Differential GPS

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maxtorque

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As a newbie, my steepest learning curve to date seems to be sorting the wheat from the chaff when it comes to gleaning information. The boating fraternity seem to have the lions share of contradictions.

The latest:

I want to buy a cheap and cheerful GPS, around the Garmin 126/128 level.

I go to jumbles/marinas/chandlery's and am told on the one hand "you need an option pack to make the GPS more accurate", to, "the differential bias signal has been made available to everyone for some time, there is no requirement to buy option packs"...all GPs are now extremely accurate.

Anyone care to muddy the waters further?




<hr width=100% size=1>WT
 
Expect that you'll find the Garmin accurate to around 5 - 10 metres almost all the time. Differential (dif) will improve that to around 2 metres or so. If selective availability is switched back on (a degredation of the signal) the accuracy will reduce to about 15 - 50 metres. However that seems unlikely, and differential was introduced to counteract this greater degree of error.

Can't really see why any pleasure user would now need dif.


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As I understand it ......

Differential GPS was created to get over the deliberate errors introduced into the GPS satellite signals by the American military. This was called "Selective Availability"

It works by having a land based based station, obviously with a know position, which then calculates any deliberately introduced error and broadcasts an error correction signal to any Differential GPS unit

I think Selective Availability was switched off a year or so ago so the signals contain no errors and any GPS set should be accurate to a few metres given good reception. Off course the yanks could switch the errors back in at any time if they felt security required it.

Some people may not realise that selective availability has been switched off and consequently differential GPS is no longer necessary.

By the way I have just bought a Garmin 126

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Yes, Chris~B4 is spot on. The differential is a totally different radio signal from the GPS signal. It is transmitted from European land stations along the coast. It gives a correction signal to make the normal GPS set more accurate. To get differential, you need a differential antenna and receiver, and then you connect this via NMEA wires to your GPS set so it can "talk" to the GPS set.

But as said there is no point in this kit anymore, for 2 reasons. First, "selective availablilty" has been turned off, so an ordinary GPS set (without differential) is now very accurate, easily good enough for boating. Second, the next generation of differential correction will be out next year. This system is WAAS and the differential signal is transmitted by satellites. An old-style differential receiver will not receive WAAS signals. Generally WAAS receivers are built into GPS sets, so if you want top gear buy WAAS-ready GPS. But still you hardly need it, ordinary bog standard GPS is easily good enough

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Although I have a DGPS set, this was bought before Bill Clinton turned off "selective availability".

You don't need DGPS; ordinary GPS is fine.

<hr width=100% size=1>Kev.

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To put in perspective. Wot GPS says is just about right. Might depend on the chart a bit. Or whether the bloke has got the D hook in the right bit of chain on the pontoon. But your boat is in the right place. Even if pontoon is not. Does this make any sence!!

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Haydn
 
Funny you should say that about the pontoon. I was using a C-Map NT cartridge and my boat was about 10-15 metres away from the pontoon. I just upgraded to a NT+ version and now I am back on my pontoon! Obivously the mapping is more accurate in these versions, you have to be careful how you "rate" GPS accuracy, a simple visual check of a chartplotter is not necessarily good enough.

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I use a garmin 128 with a gbr 21 Differential receiver for wreck fishing and it is very accurate. But when sailing i just use the 128 on its own ., It is accurate enough for all I need. If anyone wants one I would swap it for anything useful.

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I've been using a 126 for several yrs and have been v pleased w/it. Display beginning to fade a bit in places after lot of exposure to Florida sun. Disadvantage is you can't use it at home to plan routes unless you have some kind of 12v power supply setup. Another option is Garmin 12 w/mounting kit; smaller screen, cheaper, but same functions as 126, and can be carried around for planning, playing etc.
As others have mentioned, diff GPS not reqd.

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If you are into diving or fishing then either existing Differential or get WAAS enabled and wait till it's switched on everywhere - it's live in some areas including the Sates now I understand.
When fishing or diving and marking a position to return to you have 2 errors - the one when you take your position and the one when you return. This can make refinding a small wreck or pinnacle a big job! With differential the error whilst still duplicated is small enough not to matter.
WAAS is pretty much standard on the bigger units now.

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