On Nov 26, 9:06*pm, Alan Browne <alan.bro...@FreelunchVideotron.ca>
wrote:
> 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
> 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
> a display - also recording at 1/5 s.); recorded the position after the
> 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
> 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"
> for differential. *What's that all about?
>
> Could it have been from SVN 33 (EGNOS 120 / AOR-E / W15.5°)?
>
> More on that tomorrow - I'll go look for "33" to pop up - I wasn't
> 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
> is being corrected - maybe made the solution worse.
>
> --
> "I see!" said the blind man as he picked up his hammer and saw.

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:
> On Nov 26, 9:06 pm, Alan Browne<alan.bro...@FreelunchVideotron.ca>
> wrote:
>> 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
>> 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
>> a display - also recording at 1/5 s.); recorded the position after the
>> 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
>> 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"
>> for differential. What's that all about?
>>
>> Could it have been from SVN 33 (EGNOS 120 / AOR-E / W15.5°)?
>>
>> More on that tomorrow - I'll go look for "33" to pop up - I wasn't
>> 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
>> is being corrected - maybe made the solution worse.
>>
>> --
>> "I see!" said the blind man as he picked up his hammer and saw.
>
> Logic would suggest the "D" presence depends only on the IONO grid,
> regardless of ephemeris/clock corrections availability. That would

I'm not concerned too much about WAAS. I'm interested in why I'm
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.


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