{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,4]],"date-time":"2026-07-04T20:50:06Z","timestamp":1783198206208,"version":"3.54.6"},"reference-count":56,"publisher":"MDPI AG","issue":"13","license":[{"start":{"date-parts":[[2023,6,28]],"date-time":"2023-06-28T00:00:00Z","timestamp":1687910400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Atomic clocks are highly precise timing devices used in numerous Positioning, Navigation, and Timing (PNT) applications on the ground and in outer space. In recent years, however, more precise timing solutions based on optical technology have been introduced as current technology capabilities advance. State-of-the-art optical clocks\u2014predicted to be the next level of their predecessor atomic clocks\u2014have achieved ultimate uncertainty of 1 \u00d7 10\u221218 and beyond, which exceeds the best atomic clock\u2019s performance by two orders of magnitude. Hence, the successful development of optical clocks has drawn significant attention in academia and industry to exploit many more opportunities. This paper first provides an overview of the emerging optical clock technology, its current development, and characteristics, followed by a clock stability analysis of some of the successfully developed optical clocks against current Global Navigation Satellite System (GNSS) satellite clocks to discuss the optical clock potentiality in GNSS positioning. The overlapping Allan Deviation (ADEV) method is applied to estimate the satellite clock stability from International GNSS Service (IGS) clock products, whereas the optical clock details are sourced from the existing literature. The findings are (a) the optical clocks are more stable than that of atomic clocks onboard GNSS satellites, though they may require further technological maturity to meet spacecraft payload requirements, and (b) in GNSS positioning, optical clocks could potentially offer less than a 1 mm range error (clock-related) in 30 s and at least 10 times better timing performance after 900 s in contrast to the Galileo satellite atomic clocks\u2014which is determined in this study as the most stable GNSS atomic clock type used in satellite positioning.<\/jats:p>","DOI":"10.3390\/s23135998","type":"journal-article","created":{"date-parts":[[2023,6,29]],"date-time":"2023-06-29T01:43:13Z","timestamp":1688002993000},"page":"5998","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":22,"title":["Evaluating Optical Clock Performance for GNSS Positioning"],"prefix":"10.3390","volume":"23","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-3477-6635","authenticated-orcid":false,"given":"Enkhtuvshin","family":"Boldbaatar","sequence":"first","affiliation":[{"name":"School of Science (Geospatial), RMIT University, Melbourne, VIC 3001, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5523-9711","authenticated-orcid":false,"given":"Donald","family":"Grant","sequence":"additional","affiliation":[{"name":"School of Science (Geospatial), RMIT University, Melbourne, VIC 3001, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Suelynn","family":"Choy","sequence":"additional","affiliation":[{"name":"School of Science (Geospatial), RMIT University, Melbourne, VIC 3001, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0719-674X","authenticated-orcid":false,"given":"Safoora","family":"Zaminpardaz","sequence":"additional","affiliation":[{"name":"School of Science (Geospatial), RMIT University, Melbourne, VIC 3001, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lucas","family":"Holden","sequence":"additional","affiliation":[{"name":"School of Science (Geospatial), RMIT University, Melbourne, VIC 3001, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2023,6,28]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"42001","DOI":"10.1088\/1681-7575\/ab1e68","article-title":"The Coordinated Universal Time (UTC)","volume":"56","author":"Panfilo","year":"2019","journal-title":"Metrologia"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"10","DOI":"10.1186\/s43020-020-00012-0","article-title":"Precise time scales and navigation systems: Mutual benefits of timekeeping and positioning","volume":"1","author":"Tavella","year":"2020","journal-title":"Satell. Navig."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"637","DOI":"10.1103\/RevModPhys.87.637","article-title":"Optical atomic clocks","volume":"87","author":"Ludlow","year":"2015","journal-title":"Rev. Mod. Phys."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"564","DOI":"10.1038\/s41586-021-03253-4","article-title":"Frequency ratio measurements at 18-digit accuracy using an optical clock network","volume":"591","author":"Brewer","year":"2021","journal-title":"Nature"},{"key":"ref_5","unstructured":"U.S. Department of Transportation (2022, August 08). What Is Positioning, Navigation and Timing (PNT)?, Available online: https:\/\/www.transportation.gov\/pnt\/what-positioning-navigation-and-timing-pnt."},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Morton, Y.J., van Diggelen, F., Spilker, J.J., Bradford, W.P., Sherman, L., and Grace, G. (2021). Position, Navigation, and Timing Technologies in the 21st Century: Integrated Satellite Navigation, Sensor Systems, and Civil Applications\u2014Set, Wiley-IEEE Press.","DOI":"10.1002\/9781119458449"},{"key":"ref_7","doi-asserted-by":"crossref","unstructured":"Montenbruck, O., Steigenberger, P., and Hauschild, A. (2020, January 20\u201323). Comparing the \u2018Big 4\u2032\u2014A User\u2019s View on GNSS Performance. Proceedings of the 2020 IEEE\/ION Position, Location and Navigation Symposium (PLANS), Portland, OR, USA.","DOI":"10.1109\/PLANS46316.2020.9110208"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"295","DOI":"10.1007\/s10291-012-0278-4","article-title":"Short-term analysis of GNSS clocks","volume":"17","author":"Hauschild","year":"2013","journal-title":"GPS Solut."},{"key":"ref_9","doi-asserted-by":"crossref","unstructured":"Teunissen, P., and Montenbruck, O. (2017). Springer Handbook of Global Navigation Satellite Systems, Springer International Publishing AG.","DOI":"10.1007\/978-3-319-42928-1"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"49","DOI":"10.1002\/navi.215","article-title":"Compact Optical Clock with 5 \u00d7 10\u221213 Instability at 1 s","volume":"65","author":"Phelps","year":"2018","journal-title":"Navigation"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"014019","DOI":"10.1103\/PhysRevApplied.9.014019","article-title":"Compact Optical Atomic Clock Based on a Two-Photon Transition in Rubidium","volume":"9","author":"Martin","year":"2018","journal-title":"Phys. Rev. Appl."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"83","DOI":"10.1007\/s10291-021-01113-2","article-title":"Optical clock technologies for global navigation satellite systems","volume":"25","author":"Schuldt","year":"2021","journal-title":"GPS Solut."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"053443","DOI":"10.1103\/PhysRevA.98.053443","article-title":"Towards an optical clock for space: Compact, high-performance optical lattice clock based on bosonic atoms","volume":"98","author":"Origlia","year":"2018","journal-title":"Phys. Rev. A"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"4723","DOI":"10.1016\/j.asr.2020.09.012","article-title":"GNSS-grade space atomic frequency standards: Current status and ongoing developments","volume":"68","author":"Batori","year":"2021","journal-title":"Adv. Space Res."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"27","DOI":"10.1007\/s10291-020-01059-x","article-title":"Past, present and future of atomic clocks for GNSS","volume":"25","author":"Jaduszliwer","year":"2021","journal-title":"GPS Solut."},{"key":"ref_16","unstructured":"Schmittberger, B.L., and Scherer, D.R. (2021). A Review of Contemporary Atomic Frequency Standards. arXiv."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"4753","DOI":"10.1016\/j.asr.2021.03.008","article-title":"Precise orbit and Earth parameter determination supported by LEO satellites, inter-satellite links and synchronized clocks of a future GNSS","volume":"68","author":"Michalak","year":"2021","journal-title":"Adv. Space Res."},{"key":"ref_18","doi-asserted-by":"crossref","unstructured":"Giorgi, G., Kroese, B., and Michalak, G. (2019, January 2\u20139). Future GNSS constellations with optical inter-satellite links. Preliminary space segment analyses. Proceedings of the 2019 IEEE Aerospace Conference, Big Sky, MT, USA.","DOI":"10.1109\/AERO.2019.8742105"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"35","DOI":"10.1088\/0026-1394\/45\/1\/006","article-title":"GPS All in View time transfer for TAI computation","volume":"45","author":"Petit","year":"2007","journal-title":"Metrologia"},{"key":"ref_20","first-page":"555","article-title":"Optical atomic clocks","volume":"36","author":"Poli","year":"2013","journal-title":"Riv. Nuovo Cim."},{"key":"ref_21","first-page":"531","article-title":"Optical Atomic Clocks for Redefining SI Units of Time and Frequency","volume":"35","author":"Sharma","year":"2020","journal-title":"M\u0100PAN J. Metrol. Soc. India"},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"055009","DOI":"10.1088\/1681-7575\/ab3a82","article-title":"On a definition of the SI second with a set of optical clock transitions","volume":"56","author":"Lodewyck","year":"2019","journal-title":"Metrologia"},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"448","DOI":"10.1364\/OPTICA.6.000448","article-title":"Towards the optical second: Verifying optical clocks at the SI limit","volume":"6","author":"McGrew","year":"2019","journal-title":"Optica"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"41","DOI":"10.1088\/0026-1394\/4\/1\/006","article-title":"News from the International Bureau of Weights and Measures","volume":"4","author":"Terrien","year":"1968","journal-title":"Metrologia"},{"key":"ref_25","unstructured":"(2008). The International System Of Units (SI), National Institute of Standards and Technology."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"188","DOI":"10.1088\/1681-7575\/aaa302","article-title":"The CIPM list of recommended frequency standard values: Guidelines and procedures","volume":"55","author":"Riehle","year":"2018","journal-title":"Metrologia"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"1215","DOI":"10.1126\/science.1240420","article-title":"An atomic clock with 10\u221218 instability","volume":"341","author":"Hinkley","year":"2013","journal-title":"Science"},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"87","DOI":"10.1038\/s41586-018-0738-2","article-title":"Atomic clock performance enabling geodesy below the centimetre level","volume":"564","author":"McGrew","year":"2018","journal-title":"Nature"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"71","DOI":"10.1038\/nature12941","article-title":"An optical lattice clock with accuracy and stability at the 10\u201318 level","volume":"506","author":"Bloom","year":"2014","journal-title":"Nature"},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"103106","DOI":"10.1063\/1.5006962","article-title":"Compact Rb optical frequency standard with 10\u221215 stability","volume":"88","author":"Zhang","year":"2017","journal-title":"Rev. Sci. Instrum."},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Lemke, N.D., Martin, K.W., Beard, R., Stuhl, B.K., Metcalf, A.J., and Elgin, J.D. (2022). Measurement of Optical Rubidium Clock Frequency Spanning 65 Days. Sensors, 22.","DOI":"10.3390\/s22051982"},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"1297","DOI":"10.1103\/RevModPhys.78.1297","article-title":"Nobel Lecture: Passion for precision","volume":"78","year":"2006","journal-title":"Rev. Mod. Phys."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"2264","DOI":"10.1103\/PhysRevLett.85.2264","article-title":"Optical Frequency Synthesizer for Precision Spectroscopy","volume":"85","author":"Holzwarth","year":"2000","journal-title":"Phys. Rev. Lett."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"153","DOI":"10.1038\/s42005-019-0249-y","article-title":"20 years of developments in optical frequency comb technology and applications","volume":"2","author":"Fortier","year":"2019","journal-title":"Commun. Phys."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"112","DOI":"10.1007\/s00340-017-6671-5","article-title":"A compact, transportable single-ion optical clock with 7.8 \u00d7 10\u221217 systematic uncertainty","volume":"123","author":"Cao","year":"2017","journal-title":"Appl. Phys. B"},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"073601","DOI":"10.1103\/PhysRevLett.118.073601","article-title":"Transportable Optical Lattice Clock with 7 \u00d7 10\u221217 Uncertainty","volume":"118","author":"Koller","year":"2017","journal-title":"Phys. Rev. Lett."},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"680","DOI":"10.1364\/OPTICA.6.000680","article-title":"Architecture for the photonic integration of an optical atomic clock","volume":"6","author":"Newman","year":"2019","journal-title":"Optica"},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"027107","DOI":"10.1117\/1.OE.60.2.027107","article-title":"Developing a chip-scale optical clock","volume":"60","author":"Zhou","year":"2021","journal-title":"Opt. Eng."},{"key":"ref_39","unstructured":"Ghent University (2023, February 02). An Optical Atomic Clock on a Chip Unleashes a Revolution in Time Registration. Available online: https:\/\/www.ugent.be\/voor-organisaties-en-bedrijven\/en\/news-events\/news\/optical-atomic-clock-on-a-chip-unleashes-a-revolution-in-time-registration."},{"key":"ref_40","doi-asserted-by":"crossref","unstructured":"Schiller, S., Gorlitz, A., Nevsky, A., Alighanbari, S., Vasilyev, S., Abou-Jaoudeh, C., Mura, G., Franzen, T., Sterr, U., and Falke, S. (2012, January 23\u201327). The space optical clocks project: Development of high-performance transportable and breadboard optical clocks and advanced subsystems. Proceedings of the 2012 European Frequency and Time Forum, Gothenburg, Sweden.","DOI":"10.1109\/EFTF.2012.6502414"},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"9","DOI":"10.1140\/epjqt\/s40507-017-0063-y","article-title":"JOKARUS\u2014Design of a compact optical iodine frequency reference for a sounding rocket mission","volume":"4","author":"Schkolnik","year":"2017","journal-title":"EPJ Quantum Technol."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"054068","DOI":"10.1103\/PhysRevApplied.11.054068","article-title":"Iodine Frequency Reference on a Sounding Rocket","volume":"11","author":"Gutsch","year":"2019","journal-title":"Phys. Rev. Appl."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"025008","DOI":"10.1103\/RevModPhys.90.025008","article-title":"Search for new physics with atoms and molecules","volume":"90","author":"Safronova","year":"2018","journal-title":"Rev. Mod. Phys."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"487","DOI":"10.1007\/s00190-017-1075-1","article-title":"Geodetic methods to determine the relativistic redshift at the level of 10\u221218 in the context of international timescales: A review and practical results","volume":"92","author":"Denker","year":"2018","journal-title":"J. Geod."},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"064401","DOI":"10.1088\/1361-6633\/aab409","article-title":"Atomic clocks for geodesy","volume":"81","author":"Grosche","year":"2018","journal-title":"Rep. Prog. Phys."},{"key":"ref_46","doi-asserted-by":"crossref","first-page":"71","DOI":"10.1007\/s00190-020-01401-8","article-title":"Using quantum optical sensors for determining the Earth\u2019s gravity field from space","volume":"94","author":"Wu","year":"2020","journal-title":"J. Geod."},{"key":"ref_47","doi-asserted-by":"crossref","first-page":"411","DOI":"10.1038\/s41566-020-0619-8","article-title":"Test of general relativity by a pair of transportable optical lattice clocks","volume":"14","author":"Takamoto","year":"2020","journal-title":"Nat. Photonics"},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"223","DOI":"10.1038\/s41567-020-01038-6","article-title":"Intercontinental comparison of optical atomic clocks through very long baseline interferometry","volume":"17","author":"Pizzocaro","year":"2021","journal-title":"Nat. Phys."},{"key":"ref_49","doi-asserted-by":"crossref","first-page":"6896","DOI":"10.1038\/ncomms7896","article-title":"Systematic evaluation of an atomic clock at 2 \u00d7 10\u221218 total uncertainty","volume":"6","author":"Nicholson","year":"2015","journal-title":"Nat. Commun."},{"key":"ref_50","doi-asserted-by":"crossref","unstructured":"Allan, D.W., and Barnes, J.A. (1981, January 27\u201329). A Modified \u201cAllan Variance\u201d with Increased Oscillator Characterization Ability. Proceedings of the Thirty Fifth Annual Frequency Control Symposium, Philadelphia, PA, USA.","DOI":"10.1109\/FREQ.1981.200514"},{"key":"ref_51","unstructured":"William, R., and David, H. (2008). Handbook of Frequency Stability Analysis, National Institute of Standards and Technology. Special Publication (NIST SP)."},{"key":"ref_52","doi-asserted-by":"crossref","unstructured":"Howe, D.A., Allan, D.U., and Barnes, J.A. (1981, January 27\u201329). Properties of Signal Sources and Measurement Methods. Proceedings of the Thirty Fifth Annual Frequency Control Symposium, Philadelphia, PA, USA.","DOI":"10.1109\/FREQ.1981.200541"},{"key":"ref_53","doi-asserted-by":"crossref","first-page":"211","DOI":"10.1007\/s10291-008-0089-9","article-title":"Characterization of periodic variations in the GPS satellite clocks","volume":"12","author":"Senior","year":"2008","journal-title":"GPS Solut."},{"key":"ref_54","unstructured":"Allan, D., Hellwig, H., Kartaschoff, P., Vanier, J., Vig, J., Winkler, G.M.R., and Yannoni, N.F. (1988, January 1\u20133). Standard terminology for fundamental frequency and time metrology. Proceedings of the 42nd Annual Frequency Control Symposium, Baltimore, MD, USA."},{"key":"ref_55","unstructured":"IGS (2022, August 23). IGS Products. Available online: https:\/\/igs.org\/products\/#about."},{"key":"ref_56","unstructured":"Hofmann-Wellenhof, B., Lichtenegger, H., and Wasle, E. (2008). GNSS\u2014Global Navigation Satellite Systems GPS, GLONASS, Galileo, and More, Springer. [1st ed.]."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/23\/13\/5998\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T20:02:39Z","timestamp":1760126559000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/23\/13\/5998"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2023,6,28]]},"references-count":56,"journal-issue":{"issue":"13","published-online":{"date-parts":[[2023,7]]}},"alternative-id":["s23135998"],"URL":"https:\/\/doi.org\/10.3390\/s23135998","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2023,6,28]]}}}