Re: WAAS in Australia, two questions
- From: matt weber <mattheww50@xxxxxxxxxxx>
- Date: Mon, 10 Apr 2006 11:02:10 -0700
On Mon, 10 Apr 2006 13:00:18 GMT, Sam Wormley <swormley1@xxxxxxxxx>
wrote:
balt wrote:The fix is only as accurate as roughly the URA x DOP for the worst sat
Hi all,
I have a Garmin 2610 with the Aussie roadmaps. I often drive to/from
work in Sydney looking at the status page and noticed two peculiar
things recently:
1. Sporadically, especially when what seems to be within closer range
of the Sydney Harbour, I get DGPS positions, i.e. WAAS corrections are
being received and any number of the satellites received show a "D" in
their bar, apparently meaning DGSP corrections are received for those
satellites. This is usually accompanied by one almost full bar
satellite being received that will never show a filled bar, i.e. the
GPS doesn't use that satellite for navigation. Presumably, that is the
WAAS transmitting satellite (a geostationary one) with the ID's 50 and
42 (haven't seen any others here). Why would that only occur during
what appears to be harbour proximity? Does the system only use the WAAS
corrections when within a specified area for which error corrections
are received?
2. The other day I had around 6 satellites all receiving WAAS
corrections, but the position accuracy it calculated was about 35
meters, as opposed to 4-5 meters when WAAS was turned off with the same
satellites. How do you explain that? It is my understanding that the
position information should be getting better with DGPS, no?
being used in the fixed. So if one of the sats being used for a fix
has a large URA, or the geometry is bad, the accuracy may not be very
good. 4-5 meters requires WAAS on all sats in use for the fix, and
good geometry at the same time. in North American you can expect both
almost all of the time, outside North America, it is far less likely.
3. What are the WAAS plans for Australia? Is it currently in an
experimental phase?
Thanks for any clarifying information.
- Balt
SBAS Coverage
http://www.trg.soton.ac.uk/rosetta/workareas/6b_rnv/rnv_pr1_fig1.gif
http://www.trg.soton.ac.uk/rosetta/workareas/6b_rnv/rnv_pr1_background.htm
WAAS is a regional augmentation of GPS in North America.
An Essential Element of WAAS and all SBAS implementations -- spatial
correlation [with ground reference stations].
"Differential GPS involves the cooperation of two receivers, one
that's stationary and another that's roving around making position
measurements. The stationary receiver is the key. It ties all the
satellite measurements into a solid local reference." -- Trimble
Navigation Ltd.
The "reference receiver" is a network of ground stations. The WAAS
differential corrections, based on that reference network, are valid
for a WAAS (SBAS) enabled GPS receivers that is within or spatially
close to the associated reference network. One would *not* expect
valid differential corrections from a reference network that exists
on another continent.
Australia?
http://www.agcc.gov.au/downloads/Sat_nav_use_Aus.ppt
-Sam Wormley
http://edu-observatory.org/gps/dgps.html
.
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