GPS Again

[JohnL]

In PBO 451 readers forum were told again about errors in gps by an expert [FRIN]why then are we being pushed into coupling gps to vhf as a life saver ? If we cant rely on gps can we rely on GMDSS, just a thought as I need to replace my old vhf radio John L.

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[AlexL]

GPS is used by many, many items including commercial aviation navigation, and as such can be trusted to be accurate - I've read stories about GPS inaccuracy, but haven;t seen many jumbo's colliding because of it.
Also stories of GPS inaccuracy are talking about the relative inaccuracy of something which is very accurate (if that makes sense). i.e the choice is an EPIRB or DSC distress hooked to a GPS which "may" have a slight error, or a manual EPIRB which locates you to a few miles at best, or a manual Mayday from an estimated position - even with the supposed GPS problems the GPS based system is likely to be orders of magnitude better.
Most GPS 'errors' are actually human errors in typing in co-ordinates, reading co-ordinates or Datum errors.

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[dulcibella]

All the same, it isn't seamanlike to trust GPS without any cross-checks with other sources of position.

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[bedouin]

I think the level of accuracy required from the GPS for safe navigation is much greater than that required for it to be a great help with SAR.

Even if the position was a mile or two out then it would still be a great help in letting SAR know your approximate whereabouts.

What worries me is the possibility that the VHF transmits a position that is hours, or even days old, if the GPS is not switched on

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[tome]

I haven't seen the article, but provided the GPS is set to the correct datum then problems with accuracy are rare and almost always detected by the ground segment (there was a memorable glitch in early January which wasn't detected at first). Once EGNOS is fully operational, any errors will be detected and both accuracy and integrity will be assured provided you have a suitable receiver.

For the purposes of distress GPS is a highly reliable position source. Even if an incorrect datum is used, the error is unlikely to exceed 100 metres which is good enough to find me.

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[cliff]

Simple answer is get into the habit of switching the GPS on when you switch on the VHF/DSC. (As soon as you get onboard?)

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[tome]

If the position is more than 4 hours old you will receive a warning from the DSC set.

The position is time-tagged when sent by the GPS, and this time tag is transmitted. Therefore the receiving station will know the age of the fix. Better than no position at all.

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[bob_tyler]

"Once EGNOS is fully operational,"


It was supposed to be in April. Any news when it might come on stream?

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[tome]

Not sure, ESA have gone rather quiet although test transmissions continue in Mode 2. It was meant to go live in April but the last update I heard was '2004'. If I hear more I'll post.

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[bob_tyler]

This site gives a good description of the system but still quotes April 2004. Quite an interesting read however.

http:// [url]www.pocketgps.co.uk/egnos.php [/url]

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[tome]

Link doesn't work, try <A target="_blank" HREF=http://www.pocketgps.co.uk/egnos.php>this</A>

At present, the signals broadcast are part of ESTB (EGNOS System Test Bed) and from April they started to bradcast Mode 0/2 which is fully compatible with WAAS and includes differential corrections. I believe we'll now see almost continuous transmissions until the handover from ESTB to EGNOS takes place and the system is declared fully operational.

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[MarkJohnson]

My DCS set starts bleating if it has not had a GPS fix for 10 minutes.

Mj

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[JohnL]

I used gps in April alongside paper charts to sail my [new to me] boat from the Deben Suffolk to the Coquet Northumberland. The gps Garmin 182 was very accurate.The concensus of the forum seems to be, dont worry no problem but a satalite being down for one hour before it was spotted should be a cause for concern. Unless we dont believe every thing thats printed in PBO.

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[Guest]

The question is will it integrate with existing or will it require a re-program / upgrade of existing recvrs ?

Maybe I should ask my own group !!!!


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[tome]

No, it won't require any re-program provided it is WAAS capable. WAAS and EGNOS are regional implementations of SBAS (satellite broadcast augmentation systems). WAAS is North American, EGNOS European. There's also an Asian (Japanese) version whose name escapes me without looking it up. When receiver specs say 'WAAS capable' they really mean SBAS capable.

They are geo-stationary satellites which broadcast on the same frequency as the GPS satellites. They provide additional inputs to the fix, and also monitor the integrity of the solution. This is known as RAIM - remote autonomous integrity monitoring. The GPS receiver needs to be capable of handling the SBAS data, and will need additional channels. I would strongly recommend that future GPS purchasers go for an enabled set.

The upshot of this is that they provide the equivalent of differential GPS to bring the fix solution down to about 5m from the standalone 20m. More important is the RAIM function which will detect anomalies with any individual satellite and give you a circle of uncertainty in the fix. This is especially important in critical applications, eg aircraft on an approach.

Could go on, but I'm sure you'll get the drift. In a nutshell SBAS gives you corrections and the feel-good factor. Apologies to anyone who recognises my over-simplifications.

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[tome]

Then you have a very useful set which will alert you quickly to a problem, probably because you've forgotten to switch on the GPS.

Have you noticed that GPS is the only device you cannot switch on from the switch panel? Invariably you have to go to the set to turn it on, and I notice that Garmin now force you to agree to a disclaimer before issuing forth. Smacks of Bill Gates to me.

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[bob_tyler]

"they started to broadcast Mode 0/2 which is fully compatible with WAAS "

Does this mean that I can now turn on EGNOS on my 5500i Navman menu and increase the accuracy of my position on the moving chart? Mind you, how accurate is the original HO chart if the survey was done before GPS, Decca etc?

I realise that there is no guarantee that EGNOS will be transmitting continously yet. However the instruction manual for the Navman says turn WAAS/EGNOS off on the menu unless it is available as accuracy will be less (The manual is on board so this quote is from memory). Will I be able to tell if EGNOS signals are being received or not?

Much of what I read on the EGNOS site is double-dutch to me but I do understand your interpretation, I think. Many thanks.

Bob

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[tome]

Bob

You've hit the nail on the head re the original HO survey, and this is one of the dangers of an accurate fix. It won't necesarily correlate exactly with the less accurately surveyed chart. If you look at your chart, you'll see that some areas were last surveyed way back.

I cannot imagine why an SBAS enabled set would be less accurate than a standalone just because it wasn't receiving the SBAS, so would go ahead and enable it. This is opinion, so treat with caution.

Yes, you can tell if EGNOS signals are being received as your set should show that you are operating in differential mode - you'll need to check your manual as to how to see this. If it's showing differential, you are receiving EGNOS.

Tom

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[Robin]

We have a Navman 5500i and if you turn EGNOS on it IS working, though it will go off for periods and revert to non-differential and sound a beep or two each time, which is why I currently have our EGNOS turned off.

The reliability of the chart accuracy is a different question, but with differential accuracy (be it land based or EGNOS) at least the fix itself is repeatable. Repeatabilty is an excellent feature IMO, if you set WPTs on a previous track recorded on the Navman for example through a narrow channel then you can follow these with high confidence on another day - even if on the chart you might seem to stray out of the channel if the chart itself were slightly out. As an example, our Navman shows our route out of our marina to bisect the pontoon fingers and then run down the very edge of the dredged channel, but it does the very same each and every time, excellent repeatability that will be even better with EGNOS in regular use. SOG and COG readings will also be even more stable, enough to calibrate logs and swing the compass if done in a tide free area.

All of our 'key' WPTs have been checked and 'tweaked' if necessary in clear visibility, especially in critical channels such as Chenal du Four, Little Russel or harbour entrances and so on. That way in fog and with the radar as well, we have some real confidence in what the instruments tell us.



<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>