[IGSSTATION-5745] GLONASS Navigation message failure impact on IGS

Nacho Romero nacho at canaryadvancedsolutions.com
Sat Apr 12 06:22:05 PDT 2014


Author: Nacho Romero, Infrastructure Committee Chairman

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
(http://www.insidegnss.com/node/3979, 
http://gpsworld.com/glonass-gone-then-back/, 
http://gpsworld.com/altus-positioning-systems-pinpoints-cause-for-glonass-default/) 
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).

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.

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.

-- SUMMARY --

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.

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.

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.


-- ANALYSIS --

In any case it is still of interest to understand the impact over 
certain stations since some show some small effect.

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.


Almost all stations performed fine over the event (d091-092), for example:

                          GLO coverage Observed||Theoretical
14091.MGEX/NetworkQC_R.txt:For R; kour : 34.03 % 33.94 %
14092.MGEX/NetworkQC_R.txt:For R; kour : 32.73 % 34.16 %
14093.MGEX/NetworkQC_R.txt:For R; kour : 34.94 % 35.20 %
14094.MGEX/NetworkQC_R.txt:For R; kour : 32.73 % 34.46 %

14090.MGEX/NetworkQC_R.txt:For R; ous2 : 35.89 % 35.29 %
14091.MGEX/NetworkQC_R.txt:For R; ous2 : 35.98 % 35.81 %
14092.MGEX/NetworkQC_R.txt:For R; ous2 : 36.07 % 36.02 %
14093.MGEX/NetworkQC_R.txt:For R; ous2 : 35.81 % 35.98 %
14094.MGEX/NetworkQC_R.txt:For R; ous2 : 36.15 % 35.85 %

14090.MGEX/NetworkQC_R.txt:For R; brst : 35.59 % 36.76 %
14091.MGEX/NetworkQC_R.txt:For R; brst : 36.24 % 37.07 %
14092.MGEX/NetworkQC_R.txt:For R; brst : 35.37 % 36.76 %
14093.MGEX/NetworkQC_R.txt:For R; brst : 35.63 % 36.98 %
14094.MGEX/NetworkQC_R.txt:For R; brst : 34.98 % 36.37 %

14090.MGEX/NetworkQC_R.txt:For R; usn5 : 32.25 % 34.24 %
14091.MGEX/NetworkQC_R.txt:For R; usn5 : 31.60 % 33.90 %
14092.MGEX/NetworkQC_R.txt:For R; usn5 : 31.12 % 34.20 %
14093.MGEX/NetworkQC_R.txt:For R; usn5 : 31.03 % 33.46 %
14094.MGEX/NetworkQC_R.txt:For R; usn5 : 31.25 % 33.59 %


Some stations lost a few percent of GLO data on d091/092, but after 
those days they recovered fine, for example:

GLO coverage Observed||Theoretical
14090.MGEX/NetworkQC_R.txt:For R; scrz : 31.29 % 33.07 %
14091.MGEX/NetworkQC_R.txt:For R; scrz : 28.65 % 32.55 %
14092.MGEX/NetworkQC_R.txt:For R; scrz : 28.26 % 33.07 %
14093.MGEX/NetworkQC_R.txt:For R; scrz : 31.77 % 33.25 %
14094.MGEX/NetworkQC_R.txt:For R; scrz : 30.34 % 32.99 %

14090.MGEX/NetworkQC_R.txt:For R; hofn : 39.37 % 38.37 %
14091.MGEX/NetworkQC_R.txt:For R; hofn : 36.20 % 38.85 %
14092.MGEX/NetworkQC_R.txt:For R; hofn : 33.03 % 38.41 %
14093.MGEX/NetworkQC_R.txt:For R; hofn : 38.85 % 38.28 %
14094.MGEX/NetworkQC_R.txt:For R; hofn : 37.93 % 38.11 %

14090.MGEX/NetworkQC_R.txt:For R; thtg : 31.34 % 32.77 %
14091.MGEX/NetworkQC_R.txt:For R; thtg : 28.47 % 32.73 %
14092.MGEX/NetworkQC_R.txt:For R; thtg : 29.08 % 33.59 %
14093.MGEX/NetworkQC_R.txt:For R; thtg : 32.25 % 33.38 %
14094.MGEX/NetworkQC_R.txt:For R; thtg : 30.56 % 33.20 %


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.

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.

Thanks.

-- 
Best regards,
Nacho
_______________________________________________________________

Ignacio (Nacho) Romero
Aerospace Engineer, PhD
SAC @ ESA/ESOC/HSO-GN
IGS Infrastructure Committee
www.canaryadvancedsolutions.com
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