[FOSS-GPS] Questions regarding NV08C-CMS + RTKLIB

[Neil Schemenauer]

Hi,

Before I get into a long description of my problems, I want to thank
the people making open-source GNSS a possibility.  RTKLIB looks like
an amazing piece of work, thanks to T. Takasu for such a generous
gift to the world.  Also thanks to Michele Bavaro, he has been very
generous with his time as well.

I purchased two NV08C-CMS receivers from OptimalSystem.DE.  I'm
using two Tallysman TW3430 antennas.  The hardware seems to be
working but I have trouble getting a FIX solution.  I think
something is wrong with my setup but I don't know what.  My view of
the sky is pretty clear and I think my antennas are pretty good
quality.  The baseline is short, say 25 to 1000 m.

Here is one of many different sets of RTKNAVI options that I tried:

    http://arctrix.com/nas/tmp/rtk-kin2.conf

I tried AR "fix-and-hold" as well as "continuous".  I tried GPS
only, GPS+SBAS, GPS+GLO, GPS+GLO+SBAS.  I can often get a FIX
solution if I use "static" instead of kinematic, if I wait a while.
Kinematic can't seem to hold a fixed solution.  BTW, can I use
static if I am occasionally moving the rover while doing a survey?

The monitor log has errors like:

    ambiguity validate failed nb=10 ratio=1.23 s=xx/xx
    large residual (sat=XX-X ...)

I think the 's' number gets larger as I run RTKNAVI.  I sometimes
get a FIX solution for a moment when I first start RTKNAVI.  Based
on the ground track plot, it looks to be the correct fix but it
switches to FLOAT almost immediately.

Here are some NVS BINR files I saved from the base station.

    http://arctrix.com/nas/tmp/roof.zip
    http://arctrix.com/nas/tmp/roof-20131102.zip

The RINEX files for the 20131102 data is here if you want to look at
it directly:

    http://arctrix.com/nas/tmp/roof-20131102.obs
    http://arctrix.com/nas/tmp/roof-20131102.nav

I mounted the antenna on a roof, trying to stay above or away from
possible multi-path sources.  Maybe I was not entirely successful.
It seems like RTKLIB doesn't show anything about multi-path, maybe
due to a limit of the NV08C receiver?

Here is a PPP solution from the PPP tool provided by Natural
Resources Canada (uses precise GNSS orbits and clocks).  I'm getting
horizonal RMS errors of about 0.8 m, even with multiple hour
observation data.  I was hoping for better.  This is shorter
observation, from the roof.zip data I think:

    http://arctrix.com/nas/tmp/roof-ppp.pdf

In the above report, it looks like the receiver clock has a lot of
drift, maybe that doesn't matter.

Another thing I noticed this afternoon while playing with the
hardware, I don't seem to get many "NAV" messages from the
receivers.  I understand these contain ephemeris and clock data.
When I look at the RTKNAVI monitor, the "Obs(x)" count increases
continuously but for "Nav(x)" I only seem to get a few after
starting STRSRV.

That leads me to another question.  Does anyone know where the
startup/shutdown commands for the NV08C-CMS are documented?  Based
on the datasheet, I guess these may depend on the firmware loaded on
the receivers.  It would be nice to know some more details though.
For example, the Storegis tool allows the elevation mask to be set.
Maybe that's not relevant if you are reading raw data using BINR
though.

I may be on the wrong track since I'm not an expert on GNSS but this
NAV message issue seems like a problem.   Sometimes RTKNAVI will
show many GPS sat signals above 40 dB but they will all be grey.
Sometimes they will stay that way for many minutes.  It seems like
restarting some of the programs may fix, maybe restarting STRSRV.  I
think they are shown grey because there is "Obs" data but no "Nav"
data for them.  The monitor view seems to confirm this.  The
Storegis has no trouble finding many satellites and the position
seems very stable so I don't think there is something wrong with my
antenna, cable, etc.

Thanks for any direction and thanks again to the people making open
source GNSS happen.

  Neil