Re: Questions about EGNOS.

From: Eddy [UK] (one_computers_at_compuserve.com)
Date: 12/01/04 

Date: 1 Dec 2004 04:09:45 -0800

Nacho wrote in message news:<cohf6a$b2h1@news.hi.inet>...

> Hello.
> I have a new Vista C GPS with WAAS (EGNOS) and I would like to ask you
> about it.
> I have heard that EGNOS will be available in 2004, but another sites
> says that it will not work until 2005. Is it working now? I have to
> collect the alamach and I don't know what satellites are available or
> where they are.
> Also, is there any web page where I can what GPS and WAAS satellites
> can be seen from my place (Spain) and their status?
> Thanks for your help.
> Best regards

Hi Nacho,

EGNOS is currently in a test phase known as ESTB - it has yet to go
"live". One of the data bits it transmits is a "do not use" flag which
is currently set to ON. Garmins obey this flag and so ignore ESTB data.

Magellans, including my Sportrak Pro, do not obey this flag and
therefore attempt to use the ESTB data. In practice, here in the middle
of England, when ESTB signals contain valid ranging data the accuracy of
my GPSr improves - when the signals are invalid or off my GPSr functions
as normal. I have never seen the accuracy degraded by ESTB signals and
eagerly await full EGNOS live signals.

Like the American WAAS system, EGNOS data is only valid for its intended
area. WAAS covers America, EGNOS covers Europe. There have been cases
of Magellan users in the wrong area picking up ESTB signals and
consequently showing large position errors. If you want to use a
Magellan in, say, India you should disable WAAS operation to eliminate
this problem.

Every 3 months the date for EGNOS going live is put back by 3 months.
The designed performance criteria is much more rigorous than for the
earlier WAAS system, so their testing is much more rigorous. Only when
they can consistantly meet the requirements will the authorities certify
the system for full operation - this is taking it's time. The chart at
http://esamultimedia.esa.int/docs/egnos/estb/schedule.htm
shows a planned start next month but don't hold your breath.

When it goes "live" the "do not use" flag will be set to "OFF" and
Garmins will be able to benefit too.

It's intended purpose is to aid in the landing of aircraft by increasing
the accuracy of the altitude position and to provide a "quality" rating
for this value - currently GPS readings are not approved for "safety of
life" situations e.g. as above. For recreational walkers the benefit of
knowing where you are to 3m instead of 7m is debatable, but hey, the
more the merrier! For car navigation it's irrelevant: if you don't know
which side of the road you're on you shouldn't be driving.

You asked which satelites can be seen from Spain. GPS ones go in a
series of inclined orbits taking about 12 hours per orbit so all of them
will be visible from anywhere on Earth at some time each day - the ones
currently visible are constantly changing with always some above the
horizon - see here
http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
and click on
GPS Constellation

WASS and EGNOS ones are geostationary so will always appear in the same
place in the sky - like satelite TV ones. The WASS ones are over
America, the EGNOS ones are over Europe. From Spain you shouldn't see
the WASS ones.

Eddy [UK]

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