[JnA]

Speaking of sailors.. did you know that 3.14 of all sailors are pirates?

[Edster]

Ah-Haar who told ye that?
No grasp of sadistics 3.14:1 sounds like raidng party odds!

[DoctorBeaver (OP)]

Speaking of sailors.. did you know that 3.14 of all sailors are pirates?

38.62% of pirates are Black Sabbath fans.

[Edster]

You`re just paranoid!

[DoctorBeaver (OP)]

You`re just paranoid!

[JnA]

should I have said pi-rates?

[graham.d]

Coquina.rocks, I note that the article suggests that it is the receivers, not the satellites, that can be temporarily disrupted by solar flares. It seems to suggest a reduction in signal level (probably due to ionisation in the upper atmosphere) is responsible. I think this problem is of fairly short duration, will only affect some receivers, and would be likely to cause a reduction in the number of satellites "visible" to a paticular receiver. This is not good, especially for vehicles needing navigational updates on a regular, fast basis, but would be less of a problem for yachts [blow-boats indeed :-)]. I don't know anyone who sails offshore regularly who does not always take charts and traditional nav aids and, personally, I always have the right chart on the chart table at all times. I also have 2 GPS systems in use at all times so a discrepency will indicate that the results may not be reliable. I have seen problems too. Sailing off Greece last year one GPS was giving plausible but incorrect readings - about a mile out of position. I think the system had somehow lost its datum.

There are some GPS "black spots" too where, despite the number of satellites zipping about, the coverage can be insufficient to get good position at all times. I would suppose when all the national systems are on-line, and multi-system receivers are available, the potential problems and likely failures will be less. But then, if all the electronics gets zapped by lightning, it's a good idea to have traditional methods at hand.

[JimBob]

.


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A not very bright light house !

And what causes you to answer, in a misguided belief the poser of this question is brighter than the cabbage?

[DoctorBeaver (OP)]

I think I may have just been insulted  []

[lyner]

Sailing off Greece last year one GPS was giving plausible but incorrect readings - about a mile out of position. I think the system had somehow lost its datum.

graham.d
Have you come across the problem of display format?
It can show you degrees/minutes/seconds or degrees/minutes/decimal monutes.
I spend a good while trying to reconcile two, contradicting, hand helds, one time. The guy I was with had really had his confidence in GPS shattered until I sussed out the prob.

[graham.d]

No. All the GPS units I have used display decimal minutes, but I think I would have spotted a second (") symbol had it been displayed. In any case, it was the plotter that was out. Around the Ionian Islands the datum has quite an offset. Although the charts are reasonably relatively accurate (apart from the odd unmarked rock) they are out quite a bit from absolute position. When you first switch on, the GPS can easily tell you that you are moored a mile or so inland. You have to set an ofset on the datum to correct this. Once entered it should maintain this. However I can only think that something caused the GPS system to forget this offset. Switching off then on again seemed to fix it, which points to some error in the plotter. You don't really need any electronic aids in the Ionian but it was disturbing to see that the plotter did not agree with reality. Checking with a handheld confirmed that reality was correct and the plotter was out :-)

[lyner]

There's a lesson for us all then.

[JimBob]

Perhaps the difference in is in the mapping datum used for Ordinance Survey maps and naval charts. These are often NOT the same as the datums the handheld unit have loaded in them nor are the necessarily are they corrected for the GPS system itself. In North America two sets of datum are used - NAD27 & NAD83. The NAD27 is based in the Clarke Elipsoid,  the NAD83 is base on the GRS80 ellipsoid. Neither of the ellipsoid that attempt to approximate the earths surface are very good for mapping locally. The GPS signal must be interpreted with respect to the local datum used and this WILL in many case vary from the 1936 Ordnance Survey maps, which I believe were based on the Clarke Ellipsoid (That needs to be verified)

The WGS 84 datum used for navigation is 5.31 arc seconds off of the Prime Meridian at Greenwich and is not the same as either the 1936 Ordinace Survey or the NAD27

The GPS Unit must be set up withe the correct map datun for the maps and charts used. Otherwise there will be an observable difference in locations given.

[Geezer]

JimBo,

Does them nads have anything to do with yer gonads? Heard that ol' mule o' yours did a number on yours the other day.

[graham.d]

JimBob, I agree you have to set the correct datum for the chart. The point is that the plotter maybe forgot the datum. I did set it and it was OK when I started out, at some point lost its mind (though gave out a plausible position), and then only recovered following being power cycled.

[JimBob]

JimBob, I agree you have to set the correct datum for the chart. The point is that the plotter maybe forgot the datum. I did set it and it was OK when I started out, at some point lost its mind (though gave out a plausible position), and then only recovered following being power cycled.

Sounds as if the OS was written by Microsoft!

[JimBob]

JimBo,

Does them nads have anything to do with yer gonads? Heard that ol' mule o' yours did a number on yours the other day.

naw. frethak got that kick i I use a mule cup