An example of a GPS 'failure'

[tangomoon]

Re: An example of a GPS \'failure\'

That was not abrasion merely caution and I did read it he said he abandoned gps because it was giving a wrong reading have had same experience but second immediate gps was accurate. Never rely on it in dangerous waters though don't even look at it

 

[tangomoon]

Re: An example of a GPS \'failure\'

More of a fool you, I read your post as described below. You said you abandoned it....... have a nice day

 

[Benbow]

Re: An example of a GPS \'failure\'

Oh well, I suppose there is always one who has no clue what the thread is about.

A navigator should use any and every source of information available and cross check them.

I said I had a proper passage plan and was adding a GPS fix into a mix. I said that as soon as the discrepancy was spotted, we abandoned the GPS as a source of information. I would have been negligent not to have made use of GPS data. I would have been negligent to rely on it or believe it when it conflicted with other data. I was able to abandon it as source of information because I had plenty of others to work with.

That was the whole point.

[ QUOTE ]
Never rely on it in dangerous waters though don't even look at it

[/ QUOTE ]

Thankyou for that sage advice. You will forgive me, I hope, if I ignore it.

 

[Ships_Cat]

Re: An example of a GPS \'failure\'

Maybe you had a personal solar flare /forums/images/graemlins/smile.gif. Or frying eggs on board sunny side up, perhaps? - definite No-No.

But more seriously a problem that I have known happen (putting aside on board rf interference which I think is relatively common on smaller vessels with poorish electrical installations) is when the GPS is read, and assuming one is not following the track on a plotter or ECS, it can be easy to just take notice of the minutes and decimal minutes and not realise that the whole degrees have incremented or decremented by 1 (how noticable this is on the chart plot depends on how fast the vessel is going and how long between fixes).

Not only a problem with small boats, I know of a tanker that suddenly realised they were heading towards land into a bay through exactly the same thing happening.

Not saying it was the case as you seem to know what the problem was, but maybe a possibility to keep in mind for others.

John

 

[contessa26]

Re: An example of a GPS \'failure\'

Many thanks for your interesting post. I like these philosophical posts – for want of a better description – as they are ‘learning’ posts, and we are all learning, all the time (or I am anyway). I’ve (amateur) sailed in all sorts of boats, for forty years, from a toddler to the present… in all sorts of seas and conditions. Thank you for the discussion. And often on some of these discussions the thoughts of ‘There for the Grace of God go I…’, spring to mind, too…
PS: Possibly like a lot of people. I've witnessed a plotter (in this case a recent C80) give a sudden unexplained 'glitch', plotting us miles out, in our case when we were pottering just a cable off the west shore of Eileach an Naoimh... then it recovered... But anyway...
Best wishes.

 

[Phoenix of Hamble]

Re: An example of a GPS \'failure\'

And another variant, a mistake that i've personally made when overly tired after a night passage, on watch all night (don't ask!)..... and being used to working in Eastern Longitudes, suddenly finding myself in West, and going the wrong way on the chart when transferring the fix through simple 'error by rote'....

 

[BrendanS]

Re: An example of a GPS \'failure\'

You notice these 'glitches' far more on a plotter, than on a simple gps, when you can see historical track.

The position indicator zooms off momentarily. Sometimes a few hundred yards, sometimes a lot more. The log at end of day is interesting as you have a max speed of 300 knts or some other crazy number.

Happened a lot more with early equipment, and very seldom happens now.

 

[duncan]

Re: An example of a GPS \'failure\'

running 3 plotters at the helm constantly when at sea, each with independent charting and all with permenant track recording I have yet to see any of these glitches..........what am I doing wrong? /forums/images/graemlins/confused.gif

 

[BrendanS]

Re: An example of a GPS \'failure\'

No idea. But they do happen very very occasionally.

 

[Cruiser2B]

Re: An example of a GPS \'failure\'

I don't think such errors are all that rare. I think most commercially available GPS sets use a DR function to maintain their 'fix proximity' - this allows the unit to seamlessly switch between satellites without having a gap in positional info. This can also work against you where you get into a position with a poor Geographic Dilution Of Position (GDOP) - also given other names, it describes a poor angle between the LOPs from the various satellites, which give a broad fix - the DR function can displace you along the axis of the fix, well off of your actual position. Most GPS units are capable of rating the quality of the fix - it's a good idea to note that when relying on the GPS information. Keep an eye also on what your GPS is indicating for COG and SOG - if it seems off, then it probably is.

 

[Ships_Cat]

Re: An example of a GPS \'failure\'

I am one of the "unlucky" ones also Duncan.

Never seen it myself (and our GPS receiver is 10 years old so not modern), either on the display or on the ECS (the ECS has adjustable averaging though, but intention is so track draws straight not showing the helmsman's wobbles, purpose is not intended for "hiding" GPS "issues") - have even left the ECS running for days on end while the boat is in the marina so easy to see and the track always shows as a stationary small blob within the accuracy of the system. Also done it with software direct plotting to graphical output and nothing - and seen the same done by others exploring GPS accuracy and again never seen anything significant on their plots either. Not saying it doesn't happen but I suspect such due to the GPSystem are rare and fleeting.

With regard to losing fixes I can only recall the alarm going off a couple of times ever, and that has always been while anchored in steep sided valleys and bin only for a few seconds. Our GPS is always on average 2 days a week while in the marina - loss of position has never happened in 10 years.

With respect to random fluctuations I suspect some is to do with little averaging within the unit (or the setting of that if accessible or else able to be done externally in an ECS), and that plus loss of fix in the the quality of the installation (many use unshielded cable for NMEA cabling, just as one example) or the quality of other electrical/electronic installations on board which may induce rf interference or voltage excursions sufficient to drop the instrument fleetingly and have to refix - these installation matters, probably frequent in the case of fix loss.

John

 

[BrendanS]

Re: An example of a GPS \'failure\'

ho hum. It's never happpened, so not a problem. Shrug shoulders. I'll ignore it then.

 

[Ships_Cat]

Re: An example of a GPS \'failure\'

Stirrer /forums/images/graemlins/smile.gif.

I did say Not saying it doesn't happen but I suspect such due to the GPSystem are rare and fleeting.

Is true though, is not something I worry about and I take no anxious care about except for always ensuring that what I see a round me matches what the ECS is showing me. And I am diligent in that - seemingly the ECS is too 'cos I while I have never caught it out it has caught me out several times by convincing me that I was really somewhere other than where I had thought /forums/images/graemlins/smile.gif.

Also quite happy to nav in low visibility/total darkness through tightish locations relying on the ECS as long as have proven the route before (ie that it is charted correctly) - but we also always use official electronic charts, not the rip offs /forums/images/graemlins/smile.gif.

John

 

[BrendanS]

Re: An example of a GPS \'failure\'

You are bullshitting now. We've had this conversation before. Sometimes Gps plotters go mad. In the meantime, I'm quite happy to live by the consequences of following their directions absolutely until I think they are going apeshite. In which case I will take over manually.

 

[BruceDanforth]

Re: An example of a GPS \'failure\'

If it has software in it is bound to go bananas once in a while.

 

[Ships_Cat]

Re: An example of a GPS \'failure\'

You are bullshitting now.

Nope, definitely telling it as it is with me - is not a leg pull at all.

Even sitting in the marina for 2 days each week (I run the instruments when on board to keep them dry, etc) the GPS is sitting looking at me and I have never seen its position flicker more than its second decimal point of minutes one digit (the digit which is about 18.5m if in latitude).

I even downloaded VisualGPS that someone on these forums mentioned to see if I could experience some virtual excursions - but, nope, just a wandering around within the expected system error circle, so gave up.

On the ECS the electronic large scale chart shows our berth correctly and we just sit in the berth in the exact some spot with no virtual excursions, try what I may to join the doubters by having the track plotted for many many days.

At sea, which is usually around 50 to 60 days and nights (24 hour days, so 1,200-1450 hours) per year I have never experienced an issue even though the GPS is always running (plotting or as anchor alarm). Taking into account the GPS run time in the marina as well (2 days a week when we are not away cruising) then total hours per annum that GPS is running is around 3,500 hours.

You are making me feel like I am missing out on something important now, like the excitement of not being sure whether I am here or there (or maybe neither of those /forums/images/graemlins/crazy.gif).

John

 

[BruceDanforth]

Re: An example of a GPS \'failure\'

I've used handhelds that whizzed off at 100+ kts for a couple of seconds fleetingly but never experienced a systematic error. Your systems clearly have more robust software than some. Be glad.

 

[Cruiser2B]

Re: An example of a GPS \'failure\'

John,

Look at my explanation above - sitting at your berth, your GPS is applying a 0/0 DR so it will maintain a reasonably accurate position even when fix quality is low. Underway, it is not likely as easy to spot deviations.

 

[BruceDanforth]

Re: An example of a GPS \'failure\'

One end of the system is a billion dollar infrastructure, the other end is a consumer unit that costs a couple of hundred quid. People choose to suspect the infrastucture if there is a glitch....

 

[BrendanS]

Re: An example of a GPS \'failure\'

nope. When the plotter shows a line off where I am, then back again, and shows speeds of hundreds of mile per hour, I suspect everything, and resort back to knowing where I am.