{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,6,2]],"date-time":"2026-06-02T01:50:43Z","timestamp":1780365043846,"version":"3.54.1"},"reference-count":29,"publisher":"MDPI AG","issue":"2","license":[{"start":{"date-parts":[[2018,2,14]],"date-time":"2018-02-14T00:00:00Z","timestamp":1518566400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"UK Engineering and Physical Sciences Research Council","award":["EP\/L015463\/1"],"award-info":[{"award-number":["EP\/L015463\/1"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>As Global Navigation Satellite System (GNSS) signals travel through the troposphere, a tropospheric delay occurs due to a change in the refractive index of the medium. The Precise Point Positioning (PPP) technique can achieve centimeter\/millimeter positioning accuracy with only one GNSS receiver. The Zenith Tropospheric Delay (ZTD) is estimated alongside with the position unknowns in PPP. Estimated ZTD can be very useful for meteorological applications, an example is the estimation of water vapor content in the atmosphere from the estimated ZTD. PPP is implemented with different algorithms and models in online services and software packages. In this study, a performance assessment with analysis of ZTD estimates from three PPP online services and three software packages is presented. The main contribution of this paper is to show the accuracy of ZTD estimation achievable in PPP. The analysis also provides the GNSS users and researchers the insight of the processing algorithm dependence and impact on PPP ZTD estimation. Observation data of eight whole days from a total of nine International GNSS Service (IGS) tracking stations spread in the northern hemisphere, the equatorial region and the southern hemisphere is used in this analysis. The PPP ZTD estimates are compared with the ZTD obtained from the IGS tropospheric product of the same days. The estimates of two of the three online PPP services show good agreement (&lt;1 cm) with the IGS ZTD values at the northern and southern hemisphere stations. The results also show that the online PPP services perform better than the selected PPP software packages at all stations.<\/jats:p>","DOI":"10.3390\/s18020580","type":"journal-article","created":{"date-parts":[[2018,2,14]],"date-time":"2018-02-14T07:01:42Z","timestamp":1518591702000},"page":"580","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":45,"title":["Analysing the Zenith Tropospheric Delay Estimates in On-line Precise Point Positioning (PPP) Services and PPP Software Packages"],"prefix":"10.3390","volume":"18","author":[{"given":"Jorge","family":"Mendez Astudillo","sequence":"first","affiliation":[{"name":"International Doctoral Innovation Centre, University of Nottingham Ningbo China, Ningbo 315100, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8783-9666","authenticated-orcid":false,"given":"Lawrence","family":"Lau","sequence":"additional","affiliation":[{"name":"Department of Civil Engineering, University of Nottingham Ningbo China, Ningbo 315100, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yu-Ting","family":"Tang","sequence":"additional","affiliation":[{"name":"School of Geographical Sciences, University of Nottingham Ningbo China, Ningbo 315100, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Terry","family":"Moore","sequence":"additional","affiliation":[{"name":"The Nottingham Geospatial Institute, University of Nottingham, Nottingham NG7 2RD, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2018,2,14]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"457","DOI":"10.1017\/S0373463307004341","article-title":"Investigation into phase multipath mitigation techniques for high precision positioning in difficult environments","volume":"60","author":"Lau","year":"2007","journal-title":"J. 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