GPS Selective Availability?

[jackho]

Anyone know if this error has been re-introduced? I use Euronav Seapro Light with my Garmin GPS120. It has been so accurate I could almost navigate blind folded 4miles upriver into my berth. This year it has me coming up river on the shoreline to the Starboard side outside the marked channel.
Also does anyone know if there is any difference in the accuracy of a 120 vs. the newer 128 ie. 8 channel vs. 12 channel.
It was suggested I upgrade to restore my accuracy but my opinion is that the sats. above and around my location would be the same - am I correct?
Have reviewed all the YM mags and they don't seem to cover any of these points in any of the reviews - maybe I have missed something !!!!!!!!!!!!!!!!!!!!!!!!
Sorry if this is too longwinded but I eagerly await any opinions offered.

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[petery]

Could it be that your new unit came preset with a different chart datum?

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[tome]

Jackho

Check your datum as suggested. SA is off.

The clue to this is that you are now offset to starboard. Probably your set has defaulted to WGS84 when you should be using OSGB36 if on UK charts, unless you've recently bought new WGS84 charts in which case the opposite applies (GPS still on OSGB36).

Sats are in Polar orbit so constantly moving. Biggest error with GPS tends to be N-S. 8 or 12 channel makes little difference as you will mostly have 9 or less SVs in sight.

Regards
Tom

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[jackho]

Hi Tom
Thank you for your very informative reply. I suspected the error to starboard was related but nothing has changed since last season - at least that i am aware of.
My Seapro Lite charts are up to date and although I'm not too familiar with the technical aspects of chart datums my GPS is set to WGS84 and I believe the Vector charts match.
On mynext visit to the boat I will double check.
As an after thought might it be possible that my labtop screen could introduce an error?

Jackho

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[tome]

Jackho

No, your screen won't introduce an error. You're on the right track in checking your vector chart datum matches. This especially on a PC where it's easy to make an inadvertent change.

Best wishes
Tom

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[mldpt]

Hi Jackho.
I use C-Map on my plotter, if you are not zoomed right in a track line can be over land not sure why but it seems to be the case I guess that when zoomed out the detail is so small that its not possible to show all detail. Also another point I have the facility to make position correction this will also give you an error if you have inadvertantly made a correction and its not correct.
Mike.

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[qsiv]

You havent inadvertently performed a datum change on the SeaPro charts? That can cause havoc ...

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[jackho]

Replying again to share update on my nav. problem with Euronav Seaprolite charts vs. GPS.
I have now I believe proven that the charts are out. I'v checked my last season logs and also cross checked routes/waypoints stored on my labtop as part of the Seapro programme. My previous routes which I have used as an aid to navigation for two seasons now show the same difference as what I believed to be the GPS error.
I updated my charts earlier in the year for the new season and I think this has introduced the error somewhere.
Euronav agree my charts should be more accurate (presently around 50m out )
on a 1:20,000 scale and are looking into the problem.
When I get a reply I will update again but if I don't get a reply soon i will start a new topic on the forum to get more feedback.

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[Oldhand]

I agree with other respondents that you must have the GPS on a different datum to the chartplotter. Selective Availability would give random errors, not a fixed offset.

When thinking whether you should have a GPS with more channels, stop and think of the days we drew "cocked hat" fixes from RDF. If the angle between 2 stations, or satellites in the case of GPS, is low, the triangle of fix becomes very elongated. i.e. bad angle gives large error. Thus a GPS with only 6 channels which has decent software which will reject fix data from satellite(s) which give "bad angle" will almost always be more accurate than a 12 channel receiver which uses every satellite in sight including those creating "bad angles".

I think it would be safe to say that you get what you pay for in GPS receivers. Cheap models will have less developed software and thus may not be as accurate as more expensive ones.

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[tome]

GPS receivers are a bit more clever than this these days. Where there are sufficient satellites they will reject low elevation SVs and ones with a low signal strength. They also apply a weighting matrix to each SV's contribution to the fix solution based on relative geometry. Finally, they smooth the speed and position by applying an adaptive Kalman filter. With SA now off, earlier receivers are probably over-filtered.

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[hugh_nightingale]

Hi everyone, I'm back on the scene again.

On Saturday last we were coming back from Chichester and I noticed that my existing WPs were showing an error of about 0.2nm to the south.

Just a thought, on the same day Ark Royal was returning to base, in fact we followed her in. "They" could easily put in a fixed offset I believe.



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[jackho]

Thank you for your input. I have now got to the bottom of the problem (I think).
See my earlier note today. Feel free to comment.
Regards

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[tome]

They could jam your GPS if they wanted to, but couldn't put in a fixed offset. To do so would entail altering the pseudoranges from all SVs - simply not technically feasible.

You should look closer to home for the solution and it's almost certainly finger trouble. Check your datum.

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