System Position (GPS position propagated by heading and true air speed) N 34:08.17861 W118:41.55108 (34.1363101n 118.692518w)
- Reference Position (Blended GPS/DME position propagated with GPS velocities) N 34:08.17861 W118:50.54967 (34.1363101n 118.8424945w)
- DME Position (Position computed from DME range measurements) N 34:23.21876 W119:13.96316 (34.38697938n 119.2327194w)
As you suspected, the GPS position information is correct (very close to the crash location). However, the Reference and DME Positions are not. I wonder if this corner case is rare because most of the time UNS systems receive DME. I do not have a hypothesis as to why the FMS was not receiving DME data.