{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,1,13]],"date-time":"2026-01-13T01:08:21Z","timestamp":1768266501206,"version":"3.49.0"},"reference-count":50,"publisher":"MDPI AG","issue":"16","license":[{"start":{"date-parts":[[2023,8,16]],"date-time":"2023-08-16T00:00:00Z","timestamp":1692144000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"NASA","award":["WBS 936723.02.01.12.48"],"award-info":[{"award-number":["WBS 936723.02.01.12.48"]}]},{"name":"NASA","award":["WBS 880292.04.02.01.68"],"award-info":[{"award-number":["WBS 880292.04.02.01.68"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>A growing number of SmallSat\/CubeSat constellations with high-rate (50\u2013100 Hz) global navigation satellite system radio occultations (GNSS-RO) as well as low-rate (1 Hz) precise orbit determination (GNSS-POD) limb-viewing capabilities provide unprecedented spatial and temporal sampling rates for ionospheric studies. In the F-region electron density (Ne) retrieval process, instead of the conventional onion-peeling (OP) inversion, an optimal estimation (OE) inversion technique was recently developed using total electron content measurements acquired by GNSS-POD link. The new technique is applied to data acquired from the COSMIC-1, COSMIC-2, and Spire constellations. Although both OE and OP techniques use the Abel weighting function in Ne inversion, OE significantly differs in its performance, especially in the lower F- and E-regions. In this work, we evaluate and compare newly derived data sets using F2 peak properties with other space-based and ground-based observations. We determine the F2 peak Ne (NmF2) and its altitude (hmF2), and compare them with the OP-retrieved values. Good agreement is observed between the two techniques for both NmF2 and hmF2. In addition, we also utilize autoscaled F2 peak measurements from a number of worldwide Digisonde stations (\u223c30). The diurnal sensitivity and latitudinal variability of the F2 peak between the two techniques are carefully studied at these locations. Good agreement is observed between OE-retrieved NmF2 and Digisonde-measured NmF2. However, significant differences appear between OE-retrieved hmF2 and Digisonde-measured hmF2. During the daytime, Digisonde-measured hmF2 remains \u223c25\u201345 km below the OE-retrieved hmF2, especially at mid and high latitudes. We also incorporate F-region Ne measurements from two incoherent scatter radar observations at high latitudes, located in the North American (Millstone Hill) and European (EISCAT at Tromso) sectors. The radar measurements show good agreement with OE-retrieved values. Although there are several possible sources of error in the ionogram-derived Ne profiles, our further analysis on F1 and F2 layers indicates that the low Digisonde hmF2 is caused by the autoscaled method, which tends to detect a height systematically below the F2 peak when the F1 layer is present.<\/jats:p>","DOI":"10.3390\/rs15164048","type":"journal-article","created":{"date-parts":[[2023,8,16]],"date-time":"2023-08-16T10:08:09Z","timestamp":1692180489000},"page":"4048","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":8,"title":["Optimal Estimation Inversion of Ionospheric Electron Density from GNSS-POD Limb Measurements: Part II-Validation and Comparison Using NmF2 and hmF2"],"prefix":"10.3390","volume":"15","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-4275-8859","authenticated-orcid":false,"given":"Nimalan","family":"Swarnalingam","sequence":"first","affiliation":[{"name":"NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA"},{"name":"Department of Physics, The Catholic University of America, Washington, DC 20064, USA"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3490-9437","authenticated-orcid":false,"given":"Dong L.","family":"Wu","sequence":"additional","affiliation":[{"name":"NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3495-6372","authenticated-orcid":false,"given":"Daniel J.","family":"Emmons","sequence":"additional","affiliation":[{"name":"The Air Force Institute of Technology, Wright-Patterson AFB, Dayton, OH 45433, USA"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7778-3854","authenticated-orcid":false,"given":"Robert","family":"Gardiner-Garden","sequence":"additional","affiliation":[{"name":"The University of Adelaide, Adelaide 5005, Australia"}]}],"member":"1968","published-online":{"date-parts":[[2023,8,16]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"3","DOI":"10.1016\/j.jastp.2017.06.020","article-title":"Recent developments in the understanding of equatorial ionization anomaly: A review","volume":"171","author":"Balan","year":"2018","journal-title":"J. Atmos. Sol.-Terr. Phys."},{"key":"ref_2","unstructured":"Rishbeth, H., and Garriott, O.K. (1969). Introduction to Ionospheric Physics, Academic Press. [1st ed.]."},{"key":"ref_3","unstructured":"Reinisch, B.W. (2021). Digisonde 4D Technical Manual (Version 1.2.11), Lowell Digisonde International."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"477","DOI":"10.1029\/RS018i003p00477","article-title":"Automatic calculation of electron density profiles from digital ionograms. III\u2014Processing of bottomside ionograms","volume":"18","author":"Reinisch","year":"1983","journal-title":"Radio Sci."},{"key":"ref_5","unstructured":"Titheridge, J.E. (1985). Ionogram Analysis with the Generalised Program POLAN, UAG. Report UAG."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"6851","DOI":"10.1002\/2015JA021176","article-title":"Time-varying ionosonde trend: Case study of Sodankyl\u00e4 hmF2 data 1957\u20132014","volume":"120","author":"Roininen","year":"2015","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_7","first-page":"1","article-title":"The New ARTIST 5 for all Digisondes","volume":"69","author":"Galkin","year":"2008","journal-title":"Ionosonde Netw. Advis. Group Bull."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"377","DOI":"10.5047\/eps.2011.03.001","article-title":"Global Ionospheric Radio Observatory (GIRO)","volume":"63","author":"Reinisch","year":"2011","journal-title":"Earth Planets Space"},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"1824","DOI":"10.1109\/JRPROC.1958.286852","article-title":"Incoherent Scattering of Radio Waves by Free Electrons with Applications to Space Exploration","volume":"46","author":"Gordon","year":"1958","journal-title":"Proc. IRE"},{"key":"ref_10","unstructured":"Hargreaves, J.K. (1995). The Solar-Terrestrial Environment, Cambridge University Press."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"021","DOI":"10.3319\/TAO.2000.11.1.21(COSMIC)","article-title":"COSMIC System Description","volume":"11","author":"Rocken","year":"2000","journal-title":"Terr. Atmos. Ocean. Sci."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"1699","DOI":"10.1029\/2002GL014738","article-title":"Sounding of the topside ionosphere\/plasmasphere based on GPS measurements from CHAMP: Initial results","volume":"29","author":"Heise","year":"2002","journal-title":"J. Geophys. Res. Lett."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"A12309","DOI":"10.1029\/2011JA016600","article-title":"Longitudinal variations in the F region ionosphere and the topside ionosphere-plasmasphere: Observations and model simulations","volume":"116","author":"Pedatella","year":"2011","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"949","DOI":"10.1029\/1999RS900034","article-title":"Analysis and validation of GPS\/MET radio occultation data in the ionosphere","volume":"34","author":"Schreiner","year":"1999","journal-title":"Radio Sci."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"193","DOI":"10.1186\/BF03352376","article-title":"Comparisons of GPS\/MET retrieved ionospheric electron density and ground based ionosonde data","volume":"53","author":"Tsai","year":"2001","journal-title":"Earth Planets Space"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"217","DOI":"10.5194\/angeo-28-217-2010","article-title":"Error analysis of Abel retrieved electron density profiles from radio occultation measurements","volume":"28","author":"Yue","year":"2010","journal-title":"Ann. Geophys."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"8942","DOI":"10.1002\/2015JA021704","article-title":"An improved inversion for FORMOSAT-3\/COSMIC ionosphere electron density profiles","volume":"120","author":"Pedatella","year":"2015","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"825","DOI":"10.1007\/s10291-015-0491-z","article-title":"A self-contained GIM-aided Abel retrieval method to improve GNSS-radio occultation retrieved electron density profiles","volume":"20","author":"Su","year":"2016","journal-title":"GPS Solut."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"1386","DOI":"10.1002\/2016JA023027","article-title":"Ionospheric electron density inversion for Global Navigation Satellite Systems radio occultation using aided Abel inversions","volume":"122","author":"Chou","year":"2017","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_20","doi-asserted-by":"crossref","unstructured":"Wu, D.L., Emmons, D.J., and Swarnalingam, N. (2022). Global GNSS-RO Electron Density in the Lower Ionosphere. Remote Sens., 14.","DOI":"10.3390\/rs14071577"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"e28028","DOI":"10.1029\/2020JA028028","article-title":"The Early Results and Validation of FORMOSAT-7\/COSMIC-2 Space Weather Products: Global Ionospheric Specification and Ne-Aided Abel Electron Density Profile","volume":"125","author":"Lin","year":"2020","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"e30247","DOI":"10.1029\/2021JA030247","article-title":"Interhemispheric Asymmetries in Ionospheric Electron Density Responses During Geomagnetic Storms: A Study Using Space-Based and Ground-Based GNSS and AMPERE Observations","volume":"127","author":"Swarnalingam","year":"2022","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"3245","DOI":"10.3390\/rs15133245","article-title":"Optimal Estimation Inversion of Electron Density from GNSS-POD Limb Measurements: Part I\u2014Algorithm Description","volume":"15","author":"Wu","year":"2023","journal-title":"Remote Sens."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"2607","DOI":"10.5194\/angeo-27-2607-2009","article-title":"Gravity wave initiation of equatorial spread F\/plasma bubble irregularities based on observational data from the SpreadFEx campaign","volume":"27","author":"Abdu","year":"2009","journal-title":"Ann. Geophys."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"418","DOI":"10.1002\/2016SW001593","article-title":"International Reference Ionosphere 2016: From ionospheric climate to real-time weather predictions","volume":"15","author":"Bilitza","year":"2017","journal-title":"Space Weather"},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"2406","DOI":"10.1029\/JA077i013p02406","article-title":"Magnetic Apex Coordinates: A Magnetic Coordinate System for the Ionospheric F2 Laye","volume":"77","author":"VanZandt","year":"1972","journal-title":"J. Geophys. Res."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"968","DOI":"10.1029\/RS023i006p00968","article-title":"A comparison between the automatic ionogram scaling system ARTIST and the standard manual method","volume":"23","author":"Gilbert","year":"1988","journal-title":"Radio Sci."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"RS4011","DOI":"10.1029\/2005RS003440","article-title":"Quality figures and error bars for autoscaled Digisonde vertical incidence ionograms","volume":"41","author":"McNamara","year":"2006","journal-title":"Radio Sci."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"53","DOI":"10.5194\/gi-5-53-2016","article-title":"Comparison between manual scaling and Autoscala automatic scaling applied to Sodankyl\u00e4 Geophysical Observatory ionograms","volume":"5","author":"Enell","year":"2016","journal-title":"Geosci. Instrum. Methods Data Syst."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"RS4003","DOI":"10.1029\/2006RS003581","article-title":"Automatic scaling of critical frequency foF2 and MUF(3000)F2: A comparison between Autoscala and ARTIST 4.5 on Rome data","volume":"42","author":"Pezzopane","year":"2007","journal-title":"Radio Sci."},{"key":"ref_31","unstructured":"Galkin, I.A., Reinisch, B.W., Huang, X., and Khmyrov, G.M. (2013). Confidence Score of ARTIST-5 Ionogram Autoscaling, INAG. INAG Technical Memorandum."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"A09315","DOI":"10.1029\/2012JA017529","article-title":"Daytime climatology of ionospheric NmF2 and hmF2 from COSMIC data","volume":"117","author":"Burns","year":"2012","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"A09312","DOI":"10.1029\/2008JA013106","article-title":"Wave structures of the plasma density and vertical E \u00d7 B drift in low-latitude F region","volume":"113","author":"Kil","year":"2008","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"3059","DOI":"10.1002\/2014JA020820","article-title":"Interhemispheric asymmetry of the equatorial ionization anomaly in solstices observed by COSMIC during 2007\u20132012","volume":"120","author":"Luan","year":"2015","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"949","DOI":"10.1007\/s00190-011-0481-z","article-title":"Ionospheric electron density observed by FORMOSAT-3\/COSMIC over the European region and validated by ionosonde data","volume":"85","author":"Krankowski","year":"2011","journal-title":"J. Geod."},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"18","DOI":"10.1051\/swsc\/2020080","article-title":"Accuracy assessment of the quiet-time ionospheric F2 peak parameters as derived from COSMIC-2 multi-GNSS radio occultation measurements","volume":"11","author":"Cherniak","year":"2021","journal-title":"J. Space Weather Space Clim."},{"key":"ref_37","first-page":"1","article-title":"ARTIST Ionogram Autoscaling Confidence Scores: Best Practices","volume":"4","author":"Themens","year":"2022","journal-title":"URSI Radio Sci. Lett."},{"key":"ref_38","doi-asserted-by":"crossref","unstructured":"Scotto, C., and Sabbagh, D. (2020). The Accuracy of Real-time hmF2 Estimation from Ionosondes. Remote Sens., 12.","DOI":"10.3390\/rs12172671"},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"703","DOI":"10.1134\/S0016793221050078","article-title":"Errors in Estimating of the F2-Layer Peak Parameters in Automatic Systems for Processing the Ionograms in the Vertical Radio Sounding of the Ionosphere under Low Solar Activity Conditions","volume":"61","author":"Krasheninnikov","year":"2021","journal-title":"Geomagn. Aeron."},{"key":"ref_40","first-page":"283","article-title":"Evaluation of automatic ionogram scaling for use in real-time ionospheric density profile specification: Dourbes DGS-256\/ARTIST-4 performance","volume":"55","author":"Stankov","year":"2012","journal-title":"Ann. Geophys."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"RS2005","DOI":"10.1029\/2006RS003491","article-title":"Comparison of CHAMP and Digisonde plasma frequencies at Jicamarca, Peru","volume":"42","author":"McNamara","year":"2007","journal-title":"Radio Sci."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"1949","DOI":"10.1016\/j.asr.2004.06.009","article-title":"An analysis of automatically scaled F1 layer data over Grahamstown, South Africa","volume":"34","author":"Jacobs","year":"2004","journal-title":"Adv. Space Res."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"RS2S91","DOI":"10.1029\/2007RS003723","article-title":"A method for automatic scaling of F1 critical frequencies from ionograms","volume":"43","author":"Pezzopane","year":"2008","journal-title":"Radio Sci."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"23","DOI":"10.1016\/S0273-1177(00)00136-8","article-title":"Deducing topside profiles and total electron content from bottomside ionograms","volume":"27","author":"Reinisch","year":"2001","journal-title":"Adv. Space Res."},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"RS4006","DOI":"10.1029\/2005RS003367","article-title":"A study of the shape of topside electron density profile derived from incoherent scatter radar measurements over Arecibo and Millstone Hill","volume":"41","author":"Luan","year":"2006","journal-title":"Radio Sci."},{"key":"ref_46","first-page":"A10309","article-title":"Topside ionospheric effective scale heights (HT) derived with ROCSAT-1 and ground-based ionosonde observations at equatorial and midlatitude stations","volume":"114","author":"Su","year":"2009","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_47","doi-asserted-by":"crossref","first-page":"1603","DOI":"10.1002\/2017JA024817","article-title":"Topside Electron Density Representations for Middle and High Latitudes: A Topside Parameterization for E-CHAIM Based On the NeQuick","volume":"123","author":"Themens","year":"2018","journal-title":"J. Geophys. Res. Space Phys."},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"37","DOI":"10.1007\/s11214-022-00909-z","article-title":"The Ionospheric Equivalent Slab Thickness: A Review Supported by a Global Climatological Study Over Two Solar Cycles","volume":"218","author":"Pignalberi","year":"2022","journal-title":"Space Sci. Rev."},{"key":"ref_49","doi-asserted-by":"crossref","first-page":"e2022RG000792","DOI":"10.1029\/2022RG000792","article-title":"The International Reference Ionosphere Model: A Review and Description of an Ionospheric Benchmark","volume":"60","author":"Bilitza","year":"2022","journal-title":"Rev. Geophys."},{"key":"ref_50","doi-asserted-by":"crossref","first-page":"106","DOI":"10.1016\/j.jastp.2013.08.024","article-title":"Global model of the F2 layer peak height for low solar activity based on GPS radio-occultation data","volume":"104","author":"Shubin","year":"2013","journal-title":"J. Atmos. Sol.-Terr. Phys."}],"container-title":["Remote Sensing"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2072-4292\/15\/16\/4048\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T20:34:58Z","timestamp":1760128498000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2072-4292\/15\/16\/4048"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2023,8,16]]},"references-count":50,"journal-issue":{"issue":"16","published-online":{"date-parts":[[2023,8]]}},"alternative-id":["rs15164048"],"URL":"https:\/\/doi.org\/10.3390\/rs15164048","relation":{},"ISSN":["2072-4292"],"issn-type":[{"value":"2072-4292","type":"electronic"}],"subject":[],"published":{"date-parts":[[2023,8,16]]}}}