Research

39 pages, 121521 KiB

Open AccessArticle

Global GNSS-RO Electron Density in the Lower Ionosphere

by Dong L. Wu, Daniel J. Emmons and Nimalan Swarnalingam

Remote Sens. 2022, 14(7), 1577; https://doi.org/10.3390/rs14071577 - 24 Mar 2022

Cited by 7 | Viewed by 2908

Lack of instrument sensitivity to low electron density (N_e) concentration makes it difficult to measure sharp N_e vertical gradients (four orders of magnitude over 30 km) in the D/E-region. A robust algorithm is developed to retrieve global D/E [...] Read more.

Lack of instrument sensitivity to low electron density (N_e) concentration makes it difficult to measure sharp N_e vertical gradients (four orders of magnitude over 30 km) in the D/E-region. A robust algorithm is developed to retrieve global D/E-region N_e from the high-rate GNSS radio occultation (RO) data, to improve spatiotemporal coverage using recent SmallSat/CubeSat constellations. The new algorithm removes F-region contributions in the RO excess phase profile by fitting a linear function to the data below the D-region. The new GNSS-RO observations reveal many interesting features in the diurnal, seasonal, solar-cycle, and magnetic-field-dependent variations in the N_e morphology. While the D/E-region N_e is a function of solar zenith angle (χ), it exhibits strong latitudinal variations for the same χ with a distribution asymmetric about noon. In addition, large longitudinal variations are observed along the same magnetic field pitch angle. The summer midlatitude N_e and sporadic E (E_s) show a distribution similar to each other. The distribution of auroral electron precipitation correlates better with the pitch angle from the magnetosphere than from one at 100 km. Finally, a new TEC retrieval technique is developed for the high-rate RO data with a top reaching at least 120 km. For better characterization of the E- to F-transition in N_e and more accurate TEC retrievals, it is recommended to have all GNSS-RO acquisition routinely up to 220 km. Full article

(This article belongs to the Special Issue Advances of the Satellite-Based GNSS Radio Occultation (RO) Techniques and Associated Improvements in the Description and Modeling of the Atmosphere and the Thermosphere)

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23 pages, 5864 KiB

Open AccessArticle

Summer Nighttime Anomalies of Ionospheric Electron Content at Midlatitudes: Comparing Years of Low and High Solar Activities Using Observations and Tidal/Planetary Wave Features

by Yu Yin, Guillermo González-Casado, Adrià Rovira-Garcia, José Miguel Juan, Jaume Sanz and Yixie Shao

Remote Sens. 2022, 14(5), 1237; https://doi.org/10.3390/rs14051237 - 03 Mar 2022

Cited by 2 | Viewed by 1506

Abstract

In this study, midlatitude summer nighttime anomalies (MSNAs) are analyzed via observations and tidal/planetary wave features using measurements from the Formosat-3/Constellation Observing System for Meteorology, Ionosphere, and Climate (F3C) for 2007, a year with low solar activity, and 2014, a year with high [...] Read more.

In this study, midlatitude summer nighttime anomalies (MSNAs) are analyzed via observations and tidal/planetary wave features using measurements from the Formosat-3/Constellation Observing System for Meteorology, Ionosphere, and Climate (F3C) for 2007, a year with low solar activity, and 2014, a year with high solar activity. The total ionospheric electron content,

E C_{i o n}

, an integrated quantity derived from F3C measurements, was used to compare the observational data. The

E C_{i o n}

values were derived from accurate radio-occultation-retrieved electron density profiles without assuming spherical symmetry and from a model that separated the ground total electron content into the plasmaspheric and the ionospheric electron content contributions. An analysis of the

E C_{i o n}

data set confirmed that MSNAs were present in three different regions of the world for the months surrounding the local summer solstice during both 2007 and 2014. In the southern hemisphere, the so-called Weddell Sea Anomaly showed a maximum increase in

E C_{i o n}

, measured as the difference between nighttime and midday values, that was more than three times that in the northern MSNAs. For each individual MSNA, the corresponding maximum increases in electron content were similar between the two years analyzed, so they were not significantly affected by solar activity. Then, linear least-square fit to the frequency–wave number basis functions was used to derive the tidal and planetary wave components contributing to MSNAs. The main component that appears to produce the Weddell Sea Anomaly is D0, followed by SPW1, DW2, and DE1, in this order, which make secondary but still relevant contributions. The presence of MSNAs in the northern hemisphere was clearly supported by the migrating tide SW2 in combination with DE1. SW2 also supported an early morning MSNA being observed in the northern hemisphere. The main tidal and planetary wave signatures producing the MSNAs did not significantly differ between 2007 and 2014. Full article

(This article belongs to the Special Issue Advances of the Satellite-Based GNSS Radio Occultation (RO) Techniques and Associated Improvements in the Description and Modeling of the Atmosphere and the Thermosphere)

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29 pages, 11989 KiB

Open AccessArticle

Consistency and Stability of SNPP ATMS Microwave Observations and COSMIC-2 Radio Occultation over Oceans

by Xi Shao, Shu-peng Ho, Bin Zhang, Changyong Cao and Yong Chen

Remote Sens. 2021, 13(18), 3754; https://doi.org/10.3390/rs13183754 - 19 Sep 2021

Cited by 11 | Viewed by 2539

Abstract

Radio occultation (RO) sensor measurements have critical roles in numerical weather prediction (NWP) by complementing microwave and infrared sounder measurements with information of the atmospheric profiles at high accuracy, precision, and vertical resolution. This study evaluates Constellation Observing System for Meteorology, Ionosphere, and [...] Read more.

Radio occultation (RO) sensor measurements have critical roles in numerical weather prediction (NWP) by complementing microwave and infrared sounder measurements with information of the atmospheric profiles at high accuracy, precision, and vertical resolution. This study evaluates Constellation Observing System for Meteorology, Ionosphere, and Climate 2 (COSMIC-2) wet temperature and humidity data products’ consistency and stability through inter-comparison with SNPP advanced technology microwave sounder (ATMS) measurements. Through the community radiative transfer model (CRTM), brightness temperature (BT) at SNPP ATMS channels are simulated with COSMIC-2 retrieved atmospheric profiles from two versions of the University Corporation for Atmospheric Research (UCAR) wet profiles (WETprf and WETpf2) as inputs to the CRTM simulation. The analysis was focused on ATMS sounding channels CH07–14 and CH19–22 with sounding weighting function peak heights from 3.2 to 35 km. The COSMIC-2 vs. ATMS inter-comparison indicates that their BT biases are consistent, and the latitudinal difference is <0.3 K over three latitudinal regions. The differences between the two versions of UCAR COSMIC-2 wet profiles are identified and attributed to the differences in the implementation of 1DVAR retrieval algorithms. The stability between UCAR near real-time COSMIC-2 wet profile data and ATMS measurements is also well-maintained. It is demonstrated that the well-sustained quality of COSMIC-2 RO data makes itself a well-suited reference sensor to capture the calibration update of SNPP ATMS. Furthermore, the impacts of the assimilation of COSMIC-2 data into the European Centre for Medium-Range Weather Forecasts (ECMWF) model after 25 March 2020, are evaluated by trending observation-minus-background (O-B) biases, which confirms the statistically significant positive impacts of COSMIC-2 on the ECMWF reanalysis. The validation of stability and consistency between COSMIC-2 and SNPP ATMS ensures the quality of RO and microwave sounder data assimilated into the NWP models. Full article

(This article belongs to the Special Issue Advances of the Satellite-Based GNSS Radio Occultation (RO) Techniques and Associated Improvements in the Description and Modeling of the Atmosphere and the Thermosphere)

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14 pages, 5049 KiB

Open AccessArticle

Diagnostics of Es Layer Scintillation Observations Using FS3/COSMIC Data: Dependence on Sampling Spatial Scale

by Lung-Chih Tsai, Shin-Yi Su, Chao-Han Liu, Harald Schuh, Jens Wickert and Mohamad Mahdi Alizadeh

Remote Sens. 2021, 13(18), 3732; https://doi.org/10.3390/rs13183732 - 17 Sep 2021

Cited by 3 | Viewed by 1370

Abstract

The basic theory and experimental results of amplitude scintillation from GPS/GNSS radio occultation (RO) observations on sporadic E (Es) layers are reported in this study. Considering an Es layer to be not a “thin” irregularity slab on limb viewing, we characterized the corresponding [...] Read more.

The basic theory and experimental results of amplitude scintillation from GPS/GNSS radio occultation (RO) observations on sporadic E (Es) layers are reported in this study. Considering an Es layer to be not a “thin” irregularity slab on limb viewing, we characterized the corresponding electron density fluctuations as a power-law function and applied the Ryton approximation to simulate spatial spectrum of amplitude fluctuations. The scintillation index S₄ and normalized signal amplitude standard deviation S₂ are calculated depending on the sampling spatial scale. The theoretical results show that both S₄ and S₂ values become saturated when the sampling spatial scale is less than the first Fresnel zone (FFZ), and S₄ and S₂ values could be underestimated and approximately proportional to the logarithm of sampled spatial wave numbers up to the FFZ wave number. This was verified by experimental analyses using the 50 Hz and de-sampled FormoSat-3/Constellation Observing System for Meteorology, Ionosphere and Climate (FS3/COSMIC) GPS RO data in the cases of weak, moderate, and strong scintillations. The results show that the measured S₂ and S₄ values have a very high correlation coefficient of >0.97 and a ratio of ~0.5 under both complete and undersampling conditions, and complete S₄ and S₂ values can be derived by dividing the measured undersampling S₄ and S₂ values by a factor of 0.8 when using 1-Hz RO data. Full article

(This article belongs to the Special Issue Advances of the Satellite-Based GNSS Radio Occultation (RO) Techniques and Associated Improvements in the Description and Modeling of the Atmosphere and the Thermosphere)

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16 pages, 29082 KiB

Open AccessArticle

Preliminary Validation of Surface Reflections from Fengyun-3C Radio Occultation Data

by Weiwei Chen, Yongliang Xiong, Xinzhong Li, Ban Zhao, Rui Zhang and Shaoguang Xu

Remote Sens. 2021, 13(10), 1980; https://doi.org/10.3390/rs13101980 - 19 May 2021

Cited by 1 | Viewed by 1598

Abstract

Fengyun-3C (FY-3C) is a Global Navigation Satellite Systems (GNSS) Radio Occultation (RO) mission founded which was by China on 23 September 2013. In this study, under a specific temporal and spatial domain, we systematically compare FY-3C refractivity profiles with Constellation Observing System for [...] Read more.

Fengyun-3C (FY-3C) is a Global Navigation Satellite Systems (GNSS) Radio Occultation (RO) mission founded which was by China on 23 September 2013. In this study, under a specific temporal and spatial domain, we systematically compare FY-3C refractivity profiles with Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) refractivity profiles for the year 2015. The COSMIC profiles used in this study contain reflections, as identified in the Radio Occultation Meteorology Satellite Application Facility (ROM SAF) flag database. From 0 to 25 km altitude, the mean biases and relative standard deviations of the comparisons between FY-3C and COSMIC are less than 1% and 2% when COSMIC profiles present reflected signals. Radio holographic analysis is used to visualize and identify the spectra of FY-3C-reflected signals in the time-frequency domain. It is confirmed that the reflected signals in the lower troposphere and near the surface can be tracked by an FY-3C receiver. Further, most of the FY-3C events that matched with COSMIC reflected events show reflection patterns at a lower height, especially above the ocean’s surface. Under Bouguer’s rule and spherical symmetry assumptions, we reconstructed the reflected bending angle models by Abel transformation, which are valuable for reducing N-bias in the ducting layer. Three examples of FY-3C events show that the reflected bending branch is near the surface. Overall, the reflected signal of FY-3C could be used as a supplementary data portion for FY-3C atmospheric products. Full article

(This article belongs to the Special Issue Advances of the Satellite-Based GNSS Radio Occultation (RO) Techniques and Associated Improvements in the Description and Modeling of the Atmosphere and the Thermosphere)

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12 pages, 2699 KiB

Open AccessArticle

Implications of GNSS-Inferred Tropopause Altitude Associated with Terrestrial Gamma-ray Flashes

by Tao Xian, Gaopeng Lu, Hongbo Zhang, Yongping Wang, Shaolin Xiong, Qibin Yi, Jing Yang and Fanchao Lyu

Remote Sens. 2021, 13(10), 1939; https://doi.org/10.3390/rs13101939 - 16 May 2021

Cited by 5 | Viewed by 1974

Abstract

The thermal structure of the environmental atmosphere associated with Terrestrial Gamma-ray Flashes (TGFs) is investigated with the combined observations from several detectors (FERMI, RHESSI, and Insight-HXMT) and GNSS-RO (SAC-C, COSMIC, GRACE, TerraSAR-X, and MetOp-A). The geographic distributions of TGF-related tropopause altitude and climatology [...] Read more.

The thermal structure of the environmental atmosphere associated with Terrestrial Gamma-ray Flashes (TGFs) is investigated with the combined observations from several detectors (FERMI, RHESSI, and Insight-HXMT) and GNSS-RO (SAC-C, COSMIC, GRACE, TerraSAR-X, and MetOp-A). The geographic distributions of TGF-related tropopause altitude and climatology are similar. The regional TGF-related tropopause altitude in Africa and the Caribbean Sea is 0.1–0.4 km lower than the climatology, whereas that in Asia is 0.1–0.2 km higher. Most of the TGF-related tropopause altitudes are slightly higher than the climatology, while some of them have a slightly negative bias. The subtropical TGF-producing thunderstorms are warmer in the troposphere and have a colder and higher tropopause over land than the ocean. There is no significant land–ocean difference in the thermal structure for the tropical TGF-producing thunderstorms. The TGF-producing thunderstorms have a cold anomaly in the middle and upper troposphere and have stronger anomalies than the deep convection found in previous studies. Full article

(This article belongs to the Special Issue Advances of the Satellite-Based GNSS Radio Occultation (RO) Techniques and Associated Improvements in the Description and Modeling of the Atmosphere and the Thermosphere)

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17 pages, 6809 KiB

Open AccessArticle

Advanced Machine Learning Optimized by The Genetic Algorithm in Ionospheric Models Using Long-Term Multi-Instrument Observations

by Wang Li, Dongsheng Zhao, Changyong He, Andong Hu and Kefei Zhang

Remote Sens. 2020, 12(5), 866; https://doi.org/10.3390/rs12050866 - 07 Mar 2020

Cited by 23 | Viewed by 4469

Abstract

The ionospheric delay is of paramount importance to radio communication, satellite navigation and positioning. It is necessary to predict high-accuracy ionospheric peak parameters for single frequency receivers. In this study, the state-of-the-art artificial neural network (ANN) technique optimized by the genetic algorithm is [...] Read more.

The ionospheric delay is of paramount importance to radio communication, satellite navigation and positioning. It is necessary to predict high-accuracy ionospheric peak parameters for single frequency receivers. In this study, the state-of-the-art artificial neural network (ANN) technique optimized by the genetic algorithm is used to develop global ionospheric models for predicting foF2 and hmF2. The models are based on long-term multiple measurements including ionospheric peak frequency model (GIPFM) and global ionospheric peak height model (GIPHM). Predictions of the GIPFM and GIPHM are compared with the International Reference Ionosphere (IRI) model in 2009 and 2013 respectively. This comparison shows that the root-mean-square errors (RMSEs) of GIPFM are 0.82 MHz and 0.71 MHz in 2013 and 2009, respectively. This result is about 20%–35% lower than that of IRI. Additionally, the corresponding hmF2 median errors of GIPHM are 20% to 30% smaller than that of IRI. Furthermore, the ANN models present a good capability to capture the global or regional ionospheric spatial-temporal characteristics, e.g., the equatorial ionization anomaly and Weddell Sea anomaly. The study shows that the ANN-based model has a better agreement to reference value than the IRI model, not only along the Greenwich meridian, but also on a global scale. The approach proposed in this study has the potential to be a new three-dimensional electron density model combined with the inclusion of the upcoming Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC-2) data. Full article

(This article belongs to the Special Issue Advances of the Satellite-Based GNSS Radio Occultation (RO) Techniques and Associated Improvements in the Description and Modeling of the Atmosphere and the Thermosphere)

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21 pages, 5887 KiB

Open AccessArticle

Experimental Validation of GNSS Interferometric Radio Occultation

by Serni Ribó, Weiqiang Li, Estel Cardellach, Fran Fabra, Ramon Padullés, Antonio Rius and Manuel Martín-Neira

Remote Sens. 2019, 11(23), 2758; https://doi.org/10.3390/rs11232758 - 23 Nov 2019

Cited by 1 | Viewed by 2291

Abstract

In this work, we present experimental results on the interferometric radio occultation (iRO) signal processing techniques, and compare the performance to the closed-loop and open-loop processing used in conventional radio occultation measurements. We also discuss the effects of antenna beam width to mitigate [...] Read more.

In this work, we present experimental results on the interferometric radio occultation (iRO) signal processing techniques, and compare the performance to the closed-loop and open-loop processing used in conventional radio occultation measurements. We also discuss the effects of antenna beam width to mitigate inter-satellite interferences, as well as how the local oscillator stability affects the obtained Doppler estimates. The required signal processing resources are less stringent for the iRO than for conventional RO techniques. In addition, the zenith iRO has a comparable performance to the well-established RO techniques. Full article

(This article belongs to the Special Issue Advances of the Satellite-Based GNSS Radio Occultation (RO) Techniques and Associated Improvements in the Description and Modeling of the Atmosphere and the Thermosphere)

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