EGNOS

noelex

noelex

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EGNOS
My previous GPS died and I have replaced it with a Raymarine 125 GPS. The previous GPS obtained a satellite differential fix about 90% of the time, which seemed to improve the GPS stability significantly. The new 125 unit will not report a satellite differential fix despite a strong signal from the SD satellite (I am in NE Scily).
I have contacted

raymarine and they respond that the EGNOS satellite differential system is not yet operational and therefore will not be used by the 125. Is this correct ? Why did the old unit get an SD fix with better accuracy than the new unit (sold as WAAS EGNOS ready).
Thanks John
see my blog at www.amzerzo.blogspot.com
 
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I have contacted raymarine and they respond that the EGNOS satellite differential system is not yet operational and therefore will not be used by the 125. Is this correct ?

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Yes, EGNOS is not yet operational and you should not be using WAAS in Europe as it will make your fix less accurate.

Rick
 
I'm afraid Raymarine are indulging in hyperbole.

It is true that EGNOS is still being worked up, but I've been using EGNOS since the 1st satellite went up and WAAS before that in the same Central Mediterranean region.

Which confirms me in my prejudice that I'll only consider Raymarine for their autopilots - the rest of their gear being (IMHO) overpriced and obsolescent.
 
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Interesting. What are the degrees of inaccuracy you're talking about? And are they relevant in Med navigation - given the charting errors in the region?

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WAAS is a signal broadcast from a geostationary satellite that includes corrections to be applied based on the atmosphere. These measurements are for the atmosphere over the US in the case of WAAS (and logically, EGNOS will be the European equivilant). This is why you need complex ground stations for WAAS & EGNOS as they work out the corrections that need applying (and obviously need to be in the same area).

Applying the WAAS corrections whilst in Europe will cause errors - but I don't know by how much. Its reckoned that Ionospheric and Tropospheric effects give rise to errors of +/-5.5 metres - so of that order at a guess and probably worse in less than ideal conditions.

EGNOS is currently broadcasting a 'dont use' flag - I'd be surprised if any GPS receiver is using data deliberately marked as untrustworthy. I know that one of my GPS devices can be forced to use it - but I never do that when I'm relying on it as I think that would be foolish.

Rick
 
not only that, but bear in mind that a pencil line on a 1:100000 chart represents 100 metres width. So what use a 7m fix?

And yes - Raymarine are being misleading. EGNOS might not be fully up and running but it can be used (if you have a large scale chart and a thin pencil /forums/images/graemlins/grin.gif)
 
When i engage egnos/waas on my Navman it throws a periodic wobbler and claims to lose the fix..... anyone know why?
 
Thanks everybody for your replies. It appears my new GPS is fine and is not using EGNOS because of the "do not use flag".
I was obviously forcing the old gps to get an SD fix and I didn’t realize the system was not operational.
Regarding the suggestion that the improved accuracy is wasted I have to respectfully disagree. While EGNOS won’t give the substantial improvement in functionality to sailors that turning off SA did, I believe, there are some advantages to a more accurate GPS position even with inaccurate charts.
1. An anchor watch can be set with fewer false alarms. I have been in a pitch black anchorage blowing 60 knots with no reference points visible outside watching the GPS to see if we are dragging. Accuracy to +/- 3m would be of real help.
2. Returning to a charted position is more accurate. (As I am typing this I am sitting at an anchorage where the best spot is very specific due to a rapidly sloping bottom and hazards in the form of large navigational buoys. The depth sounder doesn’t fix the position as the ground slopes away both North to South and West to East. Coming into the anchorage at night I use the radar to get a distance from the navigation buoys, the depth sounder, and MK1 eyeballs to drop in the correct position, current GPS is just not quite accurate enough to be useful in this anchorage.
3. Speed. When stationary current GPS shows a speed of up to 0.3K. So I assume when moving the accuracy can be out by a similar amount. Acurate speed data can be useful even to non racing sailors.
Thanks John
 
Dear Wizard
Thanks for the info. Seems a long time considering that the system is working some of the time at least. Still considering its free I am not going to complain.
Cheers John
 
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