Loss of GPS position in recent days

[Gin]

Before I start delving about in the electrical circuitry of the boat's instrumentation which, until now at least, has been entirely reliable I thought I should perhaps ask the following.

This needs to be read in the context of two recent incidents and the only occasions, sadly on which I have been able to get afloat- i.e. no interim jaunts. The dates being 14 June and today(24 July).

My Raynav RN300 GPS plotter, repeatedly lost signal on each day which made the slave unit and the VHF/DSC throw a wobbly each time with alarms going off all over the place.

The GPS in addition, today at least, switched itself off- I think, according to my son.

It happened so many times I turned the damned system off in the end and I can't accurately recall if it rebooted of its own accord or whether it only responded when it was powered off and then back on again.

It sounds to me like an intermittent 12v supply fault but has anyone else been losing signal lately(around the Medway) which could in any way help to explain this annoying phenomenon?

 

[alan]

I had a loss of signal in the bay of Naples one day last week (Thursday I think) for about five minutes. There were satellites visible (about 8 or so) but they were showing poor quality on my RC430.

Alan.

 

[Phoenix of Hamble]

No problems all weekend in the Thames Estuary that I experienced....

 

[alienzdive]

Always carry a sextant!

 

[Oldhand]

Rest assured almost anything is possible with a Raynav RN300. Try going through the "hot keys" until you get to the "Route List", select it, scroll through the list then press "Clear" to get out of it. If this doesn't cause the RN300 to re-boot, there's somthing wrong with yours! (After the re-boot it will be "following" to the non-existant "Waypoint 000" and getting out of that mode just adds to the excitement. Raymarine's solution to all the problems on the RN300 was to stop manufacturing it rather than fix it. I've had 3 and they all developed the same problems.

Do you get interference on 490kHz Navtex reception causing garbled messages to be received? Yup, you guessed it, the RN 300 is the culprit, after 6 to 9 months of component ageing, the high level of EMI generated by the RN300 drifts right on to the 490kHz USB frequency once the unit has been switched on for half an hour or so.

Loss of fix? Easy, the unit can boot-up in a mode where it searches for a satellite constellation which is different to what should be available at that particular time and place. If you are unlucky, it can claim a fix if it can actually see 3 or more satellites which it expected but the fixes are generally high HDOP and frequently drop out. Always look at the GPS status screen after boot-up and confirm it soon tracks the majority of satellites claimed to be in view and with a low HDOP. If not re-boot until it does.

Want to moan about it to Raymarine? Buy a share and put a suitable question to the Board at the shareholders' meeting is all I can suggest. /forums/images/graemlins/frown.gif

 

[MarkJohnson12345]

Nothing to do with your part of the world, but three week's ago I was on the way to Scillies from Padstow. My handheld which is powered from the boats power supply was giving spurious readings for several hours. Speeds of 2500 knots weee seen.

Fortunately I had two other Gps's on board that were working just fine.

The handheld eventually worked Ok.

No explanation's, just the nature of the beast I guess.

 

[rwoofer]

My 3 week cruise ended last week. I never lost the GPS signal as much as I did in that 3 weeks. In all I lost about 8 times, even though plenty of satellites were in range.

 

[alienzdive]

Just a thought, the heat lately has been so hot, I wonder if the units are trying to operate outside of the manufactured operational heat range??

Some sensitive electronics have limited working temperatures

 

[Bilgediver]

I do but can never find the instructions /forums/images/graemlins/smile.gif Have great fun trying to correct the errors. Oh well it steamed many miles on a Ben line Container ship when in pre GPS mode.

 

[Oldhand]

During the last 3 weeks up to 3 satellites in the current constellation were reported on Navtex as being unusable. One assumes the "unhealthy" flag was set on these satellites but if a GPS receiver doesn't react to the healthy/unhealthy flag (and I fear many don't) then bad fixes and dropouts are very likely to occur.

Early expensive GPS recievers like the AP6/MX100 had a display page showing satellite constellation health status and any satellite could be forced unhealthy. This had the advantage that if control of a satellite had been lost without its status being set unhealthy, one could manually remove it from the position calculation to avoid problems occuring. It's a great pity not enough sailors were prepared to pay for such sophistication so decent equipment was pushed into the survey market and now we get what we pay for - cheap unsophisticated equipment.

 

[Gin]

Thanks to everyone for their input, especially MagnaCarter who has been able to eliminate local positional problems and Oldhand who has given very useful info. on the RN300, all of which now seems to point a strong finger of suspicion at the unit itself.

A bit more experimentation is called for with the plotter and if found unreliable then out it will go!

Thanks again

 

[Gin]

Do you remember this problem from previous posts?

Well, truth to tell I haven't yet followed up Oldhand's pointers but will certainly check the unit's mode when I reinstall it.

I sent the offending unit back to Raymarine, who have bench tested it for days and it has performed with a clean bill of health- I am awaiting its return as I write.

The engineer's report was read out to me and the conclusion is that either the power, or interfacing cabling(either the Seatalk circuit or the NMEA circuit) is/are the culprit(s), or even the aerial which is a small Raymarine domed job sitting on the pushpit which I haven't knocked so far as I recall.

I rerouted the Seatalk cabling over winter so this could be the cause BUT I did not interfere with the power supply to the RN300 nor did I disturb the NMEA cabling which feeds the cockpit repeater- yet the repeater showed"No Input" on each occasion the DSC went into an alarm condition. So I don't think it is my work which has created this situation, and I did first make enquiries as whether the unit could feed NMEA and Seatalk simultaneously

Clearly I need to hook everything up again and pay greater attention to what then happens but it seems to me that unless this was an intermittent unit failure or cabling problem then the only other source must be the aerial.

Does this make sense??

How do I test the aerial without going to the expense of buying a replacement- and if a replacement is necessary must this be sited in a vulnerable position outside or can it be fixed under deck somewhere??