scholarly journals Relativistic Modelling for Accurate Time Transfer via Optical Inter-Satellite Links

2021 ◽  
Author(s):  
Manuele Dassié ◽  
Gabriele Giorgi
Keyword(s):  
Orbit Determination ◽  
Electromagnetic Waves ◽  
Time Synchronization ◽  
Relativistic Effects ◽  
Satellite System ◽  
Time Transfer ◽  
Transfer Scheme ◽  
Propagation Of Light ◽  
Satellite Links ◽  
The Impact

AbstractThe DLR Institute for Communication and Navigation is currently working on a new GNSS architecture that enables accurate autonomous inter-satellite synchronization at picosecond-level. Synchronization is achieved via time transfer techniques enabled by optical inter-satellite links (OISLs), paving the way for a system in which space (orbits) and time (synchronization) can be effectively separated, leading to a high level of synchronization throughout the constellation, which in turn greatly improves accurate orbit determination. This is possible provided that relativistic effects are adequately taken into account. This work focuses on a two-way time transfer scheme based on the exchange of time stamps via optical signals, which allows the synchronization of a GNSS satellite system with respect to a defined coordinate time with picosecond-level accuracy. We analyse the impact of relativistic effects in clock offset estimation between optically linked clocks: results show that to achieve synchronization at this level of accuracy it is necessary to account for terrestrial geopotential harmonics up to the third order while the gravitational influence of additional celestial bodies can be neglected. Relativistic delays in the propagation of electromagnetic waves through spacetime are also evaluated. It is shown that for a two-way synchronization method, the Euclidean expression for the propagation of light is sufficient to achieve picosecond synchronization, provided m-level orbit determination of both satellites is available, and the hardware delays are well calibrated to the targeted accuracy. Also, we show how to practically achieve autonomous synchronization via a sequence of pair-wise synchronizations across all satellites of the constellation.

scholarly journals Impact of ECOM Solar Radiation Pressure Models on Multi-GNSS Ultra-Rapid Orbit Determination

2019 ◽  
Vol 11 (24) ◽  
pp. 3024
Author(s):  
Yang Liu ◽  
Yanxiong Liu ◽  
Ziwen Tian ◽  
Xiaolei Dai ◽  
Yun Qing ◽  
...  
Keyword(s):  
Solar Radiation ◽  
Real Time ◽  
Radiation Pressure ◽  
Orbit Determination ◽  
Precise Orbit Determination ◽  
Solar Radiation Pressure ◽  
Satellite System ◽  
Ambiguity Fixing ◽  
Global Navigation Satellite ◽  
The Impact

The Global Navigation Satellite System (GNSS) ultra-rapid precise orbits are crucial for global and wide-area real-time high-precision applications. The solar radiation pressure (SRP) model is an important factor in precise orbit determination. The real-time orbit determination is generally less accurate than the post-processed one and may amplify the instability and mismodeling of SRP models. Also, the impact of different SRP models on multi-GNSS real-time predicted orbits demands investigations. We analyzed the impact of the ECOM 1 and ECOM 2 models on multi-GNSS ultra-rapid orbit determination in terms of ambiguity resolution performance, real-time predicted orbit overlap precision, and satellite laser ranging (SLR) validation. The multi-GNSS observed orbital arc and predicted orbital arcs of 1, 3, 6, and 24 h are compared. The simulated real-time experiment shows that for GLONASS and Galileo ultra-rapid orbits, compared to ECOM 1, ECOM 2 increased the ambiguity fixing rate to 89.3% and 83.1%, respectively, and improves the predicted orbit accuracy by 9.2% and 27.7%, respectively. For GPS ultra-rapid orbits, ECOM 2 obtains a similar ambiguity fixing rate as ECOM 1 but slightly better orbit overlap precision. For BDS GEO ultra-rapid orbits, ECOM 2 obtains better overlap precision and SLR residuals, while for BDS IGSO and MEO ultra-rapid orbits, ECOM 1 obtains better orbit overlap precision and SLR residuals.

Clock modelling techniques for an enhanced GNSS orbit determination

2021 ◽  
Author(s):  
Pedro Roldan ◽  
Pierre Guerin ◽  
Julie Anton ◽  
Marco Laurenti ◽  
Sebastien Trilles
Keyword(s):  
Kalman Filter ◽  
Orbit Determination ◽  
Time Synchronization ◽  
Quadratic Model ◽  
Model Parameters ◽  
Phase Measurements ◽  
Station Network ◽  
Estimation Of Model Parameters ◽  
The Impact ◽  
Clock Models

<p>The determination of GNSS orbits is generally based on the processing of pseudorange and carrier phase measurements from a station network, with an Orbit Determination and Time Synchronization (ODTS) process. This process involves the satellite and ground station clocks as part of the GNSS measurement reconstruction. The clocks are generally estimated as a snapshot parameter, without assuming any correlation between epochs. However, the stability of satellite and some station clocks, based on technologies of hydrogen, cesium or rubidium, allows for a significant predictability. Taking advantage of this predictability the ODTS process can be improved, especially in those cases where the station network is limited or does not provide a good coverage for certain areas.</p><p>The clock modelling can be directly done by estimating additional parameters in the filter. A quadratic model is generally estimated for each clock, keeping a small snapshot contribution to account for the stochastic part and for potential deviations with respect to the theoretical behavior of the clock. The detection of this kind of deviations in the satellite and station clocks becomes a major factor for achieving a good performance with these techniques. In case the clock experiences feared events like phase or frequency jumps, the estimated clock model stops being valid and the estimation of model parameters needs to be reset.</p><p>In case a composite clock algorithm is used to provide the reference timescale for the ODTS, the estimation of clock models can rely on this algorithm. Algorithms of composite clock are generally based on a Kalman filter that estimates as part of the state vector the differences between each contributing clock and the composite timescale. These differences can be used not only to define the reference timescale of the ODTS, but also to remove the deterministic part of the clocks in the measurement reconstruction. As for the case of clock modelling, for algorithms of composite clock the detection and correction of anomalies in the contributing clocks becomes a critical point.</p><p>In this work, the integration of orbit determination, clock modelling and composite clock algorithms will be described. The impact of clock modeling techniques on the GNSS orbit determination accuracy will be presented, both considering a direct estimation of clock models in the ODTS and the estimation provided by the composite clock algorithm. These analyses will be based on NEODIS, the orbit determination software developed by Thales Alenia Space, which integrates with a Kalman filter approach GNSS orbit determination and composite clock algorithms.</p><p> </p>

scholarly journals Precise Orbit Determination of BDS-2 and BDS-3 Using SLR

2019 ◽  
Vol 11 (23) ◽  
pp. 2735 ◽  
Author(s):  
Honglei Yang ◽  
Tianhe Xu ◽  
Wenfeng Nie ◽  
Fan Gao ◽  
Meiqian Guan
Keyword(s):  
Orbit Determination ◽  
Optimal Solution ◽  
Precise Orbit Determination ◽  
Satellite System ◽  
Earth Orbit ◽  
Precise Orbit ◽  
Geo Satellite ◽  
Optimal Average ◽  
The Impact

The BeiDou Navigation Satellite System (BDS) of China is currently in the hybrid-use period of BDS-2 and BDS-3 satellites. All of them are equipped with Laser Retroreflect Arrays (LRAs) for Satellite Laser Ranging (SLR), which can directly obtain an independent, sub-centimetre level of distance measurement. The main purpose of this contribution is to use the solely SLR Normal Points (NPs) data to determinate the precise orbit of BDS-2 and BDS-3 satellites, including one Geostationary Earth Orbit (GEO), three Inclined Geo-Synchronous Orbits (ISGO), and one Medium Earth Orbit (MEO) of BDS-2 satellites, as well as four MEO of BDS-3 satellites, from 1 January to 30 June 2019. The microwave-based orbit from Wuhan University (WUM) are firstly validated to mark and eliminate the bad SLR observations in our preprocessing stage. Then, the 3-, 5-, 7-, and 9-day arc solutions are performed to investigate the impact of the different orbital arc lengths on the quality of SLR-derived orbits and test the optimal solution of the multi-day arc. Moreover, the dependency of SLR-only orbit determination accuracy on the number of SLR observations and the number of SLR sites are discussed to explore the orbit determination quality of the 3-,5-, 7-, and 9-day arc solutions. The results indicate that (1) during the half-year time span of 2019, the overall Root Mean Square (RMS) of SLR validation residuals derived from WUM is 19.0 cm for BDS-2 GEO C01, 5.2–7.3 cm for three BDS-2 IGSO, 3.4 cm for BDS-2 MEO C11, and 4.4–5.7 cm for four BDS-3 MEO satellites respectively. (2) The 9-day arc solutions present the best orbit accuracy in our multi-day SLR-only orbit determination for BDS IGSO and MEO satellites. The 9-day overlaps median RMS of BDS MEO in RTN directions are evaluated at 3.6–5.7, 12.4–21.6, and 15.6–23.9 cm respectively, as well as 5.7–9.6, 15.0–36.8, and 16.5–35.2 cm for the comparison with WUM precise orbits, while these values of BDS IGSO are larger by a factor of about 3–10 than BDS MEO orbits in their corresponding RTN directions. Furthermore, the optimal average 3D-RMS of 9-day overlaps is 0.49 and 1.89 m for BDS MEO and IGSO respectively, as well as 0.55 and 1.85 m in comparison with WUM orbits. Owing to its extremely rare SLR observations, the SLR-only orbit determination accuracy of BDS-2 GEO satellite can only reach a level of 10 metres or worse. (3) To obtain a stable and reliable SLR-only precise orbit, the 7-day to 9-day arc solutions are necessary to provide a sufficient SLR observation quantity and geometry, with more than 50–80 available SLR observations at 5–6 SLR sites that are evenly distributed, both in the Northern and Southern Hemispheres.

On the prospects of a future GNSS constellation on the global terrestrial reference frame

2020 ◽  
Author(s):  
Susanne Glaser ◽  
Grzegorz Michalak ◽  
Rolf Koenig ◽  
Benjamin Maennel ◽  
Harald Schuh
Keyword(s):  
Reference Frames ◽  
Time Synchronization ◽  
Orbital Plane ◽  
Satellite System ◽  
Low Earth Orbit ◽  
Earth Orbit ◽  
Terrestrial Reference Frames ◽  
Earth Rotation Parameters ◽  
The Impact

<p>Global terrestrial reference frames (TRFs), as one of the most important geodetic products, currently miss the imperative requirements of 1 mm accuracy and 1mm/decade long-term stability. In this study, the prospects of a future Global Navigation Satellite System (GNSS) to improve global TRFs is assessed by simulations. The future constellation, named “Kepler”, is proposed by the German Aerospace Center DLR in view of the next generation Galileo system. In addition to a contemporary Medium Earth Orbit (MEO) segment with 24 satellites in three orbital planes, Kepler consists of six Low Earth Orbit (LEO) satellites in two near polar planes, all carrying long-term stable optical clocks. The MEO satellites in one orbital plane and the LEO and MEO satellites in different planes are connected with optical two-way inter-satellite links (ISLs) as the innovative key feature. The ISLs allow very precise range measurements and time synchronization (at the picosecond-level) between the satellites. Different simulation scenarios are set up to evaluate the impact of the Kepler features on the TRF-defining parameters origin and scale as well as on the Earth rotation parameters (ERPs). The origin of a Kepler-only TRF improves considerably by factors of 8, 8, and 43 in X, Y, and Z direction, respectively, w.r.t. a Galileo-only solution. The scale realized by a Kepler-TRF shows improvements of 34% w.r.t. Galileo-only. In a combination with simulated observations of Very Long Baseline Interferometry the impact on multi-technique TRFs is assessed as well. The ERPs of both techniques are combined as global ties and benefits especially on the determination of UT1-UTC are expected.</p>

Combined Precise Orbit Determination for BDS-2 and BDS-3 Satellites

2020 ◽  
Author(s):  
Hanbing Peng ◽  
Maorong Ge ◽  
Yuanxi Yang ◽  
Harald Schuh ◽  
Roman Galas
Keyword(s):  
Orbit Determination ◽  
Precise Orbit Determination ◽  
Satellite System ◽  
Asia Pacific ◽  
Navigation Satellite ◽  
Beidou Navigation Satellite System ◽  
Precise Orbit ◽  
Product Generation ◽  
Open Service ◽  
The Impact

<p>Since November 2017, the 3rd generation BeiDou Navigation Satellite System (BDS-3) of China has stepped into an intensive build-up phase. Up to the end of 2019, there are 5 experimental and 28 operational BDS-3 satellites in the space. Besides that, 16 BDS-2 legacy satellites are still providing Positioning, Navigation and Timing (PNT) service for Asia-Pacific users. Unlike BDS-2 satellites, BDS-3 satellites will not transmit signal on frequency B2I which is one of the open service frequencies of BDS-2 and will be replaced by B2a of BDS-3. For legacy signals, only that on B1I and B3I will be transmitted by all BDS-3 satellites. Therefore, current routine scheme that generates precise orbit and clock products with B1I+B2I combination observations becomes infeasible for BDS-3. Observation combination used for product generation of BDS-2 could be switched to B1I+B3I combination as well. However, this might cause discontinuity in BDS-2 products as different hardware delays specific to signals are contained in them. In this study, combined processing of BDS-2 and BDS-3 satellites to generate consistent precise orbit and clock products is researched. To elaborate the impact of observation biases between BDS-2 and BDS-3, different combined Precise Orbit Determination (POD) processing schemes are examined. It shows that receiver biases between BDS-2 and BDS-3 should be considered in combined POD which is clear from the post-fit residuals of observations, especially from that of BDS-3 code observations. After estimating those biases between B1I+B2I of BDS-2 and B1I+B3I of BDS-3, Root-Mean-Square (RMS) of BDS-3 code observations decreases from 5.07 to 1.23 m. The results show that, biases of B1I+B3I between BDS-2 and BDS-3 are relatively small, less than 4 m for most receivers and around 1.2 m on average. But their estimates are stable with standard deviations (STDs) of 0.13 ~ 0.34 m depending on receiver types. Influences of these biases on the POD results are limited. However, biases between B1I+B2I of BDS-2 and B1I+B3I of BDS-3 are more significant, from -10 to 30 m for different receivers. Except for Septentrio receivers, quantities of those biases are basically related to the receiver types. Averages of biases from Trimble, JAVAD and Leica receivers are 18.5, 5.0 and 10.0 m, respectively. Those biases are also estimated with very small STDs, which ranges from 0.13 to 0.28 m. It is demonstrated that those receiver biases should be properly handle in combined POD processing of BDS-2 and BDS-3 satellites. As B1I+B2I is more appropriate for BDS-2, using different observation combinations for BDS-2 and BDS-3 in combined POD processing is more preferred over the scheme in which B1I+B3I is used for both BDS-2 and BDS-3.</p>

scholarly journals Orbit determination and time synchronization for BDS-3 satellites with raw inter-satellite link ranging observations

2020 ◽  
Vol 1 (1) ◽  
Author(s):  
Rengui Ruan ◽  
Xiaolin Jia ◽  
Laiping Feng ◽  
Jun Zhu ◽  
Zongbo Huyan ◽  
...  
Keyword(s):  
Orbit Determination ◽  
Time Synchronization ◽  
Satellite Link

scholarly journals General relativistic effects acting on the orbits of Galileo satellites

2021 ◽  
Vol 133 (4) ◽  
Author(s):  
K. Sośnica ◽  
G. Bury ◽  
R. Zajdel ◽  
K. Kazmierski ◽  
J. Ventura-Traveset ◽  
...  
Keyword(s):  
General Relativity ◽  
Orbit Determination ◽  
Final Height ◽  
Precise Orbit Determination ◽  
Satellite System ◽  
De Sitter ◽  
Circular Orbits ◽  
Precise Orbit ◽  
Artificial Earth Satellites ◽  
Artificial Earth

AbstractThe 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.

scholarly journals Measurement of the Sea Surface Height with Airborne GNSS Reflectometry and Delay Bias Calibration

2021 ◽  
Vol 13 (15) ◽  
pp. 3014
Author(s):  
Feng Wang ◽  
Dongkai Yang ◽  
Guodong Zhang ◽  
Jin Xing ◽  
Bo Zhang ◽  
...  
Keyword(s):  
Wind Speed ◽  
Arrival Time ◽  
Elevation Angle ◽  
Satellite System ◽  
Sea Surface Height ◽  
Antenna Pattern ◽  
Sea Surface ◽  
Observation Method ◽  
Global Navigation Satellite ◽  
The Impact

Sea surface height can be measured with the delay between reflected and direct global navigation satellite system (GNSS) signals. The arrival time of a feature point, such as the waveform peak, the peak of the derivative waveform, and the fraction of the peak waveform is not the true arrival time of the specular signal; there is a bias between them. This paper aims to analyze and calibrate the bias to improve the accuracy of sea surface height measured by using the reflected signals of GPS CA, Galileo E1b and BeiDou B1I. First, the influencing factors of the delay bias, including the elevation angle, receiver height, wind speed, pseudorandom noise (PRN) code of GPS CA, Galileo E1b and BeiDou B1I, and the down-looking antenna pattern are explored based on the Z-V model. The results show that (1) with increasing elevation angle, receiver height, and wind speed, the delay bias tends to decrease; (2) the impact of the PRN code is uncoupled from the elevation angle, receiver height, and wind speed, so the delay biases of Galileo E1b and BeiDou B1I can be derived from that of GPS CA by multiplication by the constants 0.32 and 0.54, respectively; and (3) the influence of the down-looking antenna pattern on the delay bias is lower than 1 m, which is less than that of other factors; hence, the effect of the down-looking antenna pattern is ignored in this paper. Second, an analytical model and a neural network are proposed based on the assumption that the influence of all factors on the delay bias are uncoupled and coupled, respectively, to calibrate the delay bias. The results of the simulation and experiment show that compared to the meter-level bias before the calibration, the calibrated bias decreases the decimeter level. Based on the fact that the specular points of several satellites are visible to the down-looking antenna, the multi-observation method is proposed to calibrate the bias for the case of unknown wind speed, and the same calibration results can be obtained when the proper combination of satellites is selected.

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