Help with WAAS and DGPS

[Gaulois]

On my Quest 2 I can set the GPS prefs to WAAS which, according to the manual, references fixed ground stations to give better accuracy. Is this the same as DGPS (which everyone in the yachting world was talking about a couple of years ago) or something completely different?
If it is diffrent does WAAS work in Europe. I'm fairly sure DGPS is up and running.
Cheers

 

[PRR]

On my Quest 2 I can set the GPS prefs to WAAS which, according to the manual, references fixed ground stations to give better accuracy.

WAAS is a choice on almost every unit since several years back.

Is this the same as DGPS (which everyone in the yachting world was talking about a couple of years ago) or something completely different?

Same but different. The correction information is transmitted from geostationary (always in the same position on the sky) satellites rather than via radio. WAAS therefore don't need any additional equipment and also is free to use without paying anything.

If it is diffrent does WAAS work in Europe.

It is called EGNOS in Europe and a WAAS-compatible unit will be able to receive EGNOS on this continent.

Only problem is that EGNOS is delayed, delayed and delayed. It was promised to be available in April 2005 but I've given up on looking for it now. It is in testing mode (called ESTB) so sometimes if you're lucky you will get reception. I did get corrections from it about two years ago with the units reported error actually going down from 10 to 2 meters after a while. In a newsletter from September 2005 EGNOS is said to begin early in 2006. Knock on wood.
http://esamultimedia.esa.int/docs/egnos/estb/contact.htm

The satellites are number 33, 37 and 39 on a Garmin unit and when you get reception and they transmit a valid signal you will see the letter "D" on the status page (it will take a long time with a clear view towards south) for the satellites that you are receiving corrections for.

 

[Cheesy Mike]

There used to be (probably still is) a very neat differential GPS system available in light aircraft. The stationary signal necessary for the high accuracy offered by DGPS was obtained by tuning into Classic FM. This is a perfect solution as Classic FM's transmitters are all in known locations, have nationwide coverage, and - importantly - transmit an accurate time signal as part of the RDS service.

Mike

 

[Mouse]

As far as I understand it, WAAS (or EGNOS) is not really worth using for an automotive GPS. You don't need 1m accuracy to tell which road you are on, and the WAAS signals use channels in the receiver that could better be used for locking on to more normal satellites.

I think enabling WAAS will just increase the chances of losing a satellite signal totally if your view of the sky is partially blocked, eg by trees or buildings.

 

[jimbo]

Same but different. The correction information is transmitted from geostationary (always in the same position on the sky) satellites rather than via radio. WAAS therefore don't need any additional equipment and also is free to use without paying anything.

Are you sure about this?

Garmin's Web site says:

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.

I think WAAS is land based

As to whether its any use, I agree with Mouse. I doubt the maps are accurate to that level anyway.

 

[John Armstrong]

I seem to remember PanEuropean saying that for us in the UK using the WAAS as it's so low in the sky and you get more use out of having 12 satellite options rather than 11 with one that you can't probably use.

Plus, how accurate is the roads mapping? Where I live I'm at the bottom of a road that has a turning circle, which the road maps produced by the local council don't show and therefore the Garmin ones don't and so I'm in the middle of nowhere (road wise) when I'm at my house.

BTW Accrington isn't the middle of nowhere, but you can see it from here.

 

[Mouse]

Jimbo - you're sort of right but read the info from Garmin more carefully. The WAAS signal originates at the ground stations but is then transmitted to the geostationary satellites which rebroadcast it to be picked up by your GPS receiver. Hence the use of 1 GPS channel on the receiver to get the WAAS signal, this channel is therefore no longer available for standard GPS signals.

 

[jimbo]

Ah, that makes sense. I could never see how a ground radio would correct a satellite signal.
ta

 

[ebbo]

As far as I understand it, WAAS (or EGNOS) is not really worth using for an automotive GPS. You don't need 1m accuracy to tell which road you are on, and the WAAS signals use channels in the receiver that could better be used for locking on to more normal satellites.

Mouse, I’m sure your right, since selective availability was switched off a few years back, GPS has been quite accurate enough.

 

[PRR]

I think WAAS is land based

As the original question was the difference between DGPS and WAAS/EGNOS in my answer I concentrated on the data transmission part as that is the main difference. Both systems rely on land based stations to measure the errors.

I doubt the maps are accurate to that level anyway.

Let's say that maps are 20 m wrong in one direction and the GPS 20 m in the other direction. That would in worst case lead you 40 m wrong. If the receiver with WAAS/EGNOS bring it's error down to 2 m then the worst case error would be only 22 m.

Yes, 40 m is mostly enough accuracy but if I can have 22 m without any substantial penalty I will choose that instead.

I seem to remember PanEuropean saying that for us in the UK using the WAAS as it's so low in the sky and you get more use out of having 12 satellite options rather than 11 with one that you can't probably use.

Yes it is correct that it is a trade-off but it is only very rarely that one uses all 12 channels so I will happily loose one (two on some receivers) channel to sometimes (being further up north I will have more problems than you catching the EGNOS-signal) improve accuracy. But I'm also under the impression that when EGNOS is in production those satellites will also be used for positioning so the waste will be even less then.

On older slower units there may be a performance hit using WAAS/EGNOS and since it's not in production yet (haven't checked for a while) it would presently be a waste of channels to have it turned on.

Everyone is of course free to do whatever they wish but to anyone that have the slightest technical interest I think it would be fun to test EGNOS when it goes live and see the improvement in accuracy. If you don't like it then switch it off.