[IGSMAIL-7056] an anomaly issue found in the GPS broadcast ionosphere model coefficients in the IGS navigation files

[Zhizhao Liu]

Dear GPS Broadcast Ionosphere Model Users/Colleagues,

We would like to draw your attention to an issue we identified in the IGS navigation files’ ionosphere coefficients.
We found in most time of the past 22 years, multiple sets/versions of GPS broadcast ionosphere model coefficients appeared in the IGS daily navigation files (for both combined file “brdcddd0.yyn” 
and each GNSS station’s individual navigation file), which was out of our expectation of normally one set of coefficients per day.

That finding was based on an analysis of navigation file data collected from all the IGS stations worldwide over a 22-year period (1992–2013). We found that different GPS receivers reported as many as eight 
sets/versions of ionospheric coefficients in a single day, which seemed a problem for single-frequency GPS users: which version was correction?

We took one particular day’s data as an example (June 1, 2000), we used different versions of GPS broadcast ionosphere model coefficients of that day to compute vertical total electron contents (VTEC). 
We found the largest VTEC at one of our test stations could be as large as 153.3 TECu using one set of coefficients, which was 16.36 times larger than the smallest VTEC of 9.37 TECu computed from 
using another set of coefficients. For single-frequency GPS users, if a different set of coefficients is used, large positioning and navigation errors may be caused.

Our study suggested this was an issue related to GNSS receiver models/types. It suggests that more quality controls might be taken, if possible, in the generation of daily navigation files 
(for both combined file “brdcddd0.yyn” and each GNSS station’s individual navigation file).

More information may be found at http://link.springer.com/article/10.1007/s10291-015-0448-2

Thank you very much for your attention. Hopefully it is of help to single-frequency users who employ the model to correct ionospheric errors.

George
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Dr. George LIU, Zhizhao, FHKMetS, MHKIES
Assistant Professor
Director, Hydrographic Surveying Laboratory
Secretary, IAG Sub-Commission 4.5: High-Precision GNSS Algorithms and Applications
Department of Land Surveying & Geo-Informatics (LSGI)
Faculty of Construction and Environment (FCE)
The Hong Kong Polytechnic University (PolyU)
181 Chatham Road South, Hung Hom, Kowloon, Hong Kong
P.R. China
Tel. (852)2766 5961 (biz) Tel: (852)5178 3176 (Mobile)
Fax: (852)2330 2994
E-mail: lszzliu at polyu.edu.hk (biz) zzliu2 at gmail.com (h)
http://www.lsgi.polyu.edu.hk
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