[Openstreetmap] DGPS

[Andy Robinson]

EGNOS is not up and running yet although the EGNOS test bed (ESTB) is
broadcasting intermittent and partial correction data on AOR-E PRN 120
(Gramin 33). Currently my Garmin Legend is locked on ARMETIS PRN 124 (37)
and IOR-W PRN 126 (39) which are broadcasting signals which are ignored by
my gps receiver. ARMETIS I understand will provide the initial operation for
EGNOS from hopefully early next year.

Before the ESTB was switched to IOR-E in 2003, I was getting testing
correction data for the UK satisfactorily from AOR-E. However I did not find
that the data was particularly more reliable when laying down tracks on
different days. This is most likely due to the lack of full correction data
being broadcast at the time. There is also a big restriction of course that
you have to be able to see the appropriate geostationary satellite(s).

Back in 2003, the best stated accuracy on my gps was 2m horizontally form a
5m stated best previously seen. I have seen evidence of a 1m accuracy from
elsewhere, however I think it is important to note that in reality these
stated values on the GPS should be taken with a pinch of salt. Plug your gps
into any of the real time plotting software and leave it in a clear view of
the sky for a while and you will see just how satellites coming into and out
of the field of view alter the apparent location of the receiver. I've used
SA Watch from www.huntting.com in the past to evaluate (statistically)
positional error.

I have not evaluated SisNeT yet which is the internet connectivity basis for
the correction data. This would probably be a better way of correcting gps
signals in the urban city canyons than using the EGNOS satellites and should
by the time EGNOS is fully operational, be a relatively simple process using
an internet enabled PDA and a Bluetooth gps receiver.

Of course if you want accuracy of less than 1m you need to go to commercial
differential set-ups used in land surveying etc. These do not work when
belting down the motorway at 70mph so I think we can forget about accuracy
on the move much better than 5m, even with EGNOS. However bearing in mind
that the standard 2 lane UK road is 7.3m wide and a 3 lane motorway
carriageway is 11m wide it shouldn't be too hard to be selective when
creating roads from trackpoints although it may be difficult to separate out
a pair of close parallel carriageways.

For more info on EGNOS, the ESTB and SisNeT see:
http://ravel.esrin.esa.it/docs/egnos/estb/egnos_pro.htm


Andy Robinson

-----Original Message-----
From: openstreetmap-bounces at vr.ucl.ac.uk
[mailto:openstreetmap-bounces at vr.ucl.ac.uk] On Behalf Of Simon Hewison
Sent: 17 October 2005 10:36
To: openstreetmap at vr.ucl.ac.uk
Subject: [Openstreetmap] DGPS

Just a straw poll. Does anyone here use any form of DGPS or EGNOS to 
improve accuracy?

It's a bit of a shame that the GPX tracklogs don't include estimated 
accuracy measurements, but a real time dump of the NMEA output from a 
Garmin does include such things.

-- 
Simon Hewison

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