GPS accuracy

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mick

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My Garmin Gpsmap 555 chartplotter manual tells me the instrument is normally accurate to less than 15m. In use the accuracy can be even worse. I would have thought these things would normally be much more accurate than that. Your comments would be appreciated.
 
"The errors of the GPS system are summarized in the following table. The individual values are not constant values, but are subject to variances. All numbers are approximative values.

*Ionospheric effects ± 5 meters
*Shifts in the satellite orbits ± 2.5 meter
*Clock errors of the satellites' clocks ± 2 meter
*Multipath effect ± 1 meter
*Tropospheric effects ± 0.5 meter
*Calculation- und rounding errors ± 1 meter

Altogether this sums up to an error of ± 15 meters. With the SA still activated, the error was in the range of ± 100 Meter. Corrections by systems like WAAS and EGNOS, which mainly reduce ionospheric effects, but also improve orbits and clock errors, the overall error is reduced to approximately ± 3 - 5 meters."

All from here (worth a read - they even set the satellite clocks to run slow to compensate for relatavistic effects /forums/images/graemlins/smile.gif )

Looks like yours is working fine.

Andy
 
Dont forget that charts arent uniformly accurate to anything like 15metres and that may have to be added onto any gps error.

Used to be the case that positions off cape cornwall and in the north of scotland could be inaccurate to as much as hundreds of yards. Since the charts were redrawn to WGS this has been largely eliminated but "largely" not "totally"

Interestingly the story was that all positions were initially derived by a triangulation process starting in the south east so by the time the surveyor got to (say) some headland in Cornwall, there were considerable accumulated inaccuracies. Sea positions were themselves traingulated from these inaccurate land positions.

Along comes satellite mapping and all of a sudden we know much better where all the land is. But in correcting land errors we highlight chart errors. Hence the mammoth redrawing exercise for WGS84
 
These units have an inbuilt GPS antenna, so if you are standing in front of the unit or something is shading the unit, you will reduce the number of satellites in view and for every one you lose the accuracy will drop. If the unit is below decks, then things get even worse!
 
As a relative newcomer to sailing I was returning under power to my mooring where I had found it difficult to identify my buoy from the others. When learning the basics of my Lowrance chartplotter I had fed my mooring into the system as a waypoint. I called the waypoint up and was guided directly onto my mooring. At the point of pick-up it said I was 12 feet away. It'll do for me.
 
For heaven's sake, what accuracy do you think we had before GPS, or even Decca (about 50m, though the charts were worse). If you want accuracy you can always use eyes(1.0) or even radar.
 
+/- 15m? Luxury! When I were a lad we 'ad to make do with Decca, which only seemed to work really well on sunny days with good viz. I still find it utterly awesome to have a constant supply of position data to not much more than a few boat lengths at worst.

Cue post beginning 'Decca? You were lucky....'
 
[ QUOTE ]
For heaven's sake, what accuracy do you think we had before GPS, or even Decca (about 50m, though the charts were worse). If you want accuracy you can always use eyes(1.0) or even radar.

[/ QUOTE ]

Why "For heaven's sake"? I asked a simple question looking for information, nothing else. My thanks to those who have supplied it.
 
No offence intended - it's just that the idea of setting out to sea with any notion of fixing one's position to anything less than a mile or two is too mind-boggling for people of more than a certain age. We used to sail right up to every single buoy we saw in order to make sure the name was what we were expecting; something we don't seem to need to do any more.
 
I've always had a soft spot for dubious navigation methods. Crossing from Ramsgate to Calais in fog I found it helped if I altered course 10 degrees towards the sound of whichever side the hovercraft passed.
 
Still on this same theme of accuracy, is EGNOS up and working "properly" yet? And is terrestrial dGPS still up and running?

Before SA was switched off, the dGPS signal from the Lizard LH gave a very accurate position repeatability - I'm in the Falmouth area.

The old dGPS receiver is up in the attic, along with my old Decca!! I don't know why SWMBO keeps telling me to get rid of it.

I don't think I will see Galileo working in my lifetime - especially if managed by the EU.
 
In my experience using a hand held GPS12 for Geocaching, GPS is accurate to feet if not inches.

Looking for a tupperware box in undergrowth/brambles I could walk past slowly about 6 feet from the location, and watch the arrow point at where it was, and lo it was there.

That's not my GPS finding the same place on the same day, it's someone elses GPS fixing the spot weeks/months previously and me finding it again. Other folk had found it too, they had signed the book.
 
[ QUOTE ]
I've always had a soft spot for dubious navigation methods. Crossing from Ramsgate to Calais in fog I found it helped if I altered course 10 degrees towards the sound of whichever side the hovercraft passed.

[/ QUOTE ]

Absolutely. I have listened to the dogs barking to work out where the coast was off N Anglesey in the fog.
 
Its probably more accurate than your chart! Certainly more accurate that you can interprete the chart.

With differential, you can get down to a metre, freely available from the lighthouses. Just need a receiver to pick up the signal.

Or you can borrow my surveying GPS, accurate to a golf ball. But alas is a bit pricey!
 
I remember my first "sun run sun", I was quite upset at being 5 miles out until the skipper said he would have been happy with 15 miles. I asked why he would be happy with 15 and he replied "it stops you from running into America in the dark". I learned about navigation from that, when in doubt keep well away from danger.
Stan
 
[ QUOTE ]
[ QUOTE ]
I've always had a soft spot for dubious navigation methods. Crossing from Ramsgate to Calais in fog I found it helped if I altered course 10 degrees towards the sound of whichever side the hovercraft passed.

[/ QUOTE ]

Absolutely. I have listened to the dogs barking to work out where the coast was off N Anglesey in the fog.

[/ QUOTE ]

Reminds me that the reason I bought a GPS was after a (succesfull) passage from Coverack round the Lizard to Porthleven in thick fog relying on sound bearings of the Lizard foghorn as the charted tidal stream speeds inshore round the point are too variable to allow accurate dead reckoning.
 
I've always had a soft spot for dubious navigation methods. Crossing from Ramsgate to Calais in fog I found it helped if I altered course 10 degrees towards the sound of whichever side the hovercraft passed.
***************************************************

If you ever sail across the Pacific eastbound on a great circle from Japan for the Columbie River and the weather is usual closed in winter fog and rain and no fixes or radar or GPS then you turn left when you see the muddy streak in the sea off the USA and if lucky will get over the bar and through the headlands and up the river....A well marked entrance by the muddy river in full spate.


/forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif
 
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