Re: ppp averaging and circular error of probability

[Greg Troxel]

Hans Mayer via <gpsd-users@nongnu.org> writes:

>> A nit: these really are not probabilities.  They are distances that
>> contain a fraction of points in a data set.  So in the analysis, there
>> is no probability involved at all; it's just counting.   But you are
>> correct that CEP95% must be <= CEP99%.
>>
>> The word probable shows up because this is a method to estimate the
>> underlying probability distribution of the measurement process.
>
> But isn't it that a new measured point would fall into the circle
> CEP99% with a possibility of 99% ?
>
> So it's a probability, isn't it ? Maybe my English is too bad to
> understand this.

It's more that math is difficult than English.  What I meant is that
there is a probability distribution for measurement outcomes, and things
like CEP are estimates of that (unknown) distribution.

> As antenna I use the recommended multi-band active gnss antenna
> HAB-ANN-MB-00-00 with SMA Connector from ublox.

I have one and should try some more serious tests with it.

> It's fix mounted on the top of the roof of my house.
>
> If I look for satellites with an elevation <= 1 degree I see some of
> them over a 24 h period.
>
> So it seems I can look quite near the horizon

That sounds good then.  The number of people with indoor antennas that
can just barely get lock, who then expect amazing performance, is quite
high...

>> There's also the question of datum.  You didn't explain what mode you
>> are running the receiver in.  Recent F9P firmware will default to at
>> least GPS, GLONASS and at least one of Galileo and BeiDou (fuzzy memory)
>> and I think SBAS.  Without SBAS, the receiver is probably transforming
>> the 4 systems to a common frame, and like outputting positions in
>> WGS84(G2139).  With SBAS, the outputs are in the SBAS reference frame,
>> which is probably ITRF2008 or ITRF2014 -- but it's surprisingly
>> difficult to find that information.
>
> |UBX-CFG-GNSS: |
>
> |msgVer 0 numTrkChHw 60 numTrkChUse 60 numConfigBlocks 6

wow, that really needs unmunging!

> gnssId 0 TrkCh 8 maxTrCh 16 reserved 0 Flags x11110001 GPS L1C/A L2C enabled
> gnssId 1 TrkCh 3 maxTrCh 3 reserved 0 Flags x01010001 SBAS L1C/A enabled
> gnssId 2 TrkCh 10 maxTrCh 18 reserved 0 Flags x21210001 Galileo E1 E5b enabled
> gnssId 3 TrkCh 2 maxTrCh 5 reserved 0 Flags x11110001 BeiDou B1I B2I enabled
> gnssId 5 TrkCh 0 maxTrCh 4 reserved 0 Flags x15110001 QZSS L1C/A L2C enabled
> gnssId 6 TrkCh 8 maxTrCh 12 reserved 0 Flags x11110001 GLONASS L1 L2 enabled

So you have good support for most constellations with limited to 5
BeiDou, and SBAS.  And possibly 4 channels for QZSS which if you are in
Europe you won't hear.  It's an interesting question which SBAS is in
use (ENGOS?)  and if it has correctionsfor all 4.

>> I would suggest that you record a day or so of raw data, convert to
>> RINEX, and submit to CSRS-PPP to both get a position, and look at the
>> error statistics.  Before you do, make sure that you have a stable or
>> repeatable antenna position, where it's stable to better than 1 cm.  You
>> also may want to pay attention to the "North Reference Point" (see
>> https://www.ngs.noaa.gov/ANTCAL/FAQ.xhtml  ) if you have a more serious
>> antenna, and make one up if not for repeatability.
>
> Thanks for this hint.  I will look for the antenna diagram

I don't think that antenna has a cal file, but I'd say the NRP is
opposite the cable so that the cable is south.  Or rather that's the
convention I'd adopt.  But if you can find a stated plan from u-blox or
someplace like NGS, definitely follow that.  I doubt it matters; it just
seems to be best practice to be consistent.  And, if it's fixed in place
and you don't intend to move it -- then just don't move it!

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