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Raymarine forum - [TG11] No Fix, TX timeout at fixed time

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Moored at the time of writing in Natal, in the Northeast part of Brazil, we experience every night, around 06:00 UTC the following error messages (AIS 650):

-No Fix
-TX timeout

(06:18) tx attemt failed (msg 18 no pos'n)
(06:20) tx attemt failed (msg 24A no pos'n)

We started to have the Fix problem roughly 700NM east from Maceió, on 10 degrees south, before entering port of Natal. We did not notice the pattern when sailing. Now at anchor it becomes obvious.

Can it have to do with satellite positions?

Hello Crossroads,

Yes, I think that the cause is indeed quite likely to be the particular combination, locations and signal levels of the satellites in view at the time. It might be that you have a low signal level because of the location of the antenna or a bit of background RF interference on the boat and that that's combining with the sats-in-view to produce the lost fix at that particular time of the day.

I'd suggest connecting your computer over USB to the AIS and running the ProAIS 2 software which will give diagnostic information about the GPS signal levels. Have a look at the overall signal levels at times when the AIS does have a GPS fix, and the locations in the sky, and then see what changes at the time when the unit has lost fix.

The alternative is that it is possible that the problem is interference from something on-board that's being turned on, used etc. regularly at that time in the morning (genset, watermaker on a timer for example?) If the problem were only occuring in one location then I'd add to this shore-based interference, but that clearly doesn't apply in your case. Interference with GPS is surprisingly common because it's such an incredibly low-power radio signal.

Hello Tom

Thank you for the quick reply.

We are at anchor and only the nav data from wind and underwater sensors are on. The MFD, sat phone, radar and inmersat C are off, which antennas are located on the same pole.

0.33 nm NNE of us is a huge bridge of max 103m heigh and 1782m long. For the rest no high buildings anywhere close. Mostly some mangrove bushes.

I had a look when the problem occurred via the ProAIS and discovered that normally there are 5 Navigation satellites with signal level of mostly just below 40. When the fix is lost, there are only 4 satellites. Sometimes a non-navigation satellite becomes a navigation one and the fix is back. Sometimes one nav sat shows up below the 30 before becomming stronger and the fix is back.

Could it be that around a certain time, one satellite might be on a dificult location and at sea the interferance of other instruments or construction might cause a fix loss, and at anchorage the big bridge might cause the fix loss that can last 30min.

We leave next week and I will monitor what happens at sea and when we move into the Northern hemisphere.

Thank you
Hello Crossroads,

Only seeing 4-5 satellites with comparatively low signal levels is consistent with either a poor contact to the GPS antenna (signal loss in the cabling) or high background noise. Background RF noise can come from almost any electrics or electronics - we've seen terrestrial TV tuners, voltmeters in distribution panels, LED light drivers, USB devices and chargers and more cause crippling interference in the past - and the culprit doesn't have to be something that's 'turned on' with its power button. If it's getting power it could well be 'on' and emitting noise even if it doesn't appear to be active. This is especially true of electronics which usually draw some standby current.

All this being said, cuts/splices in GPS antenna cabling or inline joiners and extension cables are common causes of this sort of problem too.

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