An example of a GPS 'failure'

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

I am happy with the concept of percieving noise in the spatiotemporal domain as a result of temporary atmospheric variation. It sounds like you are trying to model the propagation through the atmosphere as having constant characteristics: it has a chaotic element hence you will get noise in your measurements. You can model the propagation as being a simple transmission line and expect a constant delay but it will remain a simplistic model as the conditions are changing. Your concept of noise is wrong.

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we do not refer to the "slowing" (note the "") of light through a medium (delay) as being "noise" or due to "noise"

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I didn't.
 
Re: An example of a GPS \'failure\'

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BUT my GPS sounds an alarm and displays a message if it goes into DR mode.

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John,

I think you'll find your so-called "DR mode" indicates a total loss of GPS positioning data. If you're still getting signals from enough satellites, it won't go into that mode - that doesn't mean you're not getting imprecise fixes. GPS sets don't need to go into DR mode to keep a DR - they do it continuously. I have no doubt that your user manual will give some warning about "fix quality", "angularity error", "GDOP", "LDOP" or any other such euphemism. On the display, is there fix quality rating or somesuch?
 
Re: An example of a GPS \'failure\'

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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....

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You have a point. On the other hand - most of the GPS satellites are nearing or past their expected service lives. Funding cuts to the US military, as well as recent problems with NASA's shuttle program have slowed the replacement programme. Simple and cheap is often fool-proof, whereas expensive and complicated invites greater opportunity for errors to arise.
 
Re: An example of a GPS \'failure\'

that doesn't mean you're not getting imprecise fixes

All fixes from GPS (or indeed from any other nav method/system) are imprecise (and according to the laws of quantum physics always will be imprecise) and I have never claimed otherwise so is not at issue. However, it is totally irrelevant to what I have been saying.

People have been talking about seeing excursions on their GPS and/or plotter - I have said that I have never had it happen to me (and it is so) and I have always been where the GPS/ECS shows me to be.

Since you ask, various aspects of quality are available to me on both the instrument display and the ECS (# satellites, HDOP, etc) but I don't monitor them (I don't know anyone that does) and again they are totally irrelevant to what I have been saying. The ECS has user configurable track/position smoothing (which is what I think you are calling "DR mode" which is incorrect wording), and no doubt the instrument has too (as I think I may have said earlier?) but again that is totally irrelevant to what I have been saying.

I am unsure how the instrument determines actual quality of fix from the sentences from the receiver but the ECS simply uses the fix information in the GLL, RMC and GGA sentences (ie "No Fix" and Fix types from GGA, and "Active" or "Void" from GLL and RMC) - I suspect that the instrument is the same. (The ECS time duration between receiving a no fix to alarm is user configurable). But again, this is all irrelevant to what I have been saying.

What I have been saying is that whatever happens from the space segment though the user segment to the output on the displays I do not see any random or non random jumps in position on the displays, and the position shown is always correct (insofar as I have seen over the very many hours of use that I outlined) so the system proven to be reliable - more reliable than I think any human navigator is capable of achieving using any other single method. It is as simple as that - oh, again, in case missed /forums/images/graemlins/smile.gif, I don't ever see any jumps in data (that is not to say that I never will though).

In the cases where I have said I have monitored position, etc with the VisualGPS application I have sent to it the NMEA sentences direct from the GPS receiver (so no smoothing, all the data that is in the GPS nav sentences is what it gets and is only what it gets). However, perhaps sadly given the trouble it seems to be causing to some /forums/images/graemlins/smile.gif, I have never seen any jumps on that either.

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

Absolutely - and I am absolutely positive I have never claimed they are not able to be seen on the systems of others.

All I have said is that I have never seen it on the system on our boat (and, in fact, have never seen it on any other system on a boat but for me that is mainly with commercial vessels that do not use recreational equipment and which have high quality electronic/electrical installations).

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

For what - a canoe or a supertanker, or something in between? /forums/images/graemlins/smile.gif.

But just from my own personal point of view I would not use anything as the primary nav system, if nav was important to the boat (say a free ranging cruising yacht) unless it used official charts an had a decent sized screen. But that is just my personal requirement.

As a different perspective I have put smallish plotters (not Garmin) on fast commercial vessels (40 knots'ish) that operate set harbour/coastal short routes so primary nav is by eyeball and radar (regardless of what forumites seem to think is necessary, professional crews on small commercial vessels such as those do not plot their position on a chart every hour (or ever?). The plotter only to help the helmsman reobtain spacial awareness if distracted for 45 secs and the boat gone another half mile - he has the plotter to give a second frame of reference against what he sees out the window when he gets around to looking out of it again.

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

when anchored in a tiny bay, eyeball alone tells me that I'm not 5 miles away, nor doing 300knts. Quite easy really.

I can eyeball navigation too, and rarely rely on gps
 
Re: An example of a GPS \'failure\'

when anchored in a tiny bay

Now Brendan, toy boats or plastic ducks in the little bay you call your bath don't count /forums/images/graemlins/smile.gif.

I suspect most of us navigate by eyeball most of the time. What I cannot get to grips with is saily boat peeps on these forums who seem to predetermine their tracks with waypoints, etc even planning days ahead of the actual voyage, apparantly. Like the frequent comment "I plan on a notebook then transfer that to nav with the plotter".

Have never worked out how they know what the wind and sea will be doing on the day (or even a couple of hours ahead once departed), nor how the tides will be running (in places way different to the predictions and tide atlas) all of which have a big effect on routing tactics in saily boats.

Maybe someone knows the secret how one predicts exactly all these things days ahead for planning with and for putting neat little waypoints on the electronic chart - what do they do if have to tack, perhaps a waypoint here and a waypoint there, zig zag fashion and how do they know they will have to do so before departure?

/forums/images/graemlins/smile.gif

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

[ QUOTE ]
All fixes from GPS (or indeed from any other nav method/system) are imprecise (and according to the laws of quantum physics always will be imprecise)

[/ QUOTE ]

John,

While I am sure you could educate me about quantum physics, I don't think it really applies here. When I say "imprecise" I mean the fix will be less accurate than the nominal accuracy of the equipment you're using. If your WAAS or dGPS system has a stated accuracy of 10 metres 95% of the time, then it stands to reason that 5% of the time it's expected to be less accurate than 10m. That 5% takes into account, those periods where the geometry of the satellites does not favour a high-quality fix. If you've never witnessed GPS inaccuracy in the thousands of hours you've spent peering at your plotter, then bully for you. You still need to be aware of the inherent limitations of the navigation method you are using.
 
Re: An example of a GPS \'failure\'

You still need to be aware of the inherent limitations of the navigation method you are using.

I would have thought that from all I have said that I am well aware of the limitations - whatever the navigation method is.

Your comment regarding my not " witnessing GPS inaccuracy" indicates, I think, you still do not understand what I have been saying with respect to the whole system from space segment to user segment including the displays /forums/images/graemlins/frown.gif. What I have said does not mean there are no errors but just that in my own installation by the time it is displayed no wanderings of the type that have been talked about have ever shown up - whatever is presented has always fitted the real world around. As, I think, Brendan has alluded to in a couple of posts back, that does not mean that another system on board would not have shown some even though using exactly the same satellites, etc.

Never mind.

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

[ QUOTE ]
I suspect most of us navigate by eyeball most of the time. What I cannot get to grips with is saily boat peeps on these forums who seem to predetermine their tracks with waypoints, etc even planning days ahead of the actual voyage, apparantly. Like the frequent comment "I plan on a notebook then transfer that to nav with the plotter".

Have never worked out how they know what the wind and sea will be doing on the day (or even a couple of hours ahead once departed), nor how the tides will be running (in places way different to the predictions and tide atlas) all of which have a big effect on routing tactics in saily boats.

Maybe someone knows the secret how one predicts exactly all these things days ahead for planning with and for putting neat little waypoints on the electronic chart - what do they do if have to tack, perhaps a waypoint here and a waypoint there, zig zag fashion and how do they know they will have to do so before departure?


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John

I think there was an attempt here to provoke, although there is some truth in what you say. /forums/images/graemlins/smile.gif

We have many 'routes' pre-planned and stored and thousands of waypoints also stored (and shown on the plotter chart) which can be used in a route as required. Most regular trips have stored routes, so it is a question of simply selecting the required one from the list and pressing 'start'. It may be that from start to finish I don't look at the screen anyway and eyeball it or it may be I use the route data extensively. Every trip has a start position and a required finish position, so now by just accepting that you have created a 'route'. Yes of course there will be wind and tide considerations which will surely effect how you achieve that 'route' but by saying 'that is where we are going today' you have set your route regardless.

Blindly following a ground track between wpts on a route may be difficult or even impossible in the wind/tide conditions. It could also be very important to keep on track or not at all so. Even if the destination wpt is dead upwind there is a lot of information to be had from the theoretical straight groundtrack, primarily X-track error and vmg to wpt, which can help determine tacking tactics. In a narrow channel with limited space to stray one way or even both ways you may be able to use the straight groundtrack required to work out that you can maybe go 0.5ml offtrack to port but only 0.25ml to starboard, now you have some tack limits to work to. In the extreme case of a really tight route through a difficult channel in bad visibility maybe the solution will be (ugh) motorsailing but at least if the 'route' is there and stored it can be used, working it out on the fly as the fog rolls in is not a fun way to do it!

There is a huge store of useful data available from GPSs and Plotters that goes unused IMO by many who use only the basic functions of current position and destination waypoint. I know several people who only for example use the 'Go To' function and when they have 'Got To' they enter another 'Go To'. That could be dangerous if taken to the extreme in a cross tide with danger off stage left or right and for sure it isn't smart with a head wind AND cross tides.

Did I react as expected??? /forums/images/graemlins/laugh.gif

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

[ QUOTE ]
Blindly following a ground track between wpts on a route may be difficult or even impossible in the wind/tide conditions.

[/ QUOTE ]

I have seen this in a fairly new skipper. Not in any of the conditions you quote, and I guess too nervous to go anything but go along with his initial plan.

On this day it worked, but on others it may not have done.

My criticism would have been not to use paper charts enough to get a good overview on what he was doing, which I think would have led to a bit more creative thinking.

I can imagine Ships Cat's comment on this is more common than you might think.
 
Re: An example of a GPS \'failure\'

I could only expect a worthwhile comment from you Robin /forums/images/graemlins/smile.gif. Was definitely interested in "encouraging" responses so I guess that means provocative.

I don't criticise the use of waypoints and using them on the fly so to speak (either to sail to or to use bearings to for turning, etc) but I do get the impression that some, going by some previous forum comments, in sailboats decide ahead of time what their route is going to be eg Waypoint 1, Waypoint 2, etc - or if not waypoints plan "well this is the way we will go tommorrow" (or the next day, next week), which does not strike me as efficient navigation unless one actually knows what the exact conditions are going to be.

Obviously different in MoBo's, especially faster ones but there again they can be susceptable to sea conditions as to best route on the day.

Duncan - thanks for the comment /forums/images/graemlins/smile.gif.

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

An example:
In 2000 in the Bay of Naples, I lost gps signal on both the main boat system and the hand-held back-up. It didn't matter because I could see where I was.
When I went out to sea about three miles, the signal came back on both receivers. Don't know why. Never happened again.
 
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