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Old 17 October 2009, 15:56   #1
Country: Ireland
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As many of you are probaly the standard satelite signal has a built in variance which fluctuates depending on a number of factors. At worst a GPS might be out be 100 metres.

There is a system called DGPS which seems to overcome this problem. Has anyone any experience with it? Can it integrate with Raymarine?

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Old 17 October 2009, 16:24   #2
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That was years ago, most GPS now is accurate to within a couple of metres.

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Old 17 October 2009, 16:27   #3
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Does it randomly drop out, or is that just my Tomtom being c**p ? Thought I read somewhere that accuracy was going to start reducing as the satellites are starting to age and not work too well.
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Old 17 October 2009, 16:40   #4
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I've never had any trouble with tomtom or the chart plotter.
When fishing over wrecks as soon as the boat goes over the wreck on the plotter it shows on the sounder, accurate enough for me.
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Old 17 October 2009, 16:47   #5
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Ordinary GPS is now accurate enough to position within 10m or so in reality or bow, middle and stern of the larger wrecks.
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Old 17 October 2009, 18:04   #6
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DGPS was only really useful before selective availability was phased out. Back then it could have been out by 100m but generally I would say we had 30m accuracy without DGPS so it was only really useful for wreck fishermen, surveyors etc.
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Old 18 October 2009, 11:48   #7
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Although not as important now with SA switched off, I still find the improvement with the differential signal is noticeable. The other surprising thing is how quickly my plotter pickes up the dGPS signal after switching on. In the days of SA it sometimes took half an hour before it had locked onto the differential satelites (sats 33 & 44 I think). These days its a matter of minutes and most sat signal bars show a little 'd' meaning they're benefitting from the correction.

I think it's EGNOS or WAAS that we're talking about rather than the original differential system that used a ground beacon to transmit the correction to a separate aerial on the boat.
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Old 18 October 2009, 13:03   #8
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Speaking of EGNOS, I see that it has been recently launched (as a service that is) LINK.

I guess it should be a matter of switching on the WAAS function on your GPS/Plotter? Someone will be along shortly to enlighten me, I hope
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Old 19 October 2009, 11:40   #9
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From a Canadian source on the web:

selective availability
Selective Availability (SA) means that the most accurate GPS satellite signals are available only for a selected few - namely the U.S. military. Essentially, random errors are programmed into the satellite's data which limit the position accuracy of a single receiver to about 50 m. For comparison, with SA turned off (so the masses can receive the more accurate signals) it is possible (with very expensive equipment - not your average hand-held receiver) to get sub-centimetre accuracies. Well, it is a U.S. military system, so I guess they can make the rules. The need for SA will be reviewed by the U.S. Government in the year 2000.

What is Differential GPS?
Differential GPS is the civilian response to selective availability. In the quest for finding ways to get more accurate position fixes than SA allowed, the idea was hatched to use two GPS receivers instead of one. Basically, a "base-station" receiver is installed at a fixed spot where the geographic coordinates are accurately known. The second "roving" receiver is used normally and location measurements are made. However, since the base receiver is at a known and fixed position, it can read the same "inaccurate" GPS signals being received by the roving receiver and determine how much adjustment is required on each satellite's signal to have it provide the correct position. These correction factors are either transmitted directly to the roving GPS or saved for later correction of the data acquired by the second receiver. Using differential GPS, typical horizontal position accuracies of around 1 m are possible.

What's in store?
There are a couple of notable enhancements which will be finding their way into a GPS system near you within the next year or two. The first is known as "carrier-phase" GPS. This system is already employed by surveyors using very expensive equipment. Basically, a carrier-phase GPS receiver determines an approximate position fix in the usual way but then fine tunes that location by locking in on a different component of the satellite's signal. Using sophisticated equipment, surveyors can obtain sub-centimetre precision. While this is likely beyond the accuracy requirements (and expense!) for most other applications, parts of this technology are slowly migrating down into the mid-price GPS receivers.

A second notable development comes courtesy of the U.S. Federal Aviation Administration (FAA). As you might well imagine, GPS has led to a navigation renaissance in the airline (and other transportation) industry. The FAA reasoned that if differential GPS is so good, "why don't we establish our own base station to improve the position accuracy of all the GPS systems in aircraft use?" And so the Wide Area Augmentation System (WAAS) was born. The WAAS base station is carried by another satellite parked in a geostationary orbit over the continental U.S. From this location, it can transmit differential correction information to anywhere in its view, including much of southern Canada. Best of all, the signal is not scrambled and freely available to any roaming GPS that wants it (and can receive it).

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Old 20 October 2009, 07:26   #10
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Is the old land beacon transmitted DGPS signal still used, or has EGNOS / WAAS etc resulted in it being switched off?

I'm running without any correction on my trusty old Garmin 12 handheld, and as a trial drove to & from work with the "map screen" on full zoom. 19/20 times the trails overlay (or when on the motorway are sufficiently far apart & OK relative to each other to tell I'm on a different carriageway). Occasionally the new trail will shift by approx the diameter of a big roundabout, and only once ever did it go to an estimated >15m out, and that was in really cr@p weather inside a car. Having said that, "wandering trails" seem to self correct within about 10-15 mins.

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