{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,1,22]],"date-time":"2026-01-22T07:09:40Z","timestamp":1769065780021,"version":"3.49.0"},"reference-count":48,"publisher":"MDPI AG","issue":"2","license":[{"start":{"date-parts":[[2018,1,25]],"date-time":"2018-01-25T00:00:00Z","timestamp":1516838400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100012166","name":"National Key Research and Development Program of China","doi-asserted-by":"publisher","award":["2016YFB0501902"],"award-info":[{"award-number":["2016YFB0501902"]}],"id":[{"id":"10.13039\/501100012166","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["41574025"],"award-info":[{"award-number":["41574025"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["41574013"],"award-info":[{"award-number":["41574013"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["41731069"],"award-info":[{"award-number":["41731069"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]},{"name":"Spanish Ministry of Science and Innovation project","award":["CGL2015-66410-P"],"award-info":[{"award-number":["CGL2015-66410-P"]}]},{"name":"The Hong Kong RGC Joint Research Scheme","award":["E-PolyU501\/16"],"award-info":[{"award-number":["E-PolyU501\/16"]}]},{"DOI":"10.13039\/501100011354","name":"State Key Laboratory of Geo-information Engineering","doi-asserted-by":"publisher","award":["SKLGIE2015-M-2-2"],"award-info":[{"award-number":["SKLGIE2015-M-2-2"]}],"id":[{"id":"10.13039\/501100011354","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>A high-accuracy Global Ionosphere Model (GIM) is significant for precise positioning and navigating with the Global Navigation Satellite System (GNSS), as well as space weather applications. To obtain a precise GIM, it is critical to take both the ionospheric observable and mathematical model into consideration. In this contribution, the undifferenced ambiguity-fixed carrier-phase ionospheric observable is first determined from a global distribution of permanent receivers. Accuracy assessment with a co-located station experiment shows that the observational errors affecting the ambiguity-fixed carrier-phase ionospheric observables range from 0.10 to 0.35 Total Electron Content Units (TECUs, where 1 TECU     =   10   16    e \u2212  \/  m 2      and corresponds to 0.162 m on the Global Positioning System, GPS L1 frequency), indicating that the ambiguity-fixed carrier-phase ionospheric observable is over one order of magnitude more accurate than the carrier-phase leveled-code one (from 1.21 to 3.77 TECUs). Second, to better model the structure of the ionosphere, a two-layer GIM has been built based on the above carrier-phase observable. Preliminary global accuracy evaluation demonstrates that the accuracy of the two-layer GIM is below 1 TECU and about 2 TECUs during low and high solar activity periods. Third, the single-frequency point positioning experiment is adopted to test the ionosphere mitigation effects of the GIMs. Positioning results demonstrate that the single-frequency positioning accuracy can be improved by more than 30% using the undifferenced ambiguity-fixed ionospheric observable-derived two-layer GIM, compared with that using the carrier-phase leveled-code ionospheric observable-based single-layer GIM.<\/jats:p>","DOI":"10.3390\/rs10020169","type":"journal-article","created":{"date-parts":[[2018,1,25]],"date-time":"2018-01-25T12:25:49Z","timestamp":1516883149000},"page":"169","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":10,"title":["The Impacts of the Ionospheric Observable and Mathematical Model on the Global Ionosphere Model"],"prefix":"10.3390","volume":"10","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-5505-2903","authenticated-orcid":false,"given":"Wenfeng","family":"Nie","sequence":"first","affiliation":[{"name":"Institute of Space Sciences, Shandong University, 180 Wenhuaxi Road, Weihai 264209, China"},{"name":"State Key Laboratory of Geo-Information Engineering, Xi\u2019an Research Institute of Surveying and Mapping, Xi\u2019an 710054, China"},{"name":"Department of Land Surveying and Geo-Informatics, The Hong Kong Polytechnic University, Hong Kong 999077, China"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5818-6264","authenticated-orcid":false,"given":"Tianhe","family":"Xu","sequence":"additional","affiliation":[{"name":"Institute of Space Sciences, Shandong University, 180 Wenhuaxi Road, Weihai 264209, China"},{"name":"State Key Laboratory of Geo-Information Engineering, Xi\u2019an Research Institute of Surveying and Mapping, Xi\u2019an 710054, China"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7320-5029","authenticated-orcid":false,"given":"Adri\u00e0","family":"Rovira-Garcia","sequence":"additional","affiliation":[{"name":"Research Group of Astronomy and Geomatics (gAGE), Universitat Politecnica de Catalunya (UPC), Barcelona 08034, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1126-2367","authenticated-orcid":false,"given":"Jos\u00e9","family":"Zornoza","sequence":"additional","affiliation":[{"name":"Research Group of Astronomy and Geomatics (gAGE), Universitat Politecnica de Catalunya (UPC), Barcelona 08034, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8880-7084","authenticated-orcid":false,"given":"Jaume","family":"Subirana","sequence":"additional","affiliation":[{"name":"Research Group of Astronomy and Geomatics (gAGE), Universitat Politecnica de Catalunya (UPC), Barcelona 08034, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6765-2407","authenticated-orcid":false,"given":"Guillermo","family":"Gonz\u00e1lez-Casado","sequence":"additional","affiliation":[{"name":"Research Group of Astronomy and Geomatics (gAGE), Universitat Politecnica de Catalunya (UPC), Barcelona 08034, Spain"}]},{"given":"Wu","family":"Chen","sequence":"additional","affiliation":[{"name":"Department of Land Surveying and Geo-Informatics, The Hong Kong Polytechnic University, Hong Kong 999077, China"}]},{"given":"Guochang","family":"Xu","sequence":"additional","affiliation":[{"name":"Institute of Space Sciences, Shandong University, 180 Wenhuaxi Road, Weihai 264209, China"}]}],"member":"1968","published-online":{"date-parts":[[2018,1,25]]},"reference":[{"key":"ref_1","unstructured":"Hargreaves, J. 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