Very weird GPS failure/activity

[tomdmx]

On our passage from Grimsby to London we experienced a very weird GPS failure north east to the wind farms near Great Yarmouth.

Both the boat GPS and the portable hand held we have had, lost tracking, location (showed as 0), speed and other info...location then came up (then dissapeared again), speed was out though then showed 9 knts then back to 0, tracked path compass was spinning from one side to the other etc...generally no usefull information...

This lasted for a couple of hours (we reseted it,changed power sources etc) before we called the coastguard for assistance (as 12 hrs before my invertor for laptop/plotter blew up and we had no nav onboard with not much to reference against)

GPS did not work until the next morning (after being towed to safety to Grt Yr harbour) when they started to work fine once again...

One of my mates mentioned a major sun spot/flare activity which could have knocked them out, someone else suggested the wind farms...I can accept one failing but both of them at the same time and showing the wrong thing is very weird..they were both Garmin, one was a GPS 128 and the hand held an Etrex.

Any ideas/suggestions what could cause it? Obviously lessons learnt, I wont have two boat GPS systems of same brand, and I will have a secondary power source for my laptop/plotter (which works great by the way with open cpn) but I never expected two GPS systems to fail at the same time...

 

[Billjratt]

Paper traditional backup! (you'll get told all about it, I'm sure!)
Did you look at the satellite reception bars by any chance?
Solar flare activity was forecast so possibly a cause, and all the more reason to have a non-electronic substitute.
Don't think windfarms would shield enough satellites simultaneously to give any problems or we'd have heard about that phenomenon already.

 

[rib]

paper charts and one chart plotter

 

[tomdmx]

Yep had paper charts...tracked location and we though we were well clear of wind farms..(hence we did not call for help for a couple of hours) problem came as the sun was going down we spotted throught he haze a couple of wind turbines to the south west of us...which was not meant to be the case...

Coastguard said afterwards that if we continued on anythign but an east track we would have gone aground as the bar extended for 1.5 miles east which only got updated recently (and our chart was fairly old at that anyway)...

 

[rib]

expect more sun flare ups in the future,and all the sat are getting old now more reason to have up dated charts and a sound nav skills,good job your vhf worked or you would have been in trouble then

 

[fergie_mac66]

Why were you towed in ?Was it bad weather?Did the engine in or sails blow out ?Is this a thank you for being towed in or is that on a different thread?That's the details we need to here ! Gps' often lose their fix for a bit plus there are regular jamming exercises details from ofcom site

 

[tomdmx]

Totally agree with that..in the scheme of things we could have gone further east off shore and try to wing it as we had no reference points initially...I had a rough DR fix based on estimated speed and course but it was those couple of miles that made all the difference...

 

[fergie_mac66]

gps jamming

register here for list of jamming exercises and regular email updates

http://www.ofcom.org.uk/static/subscribe/gpsjamming.htm

 

[G12]

I work with GNSS systems our company experienced very few problems globally with the sun spot activity recently, and I'm talking about very fussy precise point positioning systems accurate to about 10cm. Think oil rigs and DP vessels.
Perhaps some local interferance wiped out your systems. The fact that both positions failed together would indicate something external.
Was there any military action in the area?
What sort of antenna's are you using? Internal or external? If external, how are they mounted?
I'm just about to buy a plotter for my boat, I will be getting the external antenna version because they offer a much better view of the sky. What people don't realise is that the accuracy of GPS varies continuously so the more satellites the system can see, the better. Sometimes, you might be able to see say 12 GPS satellies but they may be aligned in such a way that the fix is poor. The kit I get to use at work shows current PDOP and HDOP values as well as predictions for both values for the future. You don't get that luxury on over the counter sets though!

Interestingly, the day Gadaffi was killed, our workshop alarms went off because of high PDOP values being recorded. The yanks must have been moving their satellites about that day more than usual.

 

[tomdmx]

Incidently we did have full sat coverage..every single channel but it could have been in error since the gps was playing up...not sure if anyone military was doing anything though

 

[G12]

Did you look at the sky plot?

 

[RAI]

Hmmm.. If it was local interference, natural or man-made, you would have seen little or no satellite reception. If it were the military playing games, all bets off. The NATO ones would have informed Ofcom and issued NOTAMS.

 

[G12]

Hmmm.. If it was local interference, natural or man-made, you would have seen little or no satellite reception.

Well that's not really true..... the receiver could have been seeing a signal, just not one it can make any use of. That's why I asked about the sky plot. This will enable you to see which SV's are being used to create the position. If the receiver is just getting noise, there will be no sky plot as it won't be able to receive its almanac and ephemeris data from the SV's.

 

[G12]

This is just some of the extra data I have available at work. This particular set is operating only on GPS L1 signals with differential corrections. It is also capable of GPS/GLONASS both L1 and L2 but requires unlocking - the client pays for that if they need it.
You can see from the sky plot which SV's are being used - Green is in use, yellow is not being used in the calculation although it's visible and white is below the masking line of 7 degrees and not being used at all.
Signal strengths are under the sky plot.

 

[padge]

No jamming trials have been notified.

But there are some due 26-30 Mar in norfolk
and Lock Ewe 16-27 Apr

Aimed at jamming aircraft systems, shouldn't effect ground/sea traffic but something to watch out for.

 

[RAI]

G12 - is that a picture taken at the time you were getting no position solution displayed?

 

[lw395]

Lesson 1, GPS is not totally reliable.
Not sure whether a better system with a better, external antenna would have helped.
Lesson 2, passage plan needs to accept that.

It is not necessary to know exactly where you are to the nearest yard at all times. It is necessary to know about hazards like sandbanks and know that you are going to miss them.

I doubt this was a genuine GPS outage. I doubt the windfarm is significant. I suspect you have something generating interference on your boat, plus a marginal antenna setup.
Do you have a proper external GPS antenna?
Is it in good condition?
Did you try the handheld outside the cabin for at least 10minutes?

Do you have any switch mode power supplies on board?
Inverters, mobile phone chargers?
Was the engine running?

One thing you can do, which may or may not help in a situation like this is to switch off mobile phones. They are transmitting frequently especially if they are struggling to join a cell. Also any consumer electronics such as MP3 players etc etc.

Unfortunately, GPS signal level does vary, so if you have a marginal system, it may work fine one day and fail the next.

Still, no harm done. The Thames estuary is not a trivial place to sail in my limited experience, better to ask for assistance when it goes wrong than to blunder on. Like a lot of people on here, I started sailing offshore with not much in the electronics department, so you will get a lot of us thinking we can cope without GPS, when the reality is some of us have got a bit complacent or never really understood chartwork anyway.

 

[Bosun Higgs]

Obviously lessons learnt, I wont have two boat GPS systems of same brand, and I will have a secondary power source for my laptop/plotter (which works great by the way with open cpn) but I never expected two GPS systems to fail at the same time...

I'm just as geeky and gadget loving as the next man, and I sail with a plotter and laptop and phone gps on board as well as two other handheld GPS. Whats more I dont want to preach. But one thing I learned early on and which I always teach to my students is the importance of keeping a paper log. Position ever half hour plus such obvious detalis as weather, baro, course , etc.

What this allows me to do is to revert to paper chart navigation if I hit your situation. As it happens, that has only occurred once in 20 years in the middle of an electrical storm.

So pleas, take the paper charts and KEEP A LOG! Do you really want the embarrassment of calling the coastguard for such a trivial reason again?

 

[G12]

G12 - is that a picture taken at the time you were getting no position solution displayed?

No, the position is shown on the screen - Norway.