Position Fix Lost / GPS Alarm

[AtlanticX]

We are currently in the middle of the North Atlantic, halfway between Nova Scotia and the Azores and I’m trying to diagnose a GPS Alarm issue that is driving us crazy. Thank you Starlink for making it possible to tap into this wealth of knowledge while underway!

We replaced our old Raymarine RS125 GPS sensor with a new RS150 sensor prior to departure and in the sea trial all was good. Since getting offshore we have been struggling with two electronic issues (AIS is no longer transmitting or receiving and an intermittent and extremely annoying Position Fix Lost alarm).

Before the AIS650 quit working we were getting VSWR Fault Alarms but those have gone away. Although not ideal, I did create a hack for the loss of AIS by having our PredictWind Datahub (which is uploading and downloading AIS for us) re-broadcast the AIS data on our local NMEA network and that actually fixes it temporarily (although very inefficiently as it requires Starlink to be on all the time). I confirmed that other ships can see us and we can see them. I’m sharing the AIS issue as it may trigger ideas for someone regarding the more pressing issue, the dreaded Position Fix Lost alarm.

Several times a day, not with any regularity, the two E-Series MFD’s blare Position Fix Lost alarm and show now GPS position. Then, 2 seconds later the issue is resolve automatically. The alarm is piercing and cannot be muted thus the off watch is unable to sleep and it is becoming quite an issue. Normally an intermittent GPS warning that auto reconnects wouldn’t take priority over AIS but sleep deprivation is a real issue.

I verified the connection run to the RS150 was done with a backbone cable and proper terminating resistor given the distance between it and the nav station is over 20’. I read one article that suggested the AIS system may be broadcasting its internal GPS signal on the network which could cause issues (didn’t say what issues) but haven’t determined that is the case.

Any ideas are welcome as to what could be the root cause and how to solve.

Thank you!

[Avionics_Joe]

The VSWR Fault Alarm means you have a bad connection on the antenna. Try disconnecting all of the connectors on the AIS and antenna and inspect them for corrosion.

[Pandor]

I just went through a very similar scenario with my Garmin plotter connected to external GPS antenna. I was convinced that it had to be an issue with the antenna connection. But it turned out to be a power issue on the plotter itself. There was a bit of corrosion on the 12v connection to the back of the plotter, and that was enough to cause the GPS to keep dropping out. Not sure how your RS150 is powered, but my antenna received power from the plotter.

[AtlanticX]

Thank you, Pandor; I'll double-check the power connection just in case. Good idea.

That said, I may have solved the GPS problem. We have an E-series MFD at the nav station (the master) and the one in the cockpit, both displaying the same GPS loss error at the same time. While trying to select the GPS Data source on the master, it occurred to me that perhaps the problem is with the internal GPS of the master or slave MFD. I disabled "Internal GPS" on both MFD units and the problem has gone away! The external Raymarine RS150 sensor is providing accurate GPS for the entire boat network so we are good. Now on to fixing the AIS, I'm going to start with the antenna as suggested by Avionics Joe.