Low Elevation Angles Research Articles

Modeling, refinement and evaluation of multipath mitigation based on the hemispherical map in BDS2/BDS3 relative precise positioning

Multipath is one of the critical constraints in millimeter-level BeiDou navigation satellite system (BDS) positioning. Since multipath hemispherical map (MHM) can be conducted in relatively real-time and kinematic scenarios, it has great potential in real BDS applications. However, limited studies focus on the mitigation performance and efficiency problem based on MHM with the second-generation regional system (BDS2) and third-generation global system (BDS3). In this paper, the MHM is systematically studied in BDS2/BDS3 relative precise positioning, including the issues of MHM about the modeling, refinement, and evaluation. Specifically, at first, a rigorous modeling procedure of MHM with quality control is given, where the high-precision and high-reliability multipath corrections can be obtained. Second, based on a consecutive eleven-day designed experiment, different MHM models considering the systems, satellites, and frequencies are formed and compared comprehensively. Third, the MHM models are systematically evaluated on the positioning performance, including ambiguity resolution, float solution, and fixed solution. The results show that in MHM models, the multipath corrections for low elevation angles are more significant, and the multipath effects are usually different in different directions due to the obstacles. The behaviors of MHM models are dependent on the system, satellite, and frequency types. Besides, the MHM models are relatively stable over seven consecutive days. After applying the MHM models, the success rate of ambiguity resolution can be improved in millimeter-level positioning. Approximately 7 cm improvements can be obtained for the float solution in the 3D direction. The MHM models can improve the positioning accuracy by approximately 13% for the fixed solution especially in locations where the multipath is more severe. Moreover, the unified MHM method of the previous day with quality control can replace the refined MHM method when considering the balance between accuracy and efficiency.

Ionospheric correction of S-band tracking radar data using NavIC S-band signals in missile test range applications

AbstractIn missile test ranges, complex missions demand precise trajectory generated by radar. Both the radar and Global Navigation Satellite System (GNSS) signals are affected by atmospheric effects, degrading their accuracy and performance. The Indian Regional Navigation Satellite System/Navigation with Indian Constellation (IRNSS/NavIC) transmits signals in the S-band together with the L-band. This paper presents a novel experimental technique to improve the tracking accuracy of S-band radars using the concurrent NavIC S-band signal. The ionospheric delay using the NavIC S-band signal is calculated first, and the results are used to improve the trajectory data of simultaneously operating S-band radars. This is a unique application of the NavIC S-band signals apart from its conventional usage. During a launch mission, for low elevation angles, the ionospheric error is found to be ~130 m while at higher elevation angles the error values are found to be ~1–3 m. The concept is validated using data from a missile test mission. This report on the use of S-band GNSS signals for the correction of S-band radar range data offers a clear advantage of simplicity and accuracy.

A REVIEW OF THE EFFECTS OF LOW ELEVATION ANGLES ON THE PROPAGATION FOR SATELLITE COMMUNICATION LINK

The propagation phenomenon for satellite communication links is affected by the elevation angle of the satellite. Low elevation angles can cause signal attenuation due to increased atmospheric absorption, increased multipath fading, and rain attenuation. These effects can lead to decreased signal-to-noise ratio, reduced link availability, and degraded performance. This paper examines the effects of low elevation angle on the propagation phenomenon for satellite communication links. The paper begins by discussing the basics of satellite communication links, including the components of a link, the types of signals used, and the various propagation phenomena that can affect signal transmission. It then focuses on the effects of low elevation angle on signal propagation, including multipath fading, scintillation, and rain attenuation. The paper discusses how these effects can be mitigated through various techniques such as antenna design and signal processing. The results suggest that increasing the antenna gain and using higher frequency bands can help reduce the impact of low elevation angles on satellite communication links. Finally, it provides an overview of current research in this area and suggests potential future research directions.

Analysis of systematic biases in tropospheric hydrostatic delay models and construction of a correction model

Abstract. In the field of space geodetic techniques, such as global navigation satellite systems (GNSSs), tropospheric zenith hydrostatic delay (ZHD) is chosen as the a priori value of tropospheric total delay. Therefore, the inaccuracy of ZHD will definitely affect parameters like the wet delay and the horizontal gradient of tropospheric delay, accompanied by an indirect influence on the accuracy of geodetic parameters, if not dealt with well at low elevation angles. In fact, however, the most widely used ZHD model currently seems to contain millimeter-level biases from the precise integral method. We explored the bias of traditional ZHD models and analyzed the characteristics in different aspects on a global annual scale. It was found that biases differ significantly with season and geographical location, and the difference between the maximum and minimum values exceeds 30 mm, which should be fully considered in the field of high-precision measurement. Then, we constructed a global grid correction model, which is named ZHD_crct, based on the meteorological data of the year 2020 from the ECMWF (European Centre for Medium-Range Weather Forecasts), and it turned out that the bias of traditional models in the current year could be reduced by ∼ 50 % when the ZHD_crct was added. When we verified the effect of ZHD_crct on the biases in the next year, it worked almost the same as the former year. The mean absolute biases (MABs) of ZHD will be narrowed within ∼ 0.5 mm for most regions, and the SDs (standard deviations) will be within ∼ 0.7 mm. This improvement will be helpful for research on meteorological phenomena as well.

Improving the performance of tropospheric mapping function in low elevation angle using artificial neural network

Tropospheric delay is one of the major error sources in global navigation satellite systems (GNSS). The position accuracy determined using GNSS depends on the satellite’s geometry factor. However, in low elevation angle GNSS signals have a high tropospheric slant delay error and need a good mapping function model to cover this error. In this paper, two new tropospheric mapping functions (hydrostatic and non-hydrostatic) were developed based on an artificial neural network (ANN). The first model (Model A) was based on the day of the year (DOY), latitude and longitude. The second model (Model B) added real-time surface pressure to the parameters of Model “A.” The two developed models showed a strong correlation with the ray tracing mapping function, with a significant t-test value.The applicability of these mapping functions was tested over the Egypt region. The comparison was done with ray tracing of 5220 traces for 145 days from 2014 to 2017 as well as by assessing it (Mohamed et al., Vienna and Niell). This evaluation showed that the new model “B” agrees very well with the ray trace value. Model “B” and ray traces are on par with each other for a very low elevation angle 2⁰ the hydrostatic and non-hydrostatic mapping functions. Models “B” and “A” gave hydrostatic mean biases of 32 and 39 mm, respectively, and for non-hydrostatic, 4.5 and 4.9 mm, respectively. Vienna, Niell, and Mohamed et al. gave 46, 45, and 94 mm in hydrostatic, respectively, and 5.4, 5.2 and 9.2 mm in non-hydrostatic, respectively.

Estimating leaf moisture content at global scale from passive microwave satellite observations of vegetation optical depth

Abstract. The moisture content of vegetation canopies controls various ecosystem processes such as plant productivity, transpiration, mortality, and flammability. Leaf moisture content (here defined as the ratio of leaf water mass to leaf dry biomass, or live-fuel moisture content, LFMC) is a vegetation property that is frequently used to estimate flammability and the danger of fire occurrence and spread, and is widely measured at field sites around the globe. LFMC can be retrieved from satellite observations in the visible and infrared domain of the electromagnetic spectrum, which is however hampered by frequent cloud cover or low sun elevation angles. As an alternative, vegetation water content can be estimated from satellite observations in the microwave domain. For example, studies at local and regional scales have demonstrated the link between LFMC and vegetation optical depth (VOD) from passive microwave satellite observations. VOD describes the attenuation of microwaves in the vegetation layer. However, neither were the relations between VOD and LFMC investigated at large or global scales nor has VOD been used to estimate LFMC. Here we aim to estimate LFMC from VOD at large scales, i.e. at coarse spatial resolution, globally, and at daily time steps over past decadal timescales. Therefore, our objectives are: (1) to investigate the relation between VOD from different frequencies and LFMC derived from optical sensors and a global database of LFMC site measurements; (2) to test different model structures to estimate LFMC from VOD; and (3) to apply the best-performing model to estimate LFMC at global scales. Our results show that VOD is medium to highly correlated with LFMC in areas with medium to high coverage of short vegetation (grasslands, croplands, shrublands). Forested areas show on average weak correlations, but the variability in correlations is high. A logistic regression model that uses VOD and additionally leaf area index as predictor to account for canopy biomass reaches the highest performance in estimating LFMC. Applying this model to global VOD and LAI observations allows estimating LFMC globally over decadal time series at daily temporal sampling. The derived estimates of LFMC can be used to assess large-scale patterns and temporal changes in vegetation water status, drought conditions, and fire dynamics.

Wideband Wide-Beam Hybrid Dielectric Resonator Antenna Using Uniaxial Material

In this letter, a wideband wide-beam rectangular dielectric resonator antenna (DRA) employing uniaxial anisotropic material is presented. To the best knowledge of the authors, the uniaxial anisotropic dielectric is used in wide-beam DRAs for the first time. By tuning the uniaxial permittivity tensor of the rectangular dielectric, the radiation from the top wall of the DRA is enhanced which improves the gain at low elevation angle in the E-plane. Both the feed slot mode and the fundamental <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{TE}_{{111}}^{\mathrm{y}}$</tex-math></inline-formula> mode are merged to broaden the bandwidth. When a grounded substrate is added and four trapezoidal notches are etched on the ground, the half-power beamwidth (HPBW) in the H-plane is broadened. The measured results achieve an impedance bandwidth of 35.7% over a band of 2.65–3.8 GHz with HPBWs of at least 250° and 162° in the E- and H-planes, respectively.

Improvement of Snow Depth Inversion Derived From Terrain Tilt Correction and Multi-GNSS Measurements Using the Helmert Variance Component Estimation

As an essential part of the global water cycle, the effective detection of snow depth is of great significance to the study of climate change. Global Navigation Satellite System Interferometric Reflectometry (GNSS-IR) technology emerged as a new ground-based snow depth monitoring method. The principle is to invert the height of the reflection surface according to the SNR at a low elevation angle. However, the topographic relief in the reflection area of GNSS stations often dramatically impacts the accuracy of snow depth inversion. This study proposes a Terrain Tilt Correction (TTC) algorithm based on different azimuth intervals to address it. Taking the P351 and P676 stations in the United States as examples, the snow depth inversion research of different systems are carried out. In the P351 station with gentle terrain, the TTC algorithm can maintain stable accuracy. In the P676 station with steep terrain, the TTC algorithm can effectively compensate for the error caused by the landscape and significantly improve the accuracy of snow depth inversion. The experimental results of the two stations show that the snow depth inversion results of different systems after TTC are consistent with the in situ snow depth provided by the respective meteorological stations. Subsequently, a Helmert Variance Component Estimation (HVCE) weighting algorithm is introduced for multi-system combinations based on different systems with different snow depth inversion accuracies, and the feasibility and stability of the combination strategy are investigated. Compared with the in situ snow depth, the correlation coefficient (R) of the combined snow depth at the P351 station is 0.91, the Root Mean Square Error (RMSE) is 19.7 cm, and the Mean Error (ME) is -12.2 cm. The snow depth obtained by the combination of the multi-system at the P676 station has an R of 0.95, an RMSE of 9.9 cm, and an ME of -4.9 cm. Compared with HVCE weighting before TTC, the RMSE is reduced by 53.3%, and the absolute value of ME is reduced by 74.2%. This study is not only of great significance for high-precision snow depth monitoring, but also provides a necessary technical reference for further research on multi-system combined.

Multilayer Ionospheric Model Constrained by Physical Prior Based on GNSS Stations

The need for accurate modeling of the ionosphere plays an important role in the global navigation satellite system (GNSS) positioning. The traditional multi-layer VTEC model without prior has been used for modeling the ionospheric delay error. However, it is assumed that the electron density of the ionosphere is compressed into multiple thin layers at fixed heights in the lack of capturing ionospheric physics. In this paper, the data enhancement method by virtual observations is proposed to build the constrained multi-layer VTEC model to capture physical features from empirical ionospheric models. The extraction methods of physical knowledge have been developed by prior VTEC based on principal component analysis and model coefficients based on emulated basis function. The constrained multi-layer modeling has been verified based on simulation and real measurement of GNSS data in Yunnan, China collected from Qianxun Ground-based Augmentation System on November 3, 2021. The receiver DCB error estimated by the multi-layer model with prior constraint is significantly lower than that of the single-layer model and the traditional multi-layer model. The experimental test shows that the constrained multi-layer model achieves the accuracy of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$0.5 \,\rm {TECU}$</tex-math></inline-formula> for the independent reference station. The dSTEC of the proposed two multi-layer models are significantly lower than those of the single-layer model for low elevation angles, and the RMSE of dSTEC is reduced by 63 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> with the cutoff elevation angle of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{\circ }$</tex-math></inline-formula> . The spatial distribution of the multi-layer VTEC model shows consistency with the tomography model to verify vertical feature capturing capability. Compared with the Un-differenced and Un-combined Precise Point Positioning (UCPPP) without ionospheric constraint, the multi-layer constrained model based on the test data improves the convergence time approximately by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$36.55\%$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$18.78\%$</tex-math></inline-formula> in the horizontal (H), up (U) directions, respectively. These results demonstrate that the proposed multi-layer models not only improve ionospheric delay estimation precision but also can obtain the VTEC distribution capturing the physical characteristics of the ionosphere. The proposed multi-layer models may be valuable for the ionospheric delay modeling of satellite navigation systems in harsh variable ionospheric conditions.

Authentication for Satellite Communication Systems Using Physical Characteristics

Satellite communication networks have gained a lot of attention recently as a solution to mitigate the limitations of terrestrial networks such as stability and coverage. However, integrating satellite and terrestrial networks makes the system more vulnerable to spoofing attacks. Thus, robust and effective authentication is required. Physical layer authentication (PLA) has emerged as an alternative paradigm that uses physical characteristics to achieve authentication. In this paper, PLA is proposed for low earth orbit (LEO) satellites using the Doppler frequency shift (DS) and received power (RP) characteristics. Hypothesis testing using a threshold or machine learning (ML) is considered to discriminate between legitimate and illegitimate satellites. For ML, a one-class classification support vector machine (OCC-SVM) is employed which uses training data from only legitimate users. The performance is evaluated using real satellite data from the system tool kit (STK). Results are presented which show that the authentication rate (AR) with DS is higher than with RP at low elevation angles for both schemes, but is higher with RP at high elevation angles. Further, the ML authentication scheme provides a higher AR than the threshold scheme for a small percentage of the training data considered as outliers, but at larger percentages the OR threshold scheme is better.

A Clutter Loss Model for Satellite Communication Systems

Buildings and vegetation in the proximity of a terrestrial base station induce a significant additional loss, typically referred to as “clutter loss”, which sums up to free space loss and atmospheric attenuation. Clutter loss is essentially due to the radiowave reflection and diffraction caused by buildings and vegetation, and tends to reduce the interference between terrestrial systems, such as upper 6 GHz (U6G), and satellite systems operating in the same frequency bands. In fact, for example, at low elevation angles, the clutter loss could reach some tens of dB in the U6G band. A novel clutter loss model in urban and suburban environments for frequencies up to 10 GHz is proposed. The model relies on the Monte Carlo simulation approach presented in Report ITU-R P.2402-0, but some limitations have been removed to extend its applicability to more complex scenarios and possibly increase its accuracy for U6G systems. An analytical approach is also proposed to model the clutter loss statistics obtained by properly fitting the obtained statistics for the cities of London and Melbourne. Finally, the proposed model is validated by comparing its results to those obtained by a commercial ray tracer.

Carbon Air–Sea Flux in the Arctic Ocean from CALIPSO from 2007 to 2020

Quantified research on the Arctic Ocean carbon system is poorly understood, limited by the scarce available data. Measuring the associated phytoplankton responses to air–sea CO2 fluxes is challenging using traditional satellite passive ocean color measurements due to low solar elevation angles. We constructed a feedforward neural network light detection and ranging (LiDAR; FNN-LID) method to assess the Arctic diurnal partial pressure of carbon dioxide (pCO2) and formed a dataset of long-time-series variations in diurnal air–sea CO2 fluxes from 2001 to 2020; this study represents the first time spaceborne LiDAR data were employed in research on the Arctic air–sea carbon cycle, thus providing enlarged data coverage and diurnal pCO2 variations. Although some models replace Arctic winter Chl-a with the climatological average or interpolated Chl-a values, applying these statistical Chl-a values results in potential errors in the gap-filled wintertime pCO2 maps. The CALIPSO measurements obtained through active LiDAR sensing are not limited by solar radiation and can thus provide ‘fill-in’ data in the late autumn to early spring seasons, when ocean color sensors cannot record data; thus, we constructed the first complete record of polar pCO2. We obtained Arctic FFN-LID-fitted in situ measurements with an overall mean R2 of 0.75 and an average RMSE of 24.59 µatm and filled the wintertime observational gaps, thereby indicating that surface water pCO2 is higher in winter than in summer. The Arctic Ocean net CO2 sink has seasonal sources from some continental shelves. The growth rate of Arctic seawater pCO2 is becoming larger and more remarkable in sectors with significant sea ice retreat. The combination of sea surface partial pressure and wind speed impacts the diurnal carbon air–sea flux variability, which results in important differences between the Pacific and Atlantic Arctic Ocean. Our results show that the diurnal carbon sink is larger than the nocturnal carbon sink in the Atlantic Arctic Ocean, while the diurnal carbon sink is smaller than the nocturnal carbon sink in the Pacific Arctic Ocean.

Theory and Experiment Analysis on the Influence of Floods on a GNSS Pseudo-Range Multipath and CNR Signal Based on Two Cases Study in China

The surrounding environment of a GNSS observation station is changed during a flood, and this results in a more serious multipath than in a normal environment. Considering that the multipath error is largely related to the pseudo-range multipath and CNR (Carrier-to-noise ratio) of the GNSS signal, the influence of floods on a pseudo-range multipath and CNR is analyzed in theory and through experiment. To ensure the accuracy of the analysis results, the ground track repeat period of GPS, GLONASS, and BDS satellites is investigated from the perspective of theory and skyplots. Two real cases study collected in Zhengzhou and Xinxiang, China, in 2021, are used to demonstrate the influence of floods on a pseudo-range multipath and CNR in detail. Experimental results show that the pseudo-range multipath of a GPS satellite performs more seriously during a flood. The maximum RMS increase rate is approximately 17.85%, and the average of all other satellites with a whole arc is approximately 6.55%. In addition, the CNR of three GNSS systems performs a decrease during a flood. For GPS and GLONASS satellites, the decrease performs more seriously at a high elevation angle than that at a low elevation angle. The maximum decrease is approximately 5 dB-Hz for the GPS satellite and approximately 7 dB-Hz for the GLONASS satellite. In terms of the BDS system, the CNR of all three orbital type satellites decreases during a flood. The average decrease is approximately 2 dB-Hz for BDS MEO and GEO satellites, and about 1 dB-Hz for the BDS IGSO satellite.

A DETERMINATION OF THE MOTION BASED ON GNSS OBSERVATIONS BETWEEN 2000 AND 2021 BY USING THE IGS POINTS ON THE POLAR REGIONS

People are fascinated today more than ever by the polar regions of the Earth. One reason for this is that wide expanses of the Arctic and Antarctic have not been explored and are therefore still viewed as frontier regions. Another is that they both have very diverse histories with regard to their origins and ice formation. Their numerous aspects still pose many puzzles for science today. The regions of the Earth designated as polar are those areas located between the North or South Pole and the Arctic or Antarctic Circles, respectively. The northern polar region, called the Arctic, encompasses the Arctic Ocean and a portion of some surrounding land masses. The southern polar region, called the Antarctic, contains the continent of Antarctica and areas of the surrounding Southern Ocean. In this paper three tests (2000, 2010 and 2021) of continuous GNSS data recorded by 8 permanent International GPS Service (IGS) stations in both Polar Regions have been processed by using CSRS-PPP Software for geodetic networks. The results also show that all GNSS provide good visibility with low elevation angles, whereas with high elevation angles, which might be needed due to natural barriers, the GLONASS and other satellites provides the highest number of visible satellites. Consequently, the mean motion of the study area was found approximately 7–15 cm for horizontal components (X–Y) and 6 cm for vertical components (Ellipsoidal Height) on the eight IGS points in the both poles.

A Novel Design of Flat-Top Beamforming Anti-Multipath Antenna

This article discusses a novel design method of wide-beam circular polarization array antenna for satellite navigation. For special applications such as satellite navigation guiding aircraft landing, high-performance ground reference station antennas are required. In the system of satellite navigation guiding aircraft landing, the antenna of satellite navigation receiver needs to have the following radiation characteristics: the shape of antenna pattern is close to an ideal hemisphere, the upper half of space has uniform right-hand circular polarization gain, and the beam has the characteristic of sharp cutoff below 5 degrees of elevation. Whether the GNSS reference station antenna can have a wider beam circular polarization characteristic to receive more navigation satellites signal; whether it can suppress the multipath effect to a greater extent to reduce the multipath error; and whether it can work in a wider frequency band, so that several major satellite navigation systems work together, the above factors directly affect the satellite navigation and positioning accuracy of the entire system. The beam of this antenna has the following feature: sharp cutoff near the horizon for multipath mitigation at low elevation angles. Comprehensive design thereby accelerates system design. It owns wide beam, good circular polarization characteristics, low elevation angle sharp cutoff, and ultralow sidelobe radiation characteristics that achieve the purpose of receiving satellite navigation signal and multipath. The low sidelobe characteristic of the antenna is beneficial to suppress multipath effect. According to the theoretical knowledge of the symmetrical dipole and the four-arm helical antenna, the simple structure of the symmetrical element is fully utilized, and the printed balun easily realizes the dual-frequency operation of the antenna. Combined with the wide-beam circular polarization characteristic of the four-arm helical antenna structure, the shaped beam array antenna is designed. The wide-beam circularly polarized antenna unit not only covers the frequency band of the satellite navigation system but also satisfies the system performance and key feature requirements of the array antenna for the unit; that is, it satisfies the wide-beam circular polarization characteristics of the upper half space. According to the linear array basic theory and array antenna synthesis theory, Woodward Lawson sampling synthesis algorithm is used to synthesize the array amplitude and phase weights corresponding to the specific antenna pattern required for the Ground-Based Augmentation System (GBAS) engineering. The shaped beam array antenna consists of 21 units, of which 11 antenna units are directly fed, and the feeding ports of the rest are dummy ports. It is an equally spaced array antenna. The spacing between antenna units is close to half wavelength. They are fed according to the value corresponding to the excitation function. The array antenna is simulated by HFSS (High-Frequency Structure Simulator). Compared with traditional satellite navigation antennas, the antenna in this paper has excellent front-to-rear ratio and flat-top beam pattern. The results show that the wide-beam circularly polarized array antenna meets the engineering requirements of GBAS applications and lays the solid foundation for GBAS engineering applications.

A New Optimal Subset Selection Method of Partial Ambiguity Resolution for Precise Point Positioning

Rapid and accurate ambiguity resolution is the core of high-precision precise point positioning (PPP) data processing. However, the ambiguity parameters in PPP observation models are easily affected by atmospheric residual and gross errors, which lead to the probability of successfully fixing decreases and computational burden increases in full ambiguity resolution. Therefore, an increasing number of partial ambiguity resolution (PAR) strategies have been proposed. The selection of the optimal subset of PAR is crucial in this method. The traditional optimal subset selection method of PAR commonly leads to a single judgment criterion and weakened geometric configuration strength because the satellites with low elevation angles are often easily eliminated during the optimal subset selection. In this paper, a multi-factor constrained optimal subset selection method for PAR was proposed, which incorporates the ambiguity variance, the ambiguity dilution of precision (ADOP), satellite position dilution of precision (PDOP) and ratio test values. In order to verify the feasibility of the proposed optimal subset selection method, PAR tests under two schemes were performed for GPS/Galileo based on the static observation data of 15 Multi-GNSS Experiment (MGEX) tracking stations. The results show that, compared with the ambiguity variance sorting method, the proposed subset selection method can further improve the accuracy of the coordinate solution and the strength of geometric figure positioning. The average root mean square of the coordinate residuals is found to decrease by about 12.90%, 6.83% and 9.39% in the eastern, northern and vertical directions, respectively. The increase in the fixed epoch rate ranged from 0.87% to 33.33%, with an average of about 8.71%.

V/Ka-band LEO high-throughput satellite and integrated satellite–terrestrial network experiment system: First two years flight results

Satellite Internet technology has evolved significantly over the past few years. Low Earth orbit (LEO) satellite communication technology and its applications have received growing interest in the field of global communication, due to their advantages of faster speed, lower latency, higher throughput, etc. GalaxySpace’s first satellite (YINHE-1) has been the world’s first V/Ka-band LEO broadband satellite with a communication capacity of 24 Gbps. The satellite was launched on the 16th of January 2020, and a series of in-orbit tests and satellite–terrestrial communication experiments were carried out within the two years of orbital flight. YINHE-1 is still operating and will participate in subsequent missions developed by GalaxySpace. This paper describes the overall scheme of satellite design and presents an integrated satellite–terrestrial network experiment system. Based on the two-year in-orbit test results, the Q/V band feed beam tracking, throughput and latency, communication under low elevation angles, and user beam switching are evaluated. In addition, the experiment results of integrated satellite–terrestrial communication are analyzed in detail. It is shown that the LEO broadband satellite possesses satisfactory capability in LTE/5G backhaul and Internet applications.

The Impact of Nonzenith Elevation Angles on Ground-Based Infrared Thermodynamic Retrievals

Abstract Observing thermodynamic profiles within the planetary boundary layer is essential to understanding and predicting atmospheric phenomena because of the significant exchange of sensible and latent heat between the land and atmosphere within that layer. The Atmospheric Emitted Radiance Interferometer (AERI) is a ground-based infrared spectrometer used to obtain the vertical profiles of temperature and water vapor mixing ratio. Most AERIs are only capable of zenith views, although the Marine AERI (M-AERI) has a design that allows it to view various elevation angles. In this study, we quantify the improvement in the information content and accuracy of the retrieved profiles when nonzenith angles are included, as is common with microwave radiometer profilers. The impacts of the additional scan angles are quantified through both a synthetic study and with M-AERI observations from the ARM Cloud Aerosol Precipitation Experiment (ACAPEX) campaign. The simulation study shows that low elevation angles contain more information content for temperature whereas high elevation angles have more information content for water vapor. Outside of very humid environments, the addition of low elevation angles also results in lower root-mean-square errors when compared with high angles for both temperature and water vapor mixing ratio, although this is primarily a result of averaging multiple observations together to reduce instrument noise. Real-world results from the ACAPEX dataset indicate similar results as were found for the simulation study, although not all predicted benefits are realized because of the small sample size and observational uncertainties.

Horizontal distribution of tropospheric NO2 and aerosols derived by dual-scan multi-wavelength multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements in Uccle, Belgium

Abstract. Dual-scan ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of tropospheric nitrogen dioxide (NO2) and aerosols were carried out in Uccle (50.8∘ N, 4.35∘ E; Brussels region, Belgium) for 2 years from March 2018 to February 2020. The MAX-DOAS instrument operated in both UV and visible wavelength ranges in a dual-scan configuration consisting of two submodes: (1) an elevation scan in a fixed viewing azimuthal direction and (2) an azimuthal scan in a fixed low elevation angle (2∘). By analyzing the O4 and NO2 differential slant column density (dSCD) at six different wavelength intervals along every azimuthal direction and by applying a new optimal-estimation-based inversion approach (the so-called mapping MAX-DOAS technique), the horizontal distribution of the NO2 near-surface concentrations and vertical column densities (VCDs) as well as the aerosol near-surface extinction coefficients are retrieved along 10 azimuthal directions. The retrieved horizontal NO2 concentration profiles allow the identification of the main NO2 hotspots in the Brussels area. Correlative comparisons of the retrieved horizontal NO2 distribution were conducted with airborne, mobile, air quality model, and satellite datasets, and overall good agreement is found. The comparison with TROPOMI observations from operational and scientific data products reveals that the characterization of the horizontal distribution of tropospheric NO2 VCDs by ground-based measurements and an adequate a priori NO2 profile shape in TROPOMI retrievals lead to better consistency between satellite and ground-based datasets.

Observations and Implications of Diurnal Climatology and Trends in Direct and Diffuse Solar Radiation Over China

AbstractSolar radiation received at the Earth's surface (Rs) is comprised of two components, the direct radiation (Rd) and the diffuse radiation (Rf). Rd, the direct beam from the sun, is essential for concentrated solar power generation. Rf, scattered by atmospheric molecules, aerosols, or cloud droplets, has a fertilization effect on plant photosynthesis. But how Rd and Rf change diurnally is largely unknown owing to the lack of long‐term measurements. Taking advantage of 22 years of homogeneous hourly surface observations over China, this study documents the climatological means and evolutions in the diurnal cycles of Rd and Rf since 1993, with an emphasis on their implications for solar power and agricultural production. Over the solar energy resource region, we observe a loss of Rd which is relatively large near sunrise and sunset at low solar elevation angles when the sunrays pass through the atmosphere on a longer pathway. However, the concentrated Rd energy covering an average 10‐hr period around noon during a day is relatively unaffected. Over the agricultural crop resource region, the large amounts of clouds and aerosols scattering more of the incoming light result in Rf taking the main proportion of Rs during the whole day. Rf resources and their fertilization effect in the main crop region of China further enhances since 1993 over almost all hours of the day.