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GPS looses accuracy offshore?

Old 07-19-2005, 08:51 PM
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Default GPS looses accuracy offshore?

I have a Garmin handheld C60.
I normally have about 16ft accuracy on the boat in normal weather.
Last weekend I was 5 miles off the coast of Ocean city and was hovering over a wreck site. Seemed as if my accuracy was more like 30 ft. I never took the time to see how many satelites I had acquired but did notice it looked like my accuracy was not the usual.
Would I expect to loose WAAS signal offshore?
I have an external antenna that I did not use, should I try that next time?
Old 07-20-2005, 06:08 AM
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Default Re: GPS looses accuracy offshore?

30 feet will still get over any piece you need to anchor on. If you have a good bottom machine, 30 feet shouldn't matter.
Old 07-20-2005, 10:29 AM
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Default Re: GPS looses accuracy offshore?

What effect would being 5 miles off shore have when receiving signals from orbiting satellites? ;?

I have to guess that you may just be imagining things.
Old 07-20-2005, 12:01 PM
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Default Re: GPS looses accuracy offshore?

I may indeed be imagining things.
Only thing I could think of was the WAAS correction signals which are ground based and add more accuracy.
don't know much about how WAAS works.

Old 07-20-2005, 05:21 PM
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Default Re: GPS looses accuracy offshore?

Melnic,

WAAS is transmitted to your receiver via geostationary satellites. These are only visible to your receiver when there are no obstructions (buildings, etc). Being on the water is one of the best places to receive the WAAS signal. I doubt this was your problem.

There are many factors that could cause you receiver to say its accuracy has decreased. Also, the accuracy reading on any receiver is never right on. I really wouldn't worry about it.


"WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal."

Bill

Old 07-20-2005, 05:22 PM
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Default Re: GPS looses accuracy offshore?

You might be thinking of DGPS signals which are ground based.
Old 07-20-2005, 08:27 PM
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Default RE: GPS looses accuracy offshore?

Even with WAAS, the receiver has absolutely no idea where it "actually is" anyway to have any hope of knowing if the position is accurate or not. Estimated accuracy shown by most receivers isn't worth all that much in the actual accuracy stakes and not something to be taken seriously.

No you would not expect to loose WAAS offshore if it appeared to work fine onshore, it should work better offshore but in any case regardless of location with what sounds like an internal antenna being used then the signal affect could be very variable. An external antenna is always going to be the better option as they generally improve the integrity (reliability) of the signal without the unknown affects of obstructions etc for both GPS sats and Geo-stationary satellites used for WAAS.

Cheers, Kerry.
Old 07-21-2005, 06:17 AM
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Default Re: GPS looses accuracy offshore?

If you do a lot of wreck fishing, buy a loran.
Old 07-21-2005, 10:49 AM
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Default Re: GPS looses accuracy offshore?

Not serious enough for a loran. Just a family sport boat that I've started wreck fishing in.
Yes, I am stating the accuracy based on what it is telling me the accuracy is (+/- 30ft). On the chart plotter it shows a circle around the boat showing the accuracy range. It did seem to be repeatable within about 5ft. which was more important to me since I was marking the wreck and wanted to come back to it.
On my initial pass to find the wreck, we did not see it, but making passes about 10 ft apart, we found it on the 2nd pass.

So even though WAAS has ground based correction, those corrections are beamed back to the satellites? and then the Satellites retransmit that info?
Old 07-21-2005, 11:01 PM
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Default Re: GPS looses accuracy offshore?

....So even though WAAS has ground based correction, those corrections are beamed back to the satellites? and then the Satellites retransmit that info?....
That's about what WAAS does, ground stations collect the signals, analyses those signals, correlates this and other data and bounces it off 2 geo-stationary satellites and it all has to be done in less than 6 seconds. These 2 satellites are the bottle neck of the whole system and one need s a good clean view with absolutely no obstructions.

As for using Loran, Nah that's a little behind the times right now. Accurate (in absolute terms) it was not and repeatability wasn't what it was all made out to be.

Cheers, Kerry.

Old 07-23-2005, 08:27 AM
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Default Re: GPS looses accuracy offshore?

my ant is 12' from my display so what's the big deal ;?
Old 07-23-2005, 06:18 PM
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Default Re: GPS looses accuracy offshore?

ubettcha13 - 7/24/2005 12:27 AM

my ant is 12' from my display so what's the big deal ;?
No big deal with that as long as the antenna isn't obstructed. However the original query appears to relate to an internal antenna?

Cheers, Kerry.
Old 07-25-2005, 08:12 AM
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Default Re: GPS looses accuracy offshore?

Yes, internal antenna, handheld GPS mounted on dash.
Old 07-26-2005, 09:06 PM
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Default Re: GPS looses accuracy offshore?

I think what you are observing is a fix with and without WAAS. It may not be related to your position offshore or possibly not. The WAAS satellite that covers the Atlantic Ocean region is about 20 degrees above the horizon. The position of the receiver relative to the satellite can easily cause an obstruction by a T-Top, Canvas, etc. Plus the orientation of the receivers antenna can cause a change in received signal strength.

As far as the stated receiver accuracy or Horizon Dilution of Position (HDOP). this is based on data currently being received from the satellites (time, orbit, etc) and it not a random number - it is a statistical measurement of accuracy and represents an upper bounding of the error in a given situation. So even if the possible error in your position is 30', your true position might still be accurate to 5', but it could be as high as 30'. As a previous poster note, you are somewhere in circle bounded by the stated HDOP. You might be in the middle or you might be on the edge.

Hope this helps.

lc
Old 07-27-2005, 06:46 AM
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Default Re: GPS looses accuracy offshore?

lciummo - 7/26/2005 10:06 PM

The WAAS satellite that covers the Atlantic Ocean region is about 20 degrees above the horizon. The position of the receiver relative to the satellite can easily cause an obstruction by a T-Top, Canvas, etc.
lc
FWIW, canvas and fiberglass t-top material should not be a problem. But, a commonly overlooked problem is passengers crowding around the helm/unit.

BTW, great explanation of HDOP.
Old 07-27-2005, 09:27 AM
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Default Re: GPS looses accuracy offshore?

Kerry - 7/22/2005 12:01 AM

....So even though WAAS has ground based correction, those corrections are beamed back to the satellites? and then the Satellites retransmit that info?....
That's about what WAAS does, ground stations collect the signals, analyses those signals, correlates this and other data and bounces it off 2 geo-stationary satellites and it all has to be done in less than 6 seconds. These 2 satellites are the bottle neck of the whole system and one need s a good clean view with absolutely no obstructions.

As for using Loran, Nah that's a little behind the times right now. Accurate (in absolute terms) it was not and repeatability wasn't what it was all made out to be.

Cheers, Kerry.
Maybe not in the land down under, be here LORAN is the most repeatable system available. Loran will put me on a piece of junk almost too small to see on the bottom machine. For serious wreck fisherman, there is no substitute for a good, old fashioned LORAN C.
Old 07-27-2005, 01:21 PM
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Default Re: GPS looses accuracy offshore?

Went out last weekend. Skies were perfectly clear. About 8+ miles visibility. Error on display was about +-10'. Was consistantly picking up at least 10 sats.
I found 2 adjacent wrecks on the first pass. I recall last week visibility was like 3miles. Maybe the misty air meant the satellites near the horizon were not being recieved.
Well, at least I learned something in all these threads.
Thanks folks.

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