etrex 30 tracking - woods

1. Began into woods tracking:
10 GPS
1 WAAS
4 GLONASS - though, per the display 7 GLONASS were "visible". Houses =

and tress blocked them, I supose.
Uncertainty 3m (and briefly 2).

2. In the woods, lost WAAS and the "D" disappeared from the signal bars=20
of the GPS sats. Uncertainty went to 4 and pretty much stayed there.

7 - 9 GPS
2 - 4 GLONASS
WAAS occasional when in the few areas with a clear view south

3. After walking around for a bit, ended up at a good ground truth. Put =

both GPS' there (the other is a photo position recorder - a GPS without=20
a display - also recording at 1/5 s.); recorded the position after the=20
Garmin had re-acquired WAAS for 15 seconds. Uncertainty: 3 m.

When I got home and uploaded the tracks and waypoints.

The Garmin was off of the truth point by 7 metres. The photo tracker=20
was off by 3.

Next time I go to that truth point I'll turn off GLONASS.

Another _strange_ thing. At one point the GLONASS sat bars showed "D"=20
for differential. What's that all about?

Could it have been from SVN 33 (EGNOS 120 / AOR-E / W15.5=B0)?

More on that tomorrow - I'll go look for "33" to pop up - I wasn't=20
paying attention and it's not in the recording at all. But does seem to =

indicate that EGNOS data is leaking into the unit and the GLONASS range=20
is being corrected - maybe made the solution worse.



--=20
"I see!" said the blind man as he picked up his hammer and saw.

wrote:
ouses
outh
t
o

Logic would suggest the "D" presence depends only on the IONO grid,
regardless of ephemeris/clock corrections availability. That would
explain why not all GPS satellites get a D, even though in North
America all GPS satellites in view should have an excellent ephemeris/
clock correction.

Minimum IONO uncertainty is 3 meters, and just outside of CONUS+Alaska
it jumps quickly to 4.5 meters and higher. Perhaps the receiver
considers GIVE of 4.5 or higher too low accuracy to give the satellite
a "D" status. That doesn't mean the IONO grid is or isn't used for
those satellites without a "D" status, just that the IONO grid isn't
good enough for a "D" status.

If you look at:
http://www.nstb.tc.faa.gov/incoming/igp_plot.png

You will see that GIVE jumps quite quickly with just 300-400 km away
from a station.

Just thinking out loud. Marcelo

On 2011-11-27 10:19 , macpacheco wrote:
ses
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ut
t


to
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I'm not concerned too much about WAAS. I'm interested in why I'm=20
getting "D" on some of the GLONASS birds. I think I saw sat 33 pop up - =

it is an EGNOS bird.

More to follow on that.


--=20
"I see!" said the blind man as he picked up his hammer and saw.