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Editorial Board Members’ Collection Series: “Recent Progress of GNSS/GPS Radio Occultation Techniques”

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A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section "Satellite Missions for Earth and Planetary Exploration".

Deadline for manuscript submissions: 15 May 2024 | Viewed by 8264

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Dr. Shu-peng Ho

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Guest Editor

National Oceanic and Atmospheric Administration, NESDIS/STAR/SMCD, College Park, MD 20740-3818, USA
Interests: satellite remote sensing; GNSS applications on meteorology and climate
Special Issues, Collections and Topics in MDPI journals

Dr. Michael E. Gorbunov

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Guest Editor

A.M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, 119017 Moscow, Russia
Interests: GNSS; GPS; radio occultation
Special Issues, Collections and Topics in MDPI journals

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Dear Colleagues,

In recent decades, global navigation satellite systems (GNSS), including the global positioning system (GPS), have provided unique opportunities to sense the Earth’s environments from a variety of observing geometries with relatively low-cost sensors. The application of radio occultations (RO) for numerical weather prediction, ionosphere, and space weather has been growing. This is explained by the fact that RO observations, which are unique in some respects, allow achieving high accuracy and vertical resolution in sounding the Earth’s atmosphere and ionosphere.

GNSS-RO measurements are fundamentally self-calibrating and do not require any external calibration source. As a result, they can be assimilated into numerical weather prediction models without any bias correction and are ideally suited for long-term climate monitoring. In recent years, the potential values of GNSS reflections in a wide array of Earth science and applications, including coastal altimetry, ocean winds, and soil moisture, have garnered increasing attention. With high spatial resolutions and rapid revisit times, these measurements can complement traditional sensors at a small fraction of the cost.

This Topic Collection is aimed at the submission of both review and original research articles related to, but not limited to:

GNSS radio occultation techniques and applications;
Radio occultation theory;
Radio occultation climate data records;
Accuracy and precision of radio occultation data;
Ionosphere understanding and modeling;
Space weather and climate prediction or forecasting.

Manuscripts for this important Special Issue of Remote Sensing will be accepted by the editorial office, the editor-in-chief, and editorial board members by invitation only.

Dr. Shu-peng Ho
Dr. Michael E. Gorbunov
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Remote Sensing is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

radio occultation
ionosphere
space weather
numerical weather prediction
variational assimilation

Published Papers (7 papers)

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Research

17 pages, 6142 KiB

Open AccessArticle

Quality Assessment of the Atmospheric Radio Occultation Profiles from FY-3E/GNOS-II BDS and GPS Measurements

by Youlin He, Shaocheng Zhang, Sheng Guo and Yunlong Wu

Remote Sens. 2023, 15(22), 5313; https://doi.org/10.3390/rs15225313 - 10 Nov 2023

Cited by 1 | Viewed by 690

Abstract

The Fengyun-3E (FY-3E) satellite carrying the advanced Global Navigation Satellite System (GNSS) Radio Occultation Sounder-II (GNOS-II) is already in operation for radio occultation (RO) observation, with the BeiDou Navigation Satellite System (BDS-2 and BDS-3) and Global Positioning System (GPS) signals tracking capability. FY-3E BDS and GPS RO signals tracking capability were first evaluated by comparing their penetration depths, and then the quality of the refractivity, temperature, and specific humidity profiles was analyzed with the fifth-generation European Centre for Medium-Range Weather Forecasts reanalysis (ERA5) data. Results show the mean penetration depth of BDS occultations was 1.65 km compared to 1.09 km of GPS occultations. Between 5 and 25 km, the mean refractivity bias of the BDS (GPS) was −0.14% (0.01%) with the mean standard deviation (SD) being 1.11% (1.52%); the mean temperature biases of both were within ±0.1 K, and the mean SD of BDS was 1.1 K compared to 1.2 K for the GPS; BDS/GPS specific humidity bias was within ±0.3 g/kg with corresponding SD being less than 1.3 g/kg. Seasonal deviations of specific humidities were largest in summer and smallest in winter. Latitudinal deviations over the tropics were generally higher than in other areas. Enriched quantity and high accuracy and precision after careful calibration will promote the FY-3E RO profiles as a reliable data source for the RO community. Full article

(This article belongs to the Special Issue Editorial Board Members’ Collection Series: “Recent Progress of GNSS/GPS Radio Occultation Techniques”)

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16 pages, 7327 KiB

Open AccessArticle

Preliminary Assessment of BDS Radio Occultation Retrieval Quality and Coverage Using FY-3E GNOS II Measurements

by Congliang Liu, Mi Liao, Yueqiang Sun, Xi Wang, Jiahui Liang, Xiuqing Hu, Peng Zhang, Guanglin Yang, Yan Liu, Jinsong Wang, Weihua Bai, Qifei Du, Xiangguang Meng, Peng Hu, Guangyuan Tan, Xianyi Wang, Junming Xia, Feixiong Huang, Cong Yin, Yuerong Cai, Wei Li, Peixian Li and Gottfried Kirchengast Show full author list Hide full author list

Remote Sens. 2023, 15(20), 5011; https://doi.org/10.3390/rs15205011 - 18 Oct 2023

Viewed by 830

Abstract

The FengYun-3E Global Navigation Satellite System (GNSS) occultation sounder II (FY-3E GNOS II) was launched on 5 July 2021. For the first time, based on the new GNOS II sensor, this mission realizes radio occultation (RO) and reflectometry observations using the navigation signals from the third-generation BeiDou System (BDS-3), and it is hence important to assess and analyze the BDS-3 remote sensing performances relative to other systems. Here, we assessed FY-3E GNOS II RO atmospheric retrievals by inter-comparing with corresponding data from the NCEP FNL global atmospheric analysis and FY-3D GNOS mission. The GNOS RO data quality and consistency of the different FY-3 meteorological satellites, i.e., FY-3D and FY-3E, as well as different GNSS systems (GPS, BDS-2, BDS-3) were analyzed. We find that the FY-3E GNOS II RO data exhibit better quality than FY-3D GNOS, particularly in the number, penetration height toward surface, and global coverage by BDS RO profiles, due to the integration of BDS-2 and BDS-3. Additionally, comparing with co-located NCEP FNL analysis profiles, the mean difference (and standard deviation) of the FY-3E GNOS II RO atmospheric refractivity profile retrievals is found to be smaller than 0.2% (and 1%), in the upper troposphere and lower stratosphere, from 5 to 30 km, and remains consistent at this accuracy and precision level with the FY-3D GNOS RO data. These features provide clear evidence for a high utility of the new GNOS II RO data for weather and climate research and applications. Full article

(This article belongs to the Special Issue Editorial Board Members’ Collection Series: “Recent Progress of GNSS/GPS Radio Occultation Techniques”)

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28 pages, 9250 KiB

Open AccessArticle

Using the Commercial GNSS RO Spire Data in the Neutral Atmosphere for Climate and Weather Prediction Studies

by Shu-peng Ho, Xinjia Zhou, Xi Shao, Yong Chen, Xin Jing and William Miller

Remote Sens. 2023, 15(19), 4836; https://doi.org/10.3390/rs15194836 - 05 Oct 2023

Viewed by 1098

Abstract

Recently, the NOAA has included GNSS (Global Navigation Satellite System) Radio Occultation (RO) data as one of the crucial long-term observables for weather and climate applications. To include more GNSS RO data in its numerical weather prediction systems, the NOAA Commercial Weather Data Pilot program (CWDP) started to explore the commercial RO data available on the market. After two rounds of pilot studies, the CWDP decided to award the first Indefinite Delivery Indefinite Quantity (IDIQ) contract to GeoOptics and Spire Incs. in 2020. This study examines the quality of Spire RO data products for weather and climate applications. Spire RO data collected from commercial CubeSats are carefully compared with data from Formosa Satellite Mission 7–Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2), the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA5), and high-quality radiosonde data. The results demonstrate that, despite their generally lower Signal-Noise-Ratio (SNR), Spire RO data show a pattern of lowest penetration height similar to that of COSMIC-2. The Spire and COSMIC-2 penetration heights are between 0.6 and 0.8 km altitude over tropical oceans. Although using different GNSS RO receivers, the precision of Spire STRATOS receivers is of the same quality as those of the COSMIC-2 TriG (Global Positioning System—GPS, GALILEO, and GLObal NAvigation Satellite System—GLONASS) RO Receiver System (TGRS) receivers. Furthermore, the Spire and COSMIC-2 retrieval accuracies are quite comparable. We validate the Spire temperature and water vapor profiles by comparing them with collocated radiosonde observation (RAOB) data. Generally, over the height region between 8 km and 16.5 km, the Spire temperature profiles match those from RS41 RAOB very well, with temperature biases of <0.02 K. Over the height range from 17.8 to 26.4 km, the temperature differences are ~−0.034 K, with RS41 RAOB being warmer. We also estimate the error covariance matrix for Spire, COSMIC-2, and KOMPSAT-5. The results show that the COSMIC-2 estimated error covariance values are slightly more significant than those from Spire over the oceans at the mid-latitudes (45°N–30°N and 30°S–45°S), which may be owing to COSMIC-2 SNR being relatively lower at those latitudinal zones. Full article

(This article belongs to the Special Issue Editorial Board Members’ Collection Series: “Recent Progress of GNSS/GPS Radio Occultation Techniques”)

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17 pages, 5547 KiB

Open AccessArticle

First Galileo Single-Frequency Occultation Process and Precision Analysis of FengYun3E

by Ming Yang, Xiangguang Meng, Haoran Tian, Yueqiang Sun, Qifei Du, Weihua Bai, Bowen Wang, Xianyi Wang, Peng Hu and Guangyuan Tan

Remote Sens. 2023, 15(18), 4410; https://doi.org/10.3390/rs15184410 - 07 Sep 2023

Viewed by 745

Abstract

This article proposes a single-frequency occultation method whose core is the reconstruction of a second frequency measurement. We process the actual received Galileo E1 single-frequency occultation observation data of FengYun3E to meet the urgent need for single-frequency Galileo occultation inversion of FengYun3 E/F/G/H satellites. Galileo single-frequency occultation events are globally distributed evenly and have stable quantities. The refractive index products and dry temperature products inverted from the single-frequency occultation data are reliable at altitudes of 5–30 km. The Galileo E1 single-frequency occultation process can effectively improve the output of FengYun3E/GNOS occultation products. These results validate the feasibility and correctness of using FengYun3/GNOS for the actual Galileo single-frequency occultation process. The atmospheric occultation products of Galileo single-frequency occultation increase the quantity of global occultation products and serve as a beneficial supplement to global numerical weather prediction data sources. Full article

(This article belongs to the Special Issue Editorial Board Members’ Collection Series: “Recent Progress of GNSS/GPS Radio Occultation Techniques”)

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25 pages, 3595 KiB

Open AccessArticle

Optimal Estimation Inversion of Ionospheric Electron Density from GNSS-POD Limb Measurements: Part II-Validation and Comparison Using NmF2 and hmF2

by Nimalan Swarnalingam, Dong L. Wu, Daniel J. Emmons and Robert Gardiner-Garden

Remote Sens. 2023, 15(16), 4048; https://doi.org/10.3390/rs15164048 - 16 Aug 2023

Cited by 2 | Viewed by 1034

Abstract

A growing number of SmallSat/CubeSat constellations with high-rate (50–100 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 [...] Read more.

_{e}

) 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 N

_{e}

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 N

_{e}

(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 (∼30). 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 ∼25–45 km below the OE-retrieved hmF2, especially at mid and high latitudes. We also incorporate F-region N

_{e}

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 N

_{e}

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. Full article

(This article belongs to the Special Issue Editorial Board Members’ Collection Series: “Recent Progress of GNSS/GPS Radio Occultation Techniques”)

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28 pages, 7429 KiB

Open AccessArticle

Spire RO Thermal Profiles for Climate Studies: Initial Comparisons of the Measurements from Spire, NOAA-20 ATMS, Radiosonde, and COSMIC-2

by Xin Jing, Shu-Peng Ho, Xi Shao, Tung-Chang Liu, Yong Chen and Xinjia Zhou

Remote Sens. 2023, 15(15), 3710; https://doi.org/10.3390/rs15153710 - 25 Jul 2023

Cited by 4 | Viewed by 1654

Abstract

Global Navigation Satellite System (GNSS) Radio Occultation (RO) data play an essential role in improving numerical weather prediction (NWP) and monitoring climate change. The NOAA Commercial RO Purchase Program (CDP) purchased RO data provided by Spire Global Inc. To ensure the data quality from Spire Global Inc. is consistent with other RO missions, we need to quantify their accuracy and retrieval uncertainty carefully. In this work, Spire Wet Profile (wet temperature profile) data from 7 September 2021 to 31 October 2022, processed by the University Corporation for Atmospheric Research (UCAR), and COSMIC-2 (Constellation Observing System for Meteorology, Ionosphere, and Climate-2/Formosa Satellite Mission 7) data are evaluated through comparison with NOAA-20 Advanced Technology Microwave Sounder (ATMS) microwave sounder measurements and collocated RS41 radiosonde measurements. Through the Community Radiative Transfer Model (CRTM) simulation, we convert the Spire and COSMIC-2 RO retrievals to ATMS brightness temperature (BT) at sounding channels CH07 to CH14 (temperature channels), with weighting function peak heights from 8 km to 35 km, and CH19 to CH22 (water vapor channels), with weighting function peak heights ranging from 3.2 km to 6.7 km, and compare the simulations with the collocated NOAA-20 ATMS measurements over ocean. Using ATMS observations as references, Spire and COSMIC-2 BTs agree well with ATMS within 0.07 K for CH07-14 and 0.20 K for CH19-22. The trends between Spire and COSMIC-2 are consistent within 0.07 K/year over the oceans for ATMS CH07-CH13 and CH19-22, indicating that Spire/COSMIC-2 wet profiles are, in general, compatible with each other over oceans. The RO retrievals and RS41 radiosonde observation (RAOB) comparison shows that above 0.2 km altitude, RS41 RAOB matches Spire/COSMIC-2 temperature profiles well with a temperature difference of <0.13 K, and the trends between Spire and COSMIC-2 are consistent within 0.08 K/year over land, indicating that Spire/COSMIC-2 wet profiles are overall compatible with each other through RS41 RAOB measurements over land. In addition, the consistency of Spire and COSMIC-2 based on different latitude intervals, local times, and signal-to-noise ratios (SNRs) through ATMS was evaluated. The results show that the performance of Spire is comparable to COSMIC-2, even though COSMIC-2 has a higher SNR. The high quality of RO profiles from Spire is expected to improve short- and medium-range global numerical weather predictions and help construct consistent climate temperature records. Full article

(This article belongs to the Special Issue Editorial Board Members’ Collection Series: “Recent Progress of GNSS/GPS Radio Occultation Techniques”)

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30 pages, 11754 KiB

Open AccessArticle

Optimal Estimation Inversion of Ionospheric Electron Density from GNSS-POD Limb Measurements: Part I-Algorithm and Morphology

by Dong L. Wu, Nimalan Swarnalingam, Cornelius Csar Jude H. Salinas, Daniel J. Emmons, Tyler C. Summers and Robert Gardiner-Garden

Remote Sens. 2023, 15(13), 3245; https://doi.org/10.3390/rs15133245 - 23 Jun 2023

Cited by 3 | Viewed by 1480

Abstract

GNSS-LEO radio links from Precise Orbital Determination (POD) and Radio Occultation (RO) antennas have been used increasingly in characterizing the global 3D distribution and variability of ionospheric electron density (N_e). In this study, we developed an optimal estimation (OE) method to retrieve N_e profiles from the slant total electron content (hTEC) measurements acquired by the GNSS-POD links at negative elevation angles (ε < 0°). Although both OE and onion-peeling (OP) methods use the Abel weighting function in the N_e inversion, they are significantly different in terms of performance in the lower ionosphere. The new OE results can overcome the large N_e oscillations, sometimes negative values, seen in the OP retrievals in the E-region ionosphere. In the companion paper in this Special Issue, the HmF2 and NmF2 from the OE retrieval are validated against ground-based ionosondes and radar observations, showing generally good agreements in NmF2 from all sites. Nighttime hmF2 measurements tend to agree better than the daytime when the ionosonde heights tend to be slightly lower. The OE algorithm has been applied to all GNSS-POD data acquired from the COSMIC-1 (2006–2019), COSMIC-2 (2019–present), and Spire (2019–present) constellations, showing a consistent ionospheric N_e morphology. The unprecedented spatiotemporal sampling of the ionosphere from these constellations now allows a detailed analysis of the frequency–wavenumber spectra for the N_e variability at different heights. In the lower ionosphere (~150 km), we found significant spectral power in DE1, DW6, DW4, SW5, and SE4 wave components, in addition to well-known DW1, SW2, and DE3 waves. In the upper ionosphere (~450 km), additional wave components are still present, including DE4, DW4, DW6, SE4, and SW4. The co-existence of eastward- and westward-propagating wave4 components implies the presence of a stationary wave4 (SPW4), as suggested by other earlier studies. Further improvements to the OE method are proposed, including a tomographic inversion technique that leverages the asymmetric sampling about the tangent point associated with GNSS-LEO links. Full article

(This article belongs to the Special Issue Editorial Board Members’ Collection Series: “Recent Progress of GNSS/GPS Radio Occultation Techniques”)

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