How accurate would you like your GPS?

Really depends on the accuracy of the charts/maps you are using at the time. Personally, within 50 meters is good enough for me.
 
Pah! I worked with surveyors who used GPS to better than a centimetre accuracy, and some of my other colleagues in the geology department worked to millimetre accuracy!

Seriously, geodesists, surveyors and those studying crustal movement have been operating to these accuracies for decades. The news is that higher accuracy is becoming the norm rather than something requiring special equipment and considerable post-processing of the data (neither sets of colleagues worked in real time!)

Higher accuracy does raise several issues, though, and I doubt whether the increased accuracy is of any real benefit to yachts or even motorists.:

1) What is the accuracy of the map or chart you are using? The most accurate maps you are likely to see in general use are the 1:1250 OS base mapping, and that is good to about a metre, both absolute and relative accuracy, with an RMS error of 0.5 m.
2) What is the reference system (datum) you are measuring with respect to? At accuracies of tens of CM, there start to be issues with fixed datums like the usual WGS84 (the continents are moving around!). The Earth's pole routinely moves by amounts on this order. There are datum systems that take this into account, but you have to use a new one every year!
3) Even where is your antenna? WIth several metre accuracy it doesn't really matter, but at an accuracy of a foot it matters.
 
I have sailed in Sweden, since 2000. I can't remember if selective availability had been turned off then, but although we had GPS, I don't think I made any use of it since conning by paper chart was so much easier.

I had a h/h GPS in 1999, when Decca was on the way out, and used it to enter Braye in the fog. The position could jump 150m at times, and speed and COG similarly, so one got used to waiting for it to settle and taking a sort of average. for most purposes, 50m is adequate, but modern GPS, with its consistent position and derived parameters is more relaxing and useful. I don't need millimetre accuracy, but knowing that my speed is, say, 6.5kn is better than 'somewhere between 5 and 7.5'.
 
Sandy said:
Really depends on the accuracy of the charts/maps you are using at the time. Personally, within 50 meters is good enough for me.
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I pretty much agree. "Not too accurate" would be my response. Advertised accuracies of up to decimeters leads under-skilled or lazy navigators to place too much trust in the device which will surely not match the charted info. For me, GPS is really useful as an aid to traditional nav and as an aid in certain situations such as short tacking up a channel where I can estimate that we'll be tacking in around 3 mins - relax and watch the depth sounder, then tack.
Overly accurate instrumentation carries the risk of over-confidence.
 
I've never tried to bomb anyone. So never needed extreme accuracy.
Although I have been impressed with DGPS accuracy. Definitely much better than 50m
 
On board last weekend testing after doing an update to a chartplotter two of my GPS's disagreed by .003 mile. As the two GPS mushroom aerials in use were 15 ft apart that's a dreadful 3 foot error. Also one showed me the wrong side of the pontoon.

In Falmouth I trust GPS pretty implicitly: the charts are nearly as accurate as the electronics. I don't necessarily trust it in unfamiliar places: I've been in places in the 3rd world where GPS lets you sail past a substantial island whilst the plotter track goes well overland.
 
The benefit of super accurate GPS for a sailor is not the actual postion, which as all of the above posts point out is meaningless and possibly dangerous, it's the step-change in accuracy of the speed reading. As GPS speed (SOG etc) is calculated by the change in position / time a more accurate position allows a markedly more accurate and frquent speed reading. Not vital in cars which move fast but very vital in slow moving yachts trying to get every last ounce of drive from 4k of wind and to assess whether minor tweaks are helping.
 
Joker said:
50m? There's someone who's never sailed in Sweden.
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I've not sailed in Sweden, but am looking forward to doing so in the future. At distances less than 50m, probably 500m, I use the Mark I eyeball which I find far more accurate for the task in hand.
 
Aircraft have been able to fly GPS approaches for some time albeit with higher minima, but an improvement in the accuracy will be appreciated. Inevitably there has always been a danger relying in GPS and especially the danger of local interference or any other issues.
 
jordanbasset said:
Agree, a couple of times I have been shown to be on land but that was due to the charts rather than GPS
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When mountaineering in Scotland decades ago, I was rather surprised to see an un-mapped lochan, small lake, but the survey date was c1860.

I tested my wee Garmin handheld on one of the published OS reference points, small brass markers with a very accurate positions, quite surprised that the device was showing +/- 2m of the point.
 
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The accuracy number kind of falls out of a desire to remove sources of error. The big two error sources are ionosphere and multi-path induced, followed by constellation geometry. By combining two signals, the receiver can reduce the ionospheric induced errors and their wide bandwidths help with multi-path error reduction. Having more satellites in view leads to better geometries and more satellite reception in urban canyons. All the error sources can gang up to produce a worst time/worst place error somewhere in the world. It's not so good if you happen to need the accuracy just there but in most places, most of the time, get remarkable accuracy.
Note that surveyors' GNSS receivers are designed to be used stationary, they are actually often worse that a hand held Garmin, if the receiver is moving fast, but in return for that limitation they can provide mm accuracy with post processing.
The great thing is that GPS and Galileo are sharing frequencies and signals, so cheap receivers will produce spectacular performance, compared to a sextant anyway. I'm looking forward to owning a new toy. It will benefit in-car navigation most as that suffers badly from multi-path in built up areas. It will also benefit aircraft as the extra integrity of the signals will overcome the limitations of the old GPS L1 signal. As a sailor, that integrity is important to me too.
 
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