Waas And Egnos Important Info

[+jotne]

Here in Europe we use Egnos, so I did send the question to the ESA engineer.(European Space Agency)

http://www.esa.int

 

I did ask why do we see sometime a solid bar on the bird #33 (Egnos) with "d" and sometime a hallow bar?

WAAS works mostly the same way as Egnos

 

PS Status 2 and 3 are the two in use today. I changes several times a day.

More info about status and quality of signals found here

http://esamultimedia.esa.int/docs/egnos/estb/egnos_pro.htm

 

Here is the answer form the engineers:

---------------------------------------------------------------------

 

Dear Sir/Madam,

 

thank you very much for contacting us.

 

- status 2 means that the GEO satellite (bar #33) is not used by the receiver for the position computation. The reason is that there is no corrections broadcast for the GEO.

- status 3 (solid bar on #33 including a d) means that the GEO satellite is used for the position computation. The reason is because GEO ephemeris and GEO corrections are being broadcast in the signal.

 

These 2 status corresponds to "mode 2", with the option of "with Ranging" or "without ranging". If you look at the daily mail, the status of the ranging is mentioned in the mode 2.

 

Why we change from "Ranging" to "without ranging": when the GEO ephemeris are not accurate enough (due to a GEO manoeuvre), we discard the GEO corrections from the signal in order to achieve the best accuracy on the position provided by the receiver.

 

Your GPS receiver provide a better position when receiving ESTB or EGNOS. Of course you can trust it. The improvement on the accuracy is in the order of 5m for GPS to 1m for EGNOS.

 

For further information on ESTB/EGNOS please don't hesitate to contact us at the EGNOS Helpdesk.

 

Best regards

EGNOS Helpdesk

European Space Agency

egnos@esa.int

[peter]

...

- status 2 means that the GEO satellite (bar #33) is not used by the receiver for the position computation. The reason is that there is no corrections broadcast for the GEO.

- status 3 (solid bar on #33 including a d) means that the GEO satellite is used for the position computation. The reason is because GEO ephemeris and GEO corrections are being broadcast in the signal.

The response is correct that the hollow bar means the SBAS satellite (whether WAAS, EGNOS, or ESTB) is not being used to provide ranging data for calculating a position. Whenever there is too much uncertainty about the ephemiris the signal status is changed so signals from that satellite will not be used for ranging and possibly degrade the accuracy.

 

However, the comment about not broadcasting corrections for the GEO (SBAS) satellite is somewhat misleading. The presence of good correction data is a separate issue. There might well be a good ephemeris so the satellite can be used for ranging (i.e. solid bar), but still an absence of correction data so it wouldn't be indicated with a little 'D' for differential corrections on the signal bar.

[+GPSlug]

...

- status 2 means that the GEO satellite (bar #33) is not used by the receiver for the position computation. The reason is that there is no corrections broadcast for the GEO.

- status 3 (solid bar on #33 including a d) means that the GEO satellite is used for the position computation. The reason is because GEO ephemeris and GEO corrections are being broadcast in the signal.

The response is correct that the hollow bar means the SBAS satellite (whether WAAS, EGNOS, or ESTB) is not being used to provide ranging data for calculating a position. Whenever there is too much uncertainty about the ephemiris the signal status is changed so signals from that satellite will not be used for ranging and possibly degrade the accuracy.

 

However, the comment about not broadcasting corrections for the GEO (SBAS) satellite is somewhat misleading. The presence of good correction data is a separate issue. There might well be a good ephemeris so the satellite can be used for ranging (i.e. solid bar), but still an absence of correction data so it wouldn't be indicated with a little 'D' for differential corrections on the signal bar.

It's not quite a separate issue. Even in the best of times, the ephemeris for the GEOs are not as good as for the GPS sats. Enough so that you don't want to range off the GEO even when a low User Range Accuracy is indicated, unless the GEO is sending corrections for itself. So they could just leave out the correction and receivers wouldn't range off of it.

 

What WAAS has been doing recently to all but 134(47), and what ESTB might be doing as well, is increasing the URA and leaving it out of the corrections, but not actually indicating that ranging is impossible in the health. The health is left OK, I think, because some receivers will refuse to track the GEO at all if any health bits are set.