Galileo Satellites Research Articles

An effective interpolation strategy for mitigating the day boundary discontinuities of precise GNSS orbit products

A precise Global Navigation Satellite System (GNSS) satellite orbit is an essential prerequisite for precise point positioning (PPP). Due to the daily orbit discontinuities (DODs) between adjacent daily orbit products provided by the International GNSS Service, the interpolation accuracy at the epochs near the day boundaries cannot be as high as expected and can significantly degrade the positioning accuracy. This is especially for the emerging Beidou and Galileo satellites with larger DODs. In this study, a new interpolation strategy was proposed to mitigate the DOD effect by estimating the zero- to second-order biases between the orbits of two adjacent days. The results show that the proposed interpolation strategy outperforms the traditional interpolation strategies. The 3D orbit interpolation error for the last 10 min on each day is reduced from almost a few centimeters or even meters to millimeters, an improvement of approximately 86–99% for all satellites. PPP validation also provides concrete evidence of this improvement, as the DOD-induced satellite phase residuals are reduced significantly. Thus, the new interpolation strategy mitigates the DOD effect and helps multi-GNSS users obtain the highest interpolation accuracy at the day boundaries, which can be comparable to that at the non-day boundaries.

Determination of SLR station coordinates based on LEO, LARES, LAGEOS, and Galileo satellites

The number of satellites equipped with retroreflectors dedicated to Satellite Laser Ranging (SLR) increases simultaneously with the development and invention of the spherical geodetic satellites, low Earth orbiters (LEOs), Galileo and other components of the Global Navigational Satellite System (GNSS). SLR and GNSS techniques onboard LEO and GNSS satellites create the possibility of widening the use of SLR observations for deriving SLR station coordinates, which up to now have been typically based on spherical geodetic satellites. We determine SLR station coordinates based on integrated SLR observations to LEOs, spherical geodetic, and GNSS satellites orbiting the Earth at different altitudes, from 330 to 26,210 km. The combination of eight LEOs, LAGEOS-1/2, LARES, and 13 Galileo satellites increased the number of 7-day SLR solutions from 10–20% to even 50%. We discuss the issues of handling of range biases in multi-satellite combinations and the proper solution constraining and weighting. Weighted combination is characterized by a reduction of formal error medians of estimated station coordinates up to 50%, and the reduction of station coordinate residuals. The combination of all satellites with optimum weighting increases the consistency of station coordinates in terms of interquartile ranges by 10% of horizontal components for non-core stations w.r.t LAGEOS-only solutions.

Estimating ambiguity fixed satellite orbit, integer clock and daily bias products for GPS L1/L2, L1/L5 and Galileo E1/E5a, E1/E5b signals

Ambiguity resolution of a single receiver is becoming more and more popular for precise GNSS (Global Navigation Satellite System) applications. To serve such an approach, dedicated satellite orbit, clock and bias products are needed. However, we need to be sure whether products based on specific frequencies and signals can be used when processing measurements of other frequencies and signals. For instance, for Galileo E5a frequency, some receivers track only the pilot signal (C5Q) while some track only the pilot-data signal (C5X). We cannot compute the differences between C5Q and C5X directly since these two signals are not tracked concurrently by any common receiver. As code measurements contribute equally as phase in the Melbourne-Wuebbena (MelWub) linear combination it is important to investigate whether C5Q and C5X can be mixed in a network to compute a common satellite MelWub bias product. By forming two network clusters tracking Q and X signals, respectively, we confirm that GPS C5Q and C5X signals cannot be mixed together. Because the bias differences between GPS C5Q and C5X can be more than half of one wide-lane cycle. Whereas, mixing of C5Q and C5X signals for Galileo satellites is possible. The RMS of satellite MelWub bias differences between Q and X cluster is about 0.01 wide-lane cycles for both E1/E5a and E1/E5b frequencies. Furthermore, we develop procedures to compute satellite integer clock and narrow-lane bias products using individual dual-frequency types. Same as the finding from previous studies, GPS satellite clock differences between L1/L2 and L1/L5 estimates exist and show a periodical behavior, with a peak-to-peak amplitude of 0.7 ns after removing the daily mean difference of each satellite. For Galileo satellites, the maximum clock difference between E1/E5a and E1/E5b estimates after removing the mean value is 0.04 ns and the mean RMS of differences is 0.015 ns. This is at the same level as the noise of the carrier phase measurement in the ionosphere-free linear combination. Finally, we introduce all the estimated GPS and Galileo satellite products into PPP-AR (precise point positioning, ambiguity resolution) and Sentinel-3A satellite orbit determination. Ambiguity fixed solutions show clear improvement over float solutions. The repeatability of five ground-station coordinates show an improvement of more than 30% in the east direction when using both GPS and Galileo products. The Sentinel-3A satellite tracks only GPS L1/L2 measurements. The standard deviation (STD) of satellite laser ranging (SLR) residuals is reduced by about 10% when fixing ambiguity parameters to integer values.

General relativistic effects acting on the orbits of Galileo satellites

The first pair of satellites belonging to the European Global Navigation Satellite System (GNSS)—Galileo—has been accidentally launched into highly eccentric, instead of circular, orbits. The final height of these two satellites varies between 17,180 and 26,020 km, making these satellites very suitable for the verification of the effects emerging from general relativity. We employ the post-Newtonian parameterization (PPN) for describing the perturbations acting on Keplerian orbit parameters of artificial Earth satellites caused by the Schwarzschild, Lense–Thirring, and de Sitter general relativity effects. The values emerging from PPN numerical simulations are compared with the approximations based on the Gaussian perturbations for the temporal variations of the Keplerian elements of Galileo satellites in nominal, near-circular orbits, as well as in the highly elliptical orbits. We discuss what kinds of perturbations are detectable using the current accuracy of precise orbit determination of artificial Earth satellites, including the expected secular and periodic variations, as well as the constant offsets of Keplerian parameters. We found that not only secular but also periodic variations of orbit parameters caused by general relativity effects exceed the value of 1 cm within 24 h; thus, they should be fully detectable using the current GNSS precise orbit determination methods. Many of the 1-PPN effects are detectable using the Galileo satellite system, but the Lense–Thirring effect is not.

Maritime moving object localization and detection using global navigation smart radar system

At present, the global navigation satellite systems’ use as transmitters of maritime surveillance opportunities in passive radar systems is particularly desirable because of global coverage and accurate sources' primary advantages. This paper proposes a global navigation smart radar system for maritime moving object localization and detection. To find and detect underwater targets, signal processing algorithms were used. For finding a transmitting node that can be used during the operation, a multilayered technique is used. The new Galileo satellites and smart systems have been developed to determine the technology's viability and verify the existing signal processing algorithms. The findings show that the machine concept and its multi-service capability are simultaneously observed with two satellites.

Galileo L10 Satellites: Orbit, Clock and Signal-in-Space Performance Analysis.

The tenth launch (L10) of the European Global Navigation Satellite System Galileo filled in all orbital slots in the constellation. The launch carried four Galileo satellites and took place in July 2018. The satellites were declared operational in February 2019. In this study, we report on the performance of the Galileo L10 satellites in terms of orbital inclination and repeat period parameters, broadcast satellite clocks and signal in space (SiS) performance indicators. We used all available broadcast navigation data from the IGS consolidated navigation files. These satellites have not been reported in the previous studies. First, the orbital inclination (°) and repeat period ( s) for all four satellites are within the nominal values. The data analysis reveals also 13.5-, 27-, 177- and 354-days periodic signals. Second, the broadcast satellite clocks show different correction magnitude due to different trends in the bias component. One clock switch and several other minor correction jumps have occurred since the satellites were declared operational. Short-term discontinuities are within ±1 ps/s, whereas clock accuracy values are constantly below 0.20 m (root-mean-square—rms). Finally, the SiS performance has been very high in terms of availability and accuracy. Monthly SiS availability has been constantly above the target value of 87% and much higher in 2020 as compared to 2019. Monthly SiS accuracy has been below 0.20 m (95th percentile) and below 0.40 m (99th percentile). The performance figures depend on the content and quality of the consolidated navigation files as well as the precise reference products. Nevertheless, these levels of accuracy are well below the 7 m threshold (95th percentile) specified in the Galileo service definition document.

GREAT-UPD: An open-source software for uncalibrated phase delay estimation based on multi-GNSS and multi-frequency observations

To meet the demands of precise orbit and clock determination, high-precision positioning, and navigation applications, a software called GREAT (GNSS + Research, Application and Teaching) was designed and developed at Wuhan University. As one important module in the GREAT software, GREAT-UPD was developed for multi-GNSS and multi-frequency uncalibrated phase delay (UPD) estimation. It can provide extra-wide-lane (EWL), wide-lane (WL), and narrow-lane (NL) UPDs for GPS, GLONASS, Galileo and BDS (GREC) satellites for precise point positioning (PPP) ambiguity resolution (AR) in a multi-GNSS and multi-frequency environment. The open-source GREAT-UPD software is written in C + + 11 language following object-oriented principles and can be compiled and run on several popular operating systems, such as Windows, Linux, and Macintosh. Observations from 222 stations spanning days from DOY 091 to 120 were used to conduct multi-GNSS and multi-frequency UPD estimation and PPP AR. Results indicate that GREAT-UPD can generate stable and reliable UPD products with multi-GNSS and multi-frequency observations. After applying the UPD corrections, the multi-frequency GREC PPP AR was achieved with the averaged time to first fix of 9.0 min. The software package can be obtained at https://geodesy.noaa.gov/gps-toolbox , including the source code, user manual, batch processing scripts, example data, and some auxiliary tools.

An In-Depth Assessment of the New BDS-3 B1C and B2a Signals

An in-depth and comprehensive assessment of new observations from BDS-3 satellites is presented, with the main focus on the Carrier-to-Noise density ratio (C/N0), the quality of code and carrier phase observations for B1C and B2a signal. The signal characteristics of geosynchronous earth orbit (GEO), inclined geosynchronous satellite orbit (IGSO) and medium earth orbit (MEO) satellites of BDS-3 were grouped and compared, respectively. The evaluation results of the new B1C and B2a signals of BDS-3 were compared with the previously B1I/B2I/B3I signals and the interoperable signals of GPS, Galileo and quasi-zenith satellite system (QZSS) were compared simultaneously. As expected, the results clearly show that B1C and B2a have better signal strength and higher accuracy, including code and carrier phase observations. The C/N0 of the B2a signal is about 3 dB higher than other signals. One exception is the code observation accuracy of B3I, which value is less than 0.15 m. The carrier precision of B1C and B2a is better than that of B1I/B2I/B3I. Despite difference-in-difference (DD) observation quantity or zero-base line evaluation is adopted, while B1C is about 0.3 mm higher carrier precision than B2a. The BDS-3 MEO satellite and GPS, Galileo, and QZSS satellites have the same level of signal strength, code and phase observation accuracy at the interoperable frequency, namely 1575.42 MHz and 1176.45 MHz which are very suitable for the co-position application.

Galileo real-time orbit determination with multi-frequency raw observations

Real-time GNSS-based applications require corresponding real-time orbit products. While traditional GNSS orbits are generated with the dual-frequency IF (Ionosphere-Free) model, the increase of multi-frequency signal satellites brings new challenges for the data processing. Therefore, real-time orbit determination with the multi-frequency UC (Uncombined) model is introduced in this study considering its flexibility. With the derived mathematical model conforming to IGS (International GNSS Service) dual-frequency clock definition and one-week triple-frequency Galileo observation data from 90 IGS network stations, the convergence and accuracy of real-time orbits is assessed and the characteristics of satellite IFCB (Inter-Frequency Clock Bias) are analyzed. Results indicate that the model differences, including dual-frequency IF model, dual-frequency UC model and triple-frequency UC model, contribute to only cm-level differences with CODE (Center for Orbit Determination in Europe) final orbits after a convergence time of around 12 h. The constellation-mean RMS (Root Mean Square) differences of the converged real-time orbits with the CODE final orbits reaches about 5.0 cm, 7.0 cm and 5.0 cm for the radial, tangential and normal directions. The convergence of satellite IFCB is much faster than that of satellite orbit, which reflects a loose correlation between these two parameters. While the Galileo satellite IFCB are temporally stable, the modeling of satellite IFCB may be unreliable when over constrained and becomes even more unstable with commonly encountered datum changes. In summary, real-time GNSS orbit determination with multi-frequency raw observations is feasible and extendable with proper treatment of IFCB.

Time\u2013frequency analysis of the Galileo satellite clocks: looking for the J2 relativistic effect and other periodic variations

When observed from the ground, the frequency of the atomic clocks flying on the satellites of a Global Navigation Satellite System is referred to as apparent frequency, because it is observed through the on-board signal generation chain, the propagation path, the relativistic effects, the measurement system, and the clock estimation algorithm. As a consequence, the apparent clock frequency is affected by periodic variations of different origins such as, for example, the periodic component of the J2 relativistic effect, due to the oblateness of the earth, and the clock estimation errors induced by the orbital estimation errors. We present a detailed characterization of the periodic variations affecting the apparent frequency of the Galileo clocks, obtained by applying time–frequency analysis and other signal processing techniques on space clock data provided by the European Space Agency. In particular, we analyze one year of data from three Galileo Passive Hydrogen Masers, flying on two different orbital planes. Time–frequency analysis reveals how the spectral components of the apparent frequency change with time. For example, it confirms that the amplitude of the periodic signal due to the orbital estimation errors depends on the angle between the sun and the orbital plane. Moreover, it allows to find a more precise estimate of the amplitude of the J2 effect, in agreement with the prediction of the general theory of relativity, and it shows that such amplitude suddenly decreases when the corresponding relativistic correction is applied to the data, thus validating the analytical formula used for the correction.

Identification of Multi-Faults in GNSS Signals using RSIVIA under Dual Constellation

This publication presents the development of integrity monitoring and fault detection and exclusion (FDE) of pseudorange measurements, which are used to aid a tightly-coupled navigation filter. This filter is based on an inertial measurement unit (IMU) and is aided by signals of global navigation satellite system (GNSS). Particularly, the GNSS signals include global positioning system (GPS) and Galileo. By using GNSS signals, navigation systems suffer from signal interferences resulting in large pseudorange errors. Further, a higher number of satellites with dual-constellation increases the possibility that satellite observations contain multiple faults. In order to ensure integrity and accuracy of the filter solution, it is crucial to provide sufficient fault-free GNSS measurements for the navigation filter. For this purpose, a new hybrid strategy is applied, combining conventional receiver autonomous integrity monitoring (RAIM) and innovative robust set inversion via interval analysis (RSIVIA). To further improve the performance, as well as the computational efficiency of the algorithm, the estimated velocity and its variance from the navigation filter is used to reduce the size of the RSIVIA initial box. The designed approach is evaluated with recorded data from an extensive real-world measurement campaign, which has been carried out in GATE Berchtesgaden, Germany. In GATE, up to six Galileo satellites in orbit can be simulated. Further, the signals of simulated Galileo satellites can be manipulated to provide faulty GNSS measurements, such that the fault detection and identification (FDI) capability can be validated. The results show that the designed approach is able to identify the generated faulty GNSS observables correctly and improve the accuracy of the navigation solution. Compared with traditional RSIVIA, the designed new approach provides a more timely fault identification and is more computational efficient.

Spaceborne atomic clock performance review of BDS-3 MEO satellites

On June 23, 2020, the final satellite of the third generation Beidou satellite navigation system (BDS-3) was launched, which represents the BDS-3 is fully operational and can provide global services. Due to the fact that BDS-3 constellation mainly consists of Medium Earth Orbit (MEO) satellites, we only discussed the in-orbit atomic clock performance of MEO, and compared to that of the latest generation of GPS as well as Galileo satellites. Based on the precise satellite clock product provided by German Research Center for Geosciences (GFZ), a level of 0.2 ns fitting precision is obtained, and it is better than BDS-2. The periodic terms of satellite clock are analyzed by means of spectrum analysis. Except some mutual periodic terms, several exclusive periodic terms, approximately 8 h, 6 h and 4.8 h harmonics, are found in passive hydrogen masers (PHMs) of BDS-3, and possible reasons are given. Clock systematic characteristics, such as frequency accuracy and drift rate are examined and found that PHMs have better result than rubidium atomic clocks. Finally, frequency stability is evaluated: all BDS-3 satellite clocks can reach the order of 10-15 at the average interval of 86400 s, which is much better than BDS-2 and comparable to the latest type of GPS III as well as Galileo satellites.

Improving GPS and Galileo precise data processing based on calibration of signal distortion biases

The quality of pseudorange observations plays an important role in Global Navigation Satellite System (GNSS) data processing, especially for satellite clock estimation, differential code bias (DCB) estimation and wide-lane ambiguity resolution. It is validated that receivers with different correlator spaces and front-end designs will bring signal distortion bias (SDB) in pseudorange observations, which will affect GNSS data processing based on inhomogenous receivers, such as receivers from Multi-GNSS EXperiment (MGEX) network. We firstly evaluate the consistency of GPS and Galileo satellite clock and DCB products among different International GNSS Service (IGS) Analysis Centers (ACs). The results show that inconsistency exists in both satellite clocks and DCB products among different ACs. For example, the difference of GPS satellite DCB (C1C-C1W) between two ACs, which is free of ionospheric delay, can reach up to 0.31 ns. To improve GPS and Galileo precise data processing based on inhomogenous receivers, pseudorange biases caused by receiver signal distortion are analyzed and calibrated for 5 receiver brands and 19 receiver models. The maximum pseudorange biases of ionospheric-free combination caused by receiver SDBs can reach up to ±3 ns and ±1 ns for GPS and Galileo, respectively. Then, bias corrections of GPS and Galileo raw pseudorange observations are divided into 11 groups and estimated by each receiver group. Finally, several experiments are carried out for the validation of GPS and Galileo SDB correction models. For example, the GPS and Galileo satellite clock offset differences estimated by different receiver brands are decreased by 86.59% and 50.0% for specific receiver type, respectively. The improvement ratio of satellite DCB accuracy is related to satellite systems and signal types, and the maximum improvement can reach up to 95%. Moreover, the maximum improvement of GPS undifferenced wide-lane ambiguity resolution success rate can reach up to 22% for specific receiver types. All these results prove that the bias corrections proposed could greatly eliminate the effect of receiver SDBs on GPS and Galileo precise data processing. Hence, we suggest the corrections of SDBs should be taken into consideration for IGS data reprocessing.

Analysis of real-time multi-GNSS satellite products of Wuhan University for rapid response of precise positioning

When users need to quickly process GNSS data, they often need the satellite orbit and clock products with the minimum latency and the highest precision, and it is a good solution to receive the real-time satellite RTCM SSR correction stream to recover the precise satellite orbit and clock products in real time and then store them in an offline repository for rapid response of precise positioning. In this paper, the real-time multi-GNSS orbit and clock RTCM SSR correction stream broadcast by SSRC00WHU0 mountpoint of Wuhan University is used to recover precise satellite orbit and clock products in real time. First, the seven-day orbit files and clock files were obtained and stored locally, and compared with the final MGEX precise satellite orbit and clock products. The results show that the real-time orbit and clock products of GPS and Galileo satellites have the best accuracy, followed by GLONASS satellites and BDS satellites. The real-time orbit products can reach the accuracy level of 5 cm for GPS satellites, 8 cm for Galileo satellites, 15 cm for GLONASS satellites and 16 cm for BDS-3 satellites, and the real-time clock products can reach the accuracy level of 0.43 ns for GPS satellites, 0.44 ns for Galileo satellites, 0.91 ns for GLONASS satellites and 3.14 ns for BDS satellites. Then, the observation data of 20 IGS stations randomly distributed around the world from DOY 150 to 156 in 2021 were processed by static precise point positioning (PPP) mode using the recovered real-time products. The results show that the average positioning accuracy can reach 1.57 cm, 0.76 cm and 1.67 cm in east, north and up direction for static PPP, respectively. Finally, using the recovered real-time products and the final products, the GPS observation data collected in aviation were processed in pseudo real-time in a kinematic mode. The results show that the RMSs of positioning errors are 8.5 cm, 2.4 cm and 16.5 cm in the east, north and up direction, respectively. In addition, one-day multi-GNSS observation data at 20 IGS stations were processed in a kinematic PPP mode, and the results show that the average positioning accuracy is 3.11 cm, 2.04 cm and 4.94 cm in east, north and up directions.

First Adiabatic Invariants and Phase Space Densities for the Jovian Electron and Proton Radiation Belts—Galileo and GIRE3 Estimates

AbstractThe fluxes and phase space densities for a fixed first adiabatic invariant for high‐energy electrons and protons provide important inputs for various scientific studies for determining the physics of particle diffusion and energization. This study provides estimates of the first adiabatic invariant and phase space density based on the complete and large data base available from the Energetic Particle Detector (EPD) on Galileo for the Jovian environment. To be specific, 10 min averages of the high‐energy electron and proton data are used to compute differential flux spectra versus energy between L = 8 and 25 over the mission. These spectra provide estimates of the differential fluxes and phase space density for constant first adiabatic invariants between 102 and 105 MeV/G. As would be expected, the electron and proton fluxes and phase space densities generally trend lower as the planet is approached. The results indicate that, whereas the overall trends for each orbit are consistent, detailed orbit to orbit variations can be observed. Galileo orbit C22 is presented as an example of deviations from the mean downward trend. To validate the Galileo results and extend the findings into L = 3, the GIRE3 model was also used to compute the fluxes and phase space densities for constant first adiabatic invariant versus L‐shell. Comparison between GIRE3 and EPD demonstrates that the model adequately reproduces the EPD data trends and they consistently show additional variations near Io. This provides proof that the GIRE3 is a useful starting point for diffusion analyses and similar studies.

Characterization of biases between BDS-3 and BDS-2, GPS, Galileo and GLONASS observations and their effect on precise time and frequency transfer

With the development of the Chinese BeiDou Navigation Satellite System (BDS), from a regional system BDS-2 to a global system BDS-3, more satellites can now be employed in the domain of globally precise time and frequency transfer. However, if the current BDS-3 and BDS-2, Global Positioning System (GPS), Galileo and GLObal NAvigation satellite system (GLONASS) are combined for multi-global navigation satellite system (multi-GNSS) time transfer, the characteristics of inter-system biases (ISBs) remain unclear. In this study, we analyzed the characteristics of ISBs between BDS-3 and BDS-2, GPS, Galileo and GLONASS, and revealed that their ISB series is not constant and that all the daily ISB series of the multi-GNSS show some systematic characteristics among different stations. Therefore, three stochastic models for ISBs of white noise process, random constant process and random walk process expressed as ‘White’, ‘Hour’ and ‘Walk’, respectively, were employed to assess the performance of the time and frequency transfer using the time link accuracy and frequency stability indicators. The results show that the models for the ‘Hour’ and ‘Walk’ schemes are notably better than that for the ‘White’ scheme by 10.4% and 16.8%, respectively, for the least noisy time link of WTZZ-BRCH. Furthermore, the improvements in the ‘Hour’ and ‘Walk’ schemes are 2.1% and 3.5%, respectively, for the TP01-BRCH time link, when compared with that of the ‘White’ scheme. With respect to frequency stability, the ‘Hour’ and ‘Walk’ schemes also perform better in terms of frequency stability than the ‘White’ scheme at different time intervals. Similarly, the improvements over the ‘White’ scheme are 5.05% and 4.97% for the TP01-BRCH time link and are 24.01% and 32.48% for the WTZZ-BRCH time link.

Visibility Anomaly of GNSS Satellite and Support from Regional Systems

In a multi-GNSS (global navigation satellite system) environment with operational GPS, GLONASS and Galileo Satellites, the Asia–Oceania region is expected to get better benefits of a large number of GNSS satellites for use. However, it is witnessed that during some parts of the day, no GNSS satellite is present above 60° elevation angle from many parts of the earth, including India. Real-time data from India show the regular absence of usable GPS satellites above 60° elevation angles during some parts of the day; addition of GLONASS and Galileo satellites does not improve the situation much. From Burdwan, West Bengal, India at least twice a day, no GNSS satellite is found above 60° elevation angles for more than 30 min. Simulation study for scattered places of India and data from IGS Centres confirm similar observations, except for the extreme northern region. The global scenario also supports these observations, while the individual operator’s country is free from the problem using their own navigational system. The consequences of the problem affect GNSS-based solutions; for locations with obstruction of GNSS signals from low elevation angles, the concurrent occurrence of this incidence poses a threat for seamless GNSS-based navigation through intermittent loss of solution and degraded solution quality. Regional navigation satellite systems (RNSS) help mitigate this problem within the respective service regions. For a large part of the globe, the problem may be allayed using GNSS–RNSS hybrid operation. The result would be important for location-specific GNSS mission planning in strategic, life and safety-critical applications.

Seasonal analysis of Klobuchar and NeQuick G single-frequency ionospheric models performance in 2018

For single frequency positioning, the ionospheric effects can be minimized by using an ionospheric model, e.g., the Klobuchar or the NeQuick G. These models, respectively associated with the Global Positioning System (GPS) and Galileo systems, are based on a set of transmitted coefficients that describe the background ionospheric behavior and can be used to estimate the ionospheric delay at any time and location on the globe. We evaluate the accuracy and seasonal behavior of the Klobuchar and the NeQuick G models at different latitudes during the solar minimum year of 2018. Calibrated ionospheric Slant Total Electron Content (STEC) values from 33 MGEX (The Multi-GNSS Experiment) stations tracking GPS and Galileo satellites are used as a reference for comparison. The evaluation results show that, for low solar activity conditions (Winter Solstice), the Klobuchar model usually overestimates the ionospheric delay measured on the L1 frequency, with a mean bias of 0.93 m, and performs better than NeQuick G in regions and periods of high ionospheric activity (Spring and Autumn Equinoxes). NeQuick G usually underestimates the delay, with a mean bias of −0.14 m, performing better than Klobuchar in regions and periods of low ionospheric activity. Overall, Klobuchar and NeQuick G presented a modelling error RMS of 1.54 m and 1.19 m respectively.

Determination of precise Galileo orbits using combined GNSS and SLR observations

Galileo satellites are equipped with laser retroreflector arrays for satellite laser ranging (SLR). In this study, we develop a methodology for the GNSS-SLR combination at the normal equation level with three different weighting strategies and evaluate the impact of laser observations on the determined Galileo orbits. We provide the optimum weighting scheme for precise orbit determination employing the co-location onboard Galileo. The combined GNSS-SLR solution diminishes the semimajor axis formal error by up to 62%, as well as reduces the dependency between values of formal errors and the elevation of the Sun above the orbital plane—the β angle. In the combined solution, the standard deviation of the SLR residuals decreases from 36.1 to 29.6 mm for Galileo-IOV satellites and |β|> 60°, when compared to GNSS-only solutions. Moreover, the bias of the Length-of-Day parameter is 20% lower for the combined solution when compared to the microwave one. As a result, the combination of GNSS and SLR observations provides promising results for future co-locations onboard the Galileo satellites for the orbit determination, realization of the terrestrial reference frames, and deriving geodetic parameters.

Galileo: a consolidação do sistema de posicionamento europeu

O Galileo é a contribuição da União Europeia ao GNSS (ingl. Global Navigation Satellite Systems – Sistemas de Navegação Global por Satélite) e está próximo da declaração da fase operacional completa, que deve ocorrer no final de 2020 ou início de 2021. Este sistema começou a ser concebido na década de 90, após a decisão do governo americano em não permitir que outras nações participassem da construção e manutenção do sistema NAVSTAR – GPS (ingl. Global Positioning System – Sistema de Posicionamento Global). O sistema Galileo é a primeira contribuição civil para o GNSS e foi desenvolvido de forma a ser independente dos outros sistemas nos segmentos espacial, de controle e operacional. Além disso, está sendo desenvolvido para ser interoperável e compatível com os outros GNSS, em especial o GPS. Nos últimos anos, o desenvolvimento do Galileo fez progressos significativos. A constelação atual compreende um total de 26 satélites orbitando a Terra, 22 operacionais, dos quais três pertencentes à primeira geração de satélites de validação de órbita, e a infraestrutura de controle terrestre está em pleno funcionamento. Para o usuário, são transmitidos sinais em três frequências E1, E5 e E6. Os sinais em E1 e E5 são transmitidos nas mesmas frequências que os sinais GPS L1 e L5 e ambos sistemas usam princípios de modulação equivalentes. Isso é benéfico pois proporciona uma melhor cobertura e maior robustez para usuários que podem utilizar os sistemas de forma combinada. Além disso, o Galileo oferece vários novos serviços específicos, como o serviço aberto, o serviço de alta acurácia e de busca e resgate. Como o sistema Galileo está atualmente em fase final de implantação, faz-se necessário na literatura brasileira, um artigo que trate exclusivamente desse sistema, este artigo apresenta o estado da arte do sistema Galileo (julho de 2020). Resultados iniciais demonstraram que o Galileo tem acurácia comparável ao GPS, no posicionamento por ponto simples.