<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"><meta http-equiv="Content-Type" content="text/html charset=windows-1252"><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div>Dear GPS Broadcast Ionosphere Model Users/Colleagues,</div><div><br></div><div>We would like to draw your attention to an issue we identified in the IGS navigation files’ ionosphere coefficients.</div><div>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” </div><div>and each GNSS station’s individual navigation file), which was out of our expectation of normally one set of coefficients per day.</div><div><br></div><div>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 </div><div>sets/versions of ionospheric
coefficients in a single day, which seemed a problem for single-frequency GPS users: which version was correction?</div><div><br></div><div>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). </div><div>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 </div><div>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.</div><div><br></div><div>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 </div><div>(for both combined file “brdcddd0.yyn” and each GNSS station’s individual navigation file).</div><div><br></div><div>More information may be found at <a href="http://link.springer.com/article/10.1007/s10291-015-0448-2">http://link.springer.com/article/10.1007/s10291-015-0448-2</a></div><div><br></div><div>Thank you very much for your attention. Hopefully it is of help to single-frequency users who employ the model to correct ionospheric errors.</div><div><br></div><div>George</div><div apple-content-edited="true">
<div style="color: rgb(0, 0, 0); letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div style="orphans: 2; widows: 2;">****************************************************************************************************</div><div style="orphans: 2; widows: 2;">Dr. George LIU, Zhizhao, FHKMetS, MHKIES</div><div style="orphans: 2; widows: 2;">Assistant Professor</div><div style="orphans: 2; widows: 2;">Director, Hydrographic Surveying Laboratory</div><div style="orphans: 2; widows: 2;">Secretary, IAG Sub-Commission 4.5: High-Precision GNSS Algorithms and Applications</div><div style="orphans: 2; widows: 2;">Department of Land Surveying & Geo-Informatics (LSGI)</div><div style="orphans: 2; widows: 2;">Faculty of Construction and Environment (FCE)</div><div style="orphans: 2; widows: 2;">The Hong Kong Polytechnic University (PolyU)</div><div style="orphans: 2; widows: 2;">181 Chatham Road South, Hung Hom, Kowloon, Hong Kong</div><div style="orphans: 2; widows: 2;">P.R. China</div><div style="orphans: 2; widows: 2;">Tel. (852)2766 5961 (biz) Tel: (852)5178 3176 (Mobile)</div><div style="orphans: 2; widows: 2;">Fax: (852)2330 2994</div><div style="orphans: 2; widows: 2;">E-mail: <a href="mailto:lszzliu@polyu.edu.hk">lszzliu@polyu.edu.hk</a> (biz) <a href="mailto:zzliu2@gmail.com">zzliu2@gmail.com</a> (h)</div><div style="orphans: 2; widows: 2;"><a href="http://www.lsgi.polyu.edu.hk">http://www.lsgi.polyu.edu.hk</a></div><div style="orphans: 2; widows: 2;">****************************************************************************************************</div></div>
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