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<font size="+1">Author: Nacho Romero, Infrastructure Committee
Chairman<br>
<br>
Last April 1st, 2014 (d091) the GLONASS (GLO) constellation
suffered an event as the uploaded navigation messages had an
incorrect applicability times, this has been reported in the trade
press as you are well aware </font><br>
<font size="+1"><font size="+1">(<a class="moz-txt-link-freetext"
href="http://www.insidegnss.com/node/3979">http://www.insidegnss.com/node/3979</a>,
<a class="moz-txt-link-freetext"
href="http://gpsworld.com/glonass-gone-then-back/">http://gpsworld.com/glonass-gone-then-back/</a>,
<a class="moz-txt-link-freetext"
href="http://gpsworld.com/altus-positioning-systems-pinpoints-cause-for-glonass-default/">http://gpsworld.com/altus-positioning-systems-pinpoints-cause-for-glonass-default/</a>)
the constellation continued emmitting navigation signals but the
navigation message was incorrect, so some navigation users were
affected if the incorrect navigation messages were not ignored,
this lasted until well into April 2nd (d092).<br>
<br>
I have taken the IGS data from the Data Centers (DCs) in Rinex 2
and Rinex 3 to analyze what the impact of the GLO problems has
been on the IGS; on the data holdings, on the GLO constellation
coverage, and on the IGS products.<br>
<br>
This is a thorough but not complete or conclusive look into this
issue, but after more than a week since the event I wanted to
report to the IGS community about it with some hard facts. If
interested please contact me and we can plan a more detailed
study if needed.<br>
<br>
-- SUMMARY --<br>
<br>
The conclusion is that the IGS was not affected; we have good
amounts of GLO data over the period in the DCs and good GLO
products.<br>
<br>
We continue to have enough data coverage for GLO, and regular
product generation were not at all affected. Either the
receivers ignored the incorrect navigation messages or were
unaffected due to internal RAIMS checks. Thus the IGS network
is stable due to its heterogeneity, as we have always
advocated. This diversity of equipment in the network continues
to ensure that we have a resilient IGS.<br>
<br>
This does not mean we did not have problems ... GLO coverage did
suffer slightly (0-12 % per satellite on day 092; April 2nd) as
can be seen from the Analysis Center processing reports for the
Finals of last week, but we have so many stations providing data
that it did not affect the GLO product generation. From those
same Analysis reports it is clear that the availability of GPS
observations was not affected.<br>
<br>
<br>
-- ANALYSIS --<br>
<br>
In any case it is still of interest to understand the impact
over certain stations since some show some small effect.<br>
<br>
I have projected below the expected tracking down to 3 deg
elevation of GLO satellite data from each MGEX station (using
MGEX to ensure many IGS Glonass tracking stations), and then
compared to the actual observations contained in the downloaded
data files. The figures below report the percent of the 24 hr
GLO constellation orbits covered, so ... for the entire GLO
constellation over 24 hr what should a station see in theory
down to 3 deg, versus what it actually observes! Many stations
can track down to 0 deg so they have more observations than
theorized to 3 deg but that is not significant in this analysis.<br>
<br>
<br>
Almost all stations performed fine over the event (d091-092),
for example:<br>
<br>
GLO coverage Observed||Theoretical<br>
14091.MGEX/NetworkQC_R.txt:For R; kour : 34.03 % 33.94 % <br>
14092.MGEX/NetworkQC_R.txt:For R; kour : 32.73 % 34.16 % <br>
14093.MGEX/NetworkQC_R.txt:For R; kour : 34.94 % 35.20 % <br>
14094.MGEX/NetworkQC_R.txt:For R; kour : 32.73 % 34.46 % <br>
<br>
14090.MGEX/NetworkQC_R.txt:For R; ous2 : 35.89 % 35.29 %
<br>
14091.MGEX/NetworkQC_R.txt:For R; ous2 : 35.98 % 35.81 %
<br>
14092.MGEX/NetworkQC_R.txt:For R; ous2 : 36.07 % 36.02 %
<br>
14093.MGEX/NetworkQC_R.txt:For R; ous2 : 35.81 % 35.98 %
<br>
14094.MGEX/NetworkQC_R.txt:For R; ous2 : 36.15 % 35.85 %
<br>
<br>
14090.MGEX/NetworkQC_R.txt:For R; brst : 35.59 % 36.76 %
<br>
14091.MGEX/NetworkQC_R.txt:For R; brst : 36.24 % 37.07 %
<br>
14092.MGEX/NetworkQC_R.txt:For R; brst : 35.37 % 36.76 %
<br>
14093.MGEX/NetworkQC_R.txt:For R; brst : 35.63 % 36.98 %
<br>
14094.MGEX/NetworkQC_R.txt:For R; brst : 34.98 % 36.37 %
<br>
</font></font><font size="+1"><font size="+1"><font size="+1"><font
size="+1"><br>
14090.MGEX/NetworkQC_R.txt:For R; usn5 : 32.25 % 34.24 % <br>
14091.MGEX/NetworkQC_R.txt:For R; usn5 : 31.60 % 33.90 % <br>
14092.MGEX/NetworkQC_R.txt:For R; usn5 : 31.12 % 34.20 % <br>
14093.MGEX/NetworkQC_R.txt:For R; usn5 : 31.03 % 33.46 % <br>
14094.MGEX/NetworkQC_R.txt:For R; usn5 : 31.25 % 33.59 % <br>
<br>
<br>
Some stations lost a few percent of GLO data on d091/092,
but after those days they recovered fine, for example:<br>
<br>
</font></font></font></font><font size="+1"><font size="+1"><font
size="+1"><font size="+1"><font size="+1"><font size="+1">
GLO </font></font></font></font></font></font><font
size="+1"><font size="+1"><font size="+1"><font size="+1"><font
size="+1"><font size="+1"><font size="+1"><font size="+1">coverage</font></font>
Observed||Theoretical<br>
</font></font>14090.MGEX/NetworkQC_R.txt:For R; scrz :
31.29 % 33.07 % <br>
14091.MGEX/NetworkQC_R.txt:For R; scrz : 28.65 % 32.55 % <br>
14092.MGEX/NetworkQC_R.txt:For R; scrz : 28.26 % 33.07 % <br>
14093.MGEX/NetworkQC_R.txt:For R; scrz : 31.77 % 33.25 % <br>
14094.MGEX/NetworkQC_R.txt:For R; scrz : 30.34 % 32.99 % <br>
<br>
14090.MGEX/NetworkQC_R.txt:For R; hofn : 39.37 % 38.37 % <br>
14091.MGEX/NetworkQC_R.txt:For R; hofn : 36.20 % 38.85 % <br>
14092.MGEX/NetworkQC_R.txt:For R; hofn : 33.03 % 38.41 % <br>
14093.MGEX/NetworkQC_R.txt:For R; hofn : 38.85 % 38.28 % <br>
14094.MGEX/NetworkQC_R.txt:For R; hofn : 37.93 % 38.11 % <br>
<br>
14090.MGEX/NetworkQC_R.txt:For R; thtg : 31.34 % 32.77 % <br>
14091.MGEX/NetworkQC_R.txt:For R; thtg : 28.47 % 32.73 % <br>
14092.MGEX/NetworkQC_R.txt:For R; thtg : 29.08 % 33.59 % <br>
14093.MGEX/NetworkQC_R.txt:For R; thtg : 32.25 % 33.38 % <br>
14094.MGEX/NetworkQC_R.txt:For R; thtg : 30.56 % 33.20 % <br>
<br>
<br>
No effect in the GPS tracking of the MGEX stations is
detected even when the GLO tracking is reduced a few percent
over the 24 hr period, so this is a very minor effect on the
days of the event of very limited consequence taking
everything into consideration.<br>
<br>
some stations over the affected days submitted partial Rinex
files (zim2, pado, etc) but its unclear if the partial files
are due to the GLO problem or not, so I have not considered
that issue, since in any case all the files a day after the
event are fine again. <br>
<br>
Thanks.<br>
</font></font></font></font>
<pre class="moz-signature" cols="72">--
Best regards,
Nacho
_______________________________________________________________
Ignacio (Nacho) Romero
Aerospace Engineer, PhD
SAC @ ESA/ESOC/HSO-GN
IGS Infrastructure Committee
<a class="moz-txt-link-abbreviated" href="http://www.canaryadvancedsolutions.com">www.canaryadvancedsolutions.com</a>
_______________________________________________________________
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