Subject: eTrex Venture WAAS observations from this afternoon
Newsgroups: sci.geo.satellite-nav
Date: Thu, 22 Mar 2001 05:08:14 GMT

First, I should say I'm not WAAS expert, and I've only
had my Venture, which is my first GPSR, for a couple
of weeks.
Secondly, the following observations are based on using
the Venture by itself - in other words, there is no
attached laptop recording/analysing the NMEA data.
If anyone in the Vancouver area is interested, I've
put some info about where I did the test at the end of
this posting.

1)
Despite my posting the other day, which was based
on a phone call to Garmin, at 3PM PDT today, the
Venture acquired the WAAS signal from SV(space vehicle)
#35 in less than 1 minute, and, based on the "D" appearing
in the signal strength bars, was providing  correction data
for multiple SVs also within the minute.
This seems to contradict the notion that it takes
5-10 minutes to download a new ionaspheric(sp?) model.
The Venture had not previously been powered on today at all.
2)
The "D" that appears in the SV signal strength bars is
'normally' at the bottom of the bar, but, when the signal
strength is low, the "D" appears above the top of the
signal strength bar.
3)
I have seen mentioned by others that the Garmin units
seem to 'extrapolate' for up to 30 seconds.
This may explain why, even when the WASS #35 SV signal
strength dropped to zero, some of the other SV signal
strength bars still showed a "D". These did disappear
after "a while"(probably less than 30 seconds).
4)
The signal strength bars seemed to react quicker that
some other SV-related information, as indicated above.
I also found that to get a signal from the WAAS SV #35,
which was very low on the horizon to the East, that I
needed to have the Venture oriented horizontally, with
its long axis running North/South. Holding it vertically,
or pointed to the East didn't seem to be able to acquire
a signal for SV #35.
5)
The Garmin support rep confirmed that the Venture
(and therefore probably most other 12-channel GPSRs?)
'gives up' one of the channels in order to track the
WAAS SV. He indicated that this isn't an issue, and
that he had only seen a few times in three years when
he had actually been tracking 12 SVs at once.
This afternoon, I was able to routinely get 9-10 SVs,
and a few times, I had all 12, which included the
WAAS SV(#35). When this happened, I had all the signal
strength bars showing solid, with strength of from
1 to 4(0 is no bar, 4 is the max).
The SVs I was tracking at about 4PM where 03, 06, 10,
17, 18, 21, 22, 23, 26, 28, 35, 47.
6)
These observations were done in/around the Jericho
sailing centre parking lot.
The UTM coordinates from the Venture are:
10 U 0485369 5457944
I parked my vehicle on the south side of the lot,
about 15 parking spaces to the west of the tennis
courts, facing south.
The initial fix was obtained by placing the Venture
on the hood of my vehicle, pointing North.
Both the 3PM and 4PM information was from this location.
Walking around the sailing compound with the Venture
provided various results, but the best fix seemed to
be when the Venture was at the above position.
Even postions I thought might be better, where there
appeared to be an almost unobstructed view to the East
horizon, didn't seem to give as good results.
This included the outer end of the Jericho pier.