A New Approach to Improve Satellite Clock Estimates, Removing the Inter-day Jumps

Many sophisticated Global Navigation Satellite System (GNSS ) applications require high-precision satellite orbit and clock products. The GNSS orbits and clocks are usually derived from the analysis of tracking data collected by a globally distributed GNSS receiver network. The estimation process adjusts parameters for the satellite orbits, transmitter and receiver clocks, station positions, tropospheric delays, Earth orientation, intersystem and interfrequency biases, and carrier-phase ambiguities. The estimation requires detailed modeling of geophysical processes, atmospheric and relativistic effects, receiver tracking modes, antenna phase centers, spacecraft properties, and attitude control algorithms. This chapter describes precise orbit and clock determination of the GNSS constellations as performed by the analysis centers of the International GNSS Service, including models, estimation strategies, products, and the combination of orbit and clock solutions.

Assessment of L5 frequency on real-time GPS orbit and clock estimation

Robust ambiguity datum for real-time GNSS satellite integer clock estimation

Ground-based GNSS ZTD/IWV estimation system for numerical weather prediction in challenging weather conditions

Acousto-optic devices using lead molybdate and related compounds: Bell Telephone Labs Inc US patent 3617931 (1971) filed 5 May 1969

The International GNSS Service (IGS) is an international activity involving more than 200 participating organisations in over 80 countries with a track record of one and a half decades of successful operations. The IGS is a service of the International Association of Geodesy (IAG). It primarily supports scientific research based on highly precise and accurate Earth observations using the technologies of Global Navigation Satellite Systems (GNSS), primarily the US Global Positioning System (GPS). The mission of the IGS is “to provide the highest-quality GNSS data and products in support of the terrestrial reference frame, Earth rotation, Earth observation and research, positioning, navigation and timing and other applications that benefit society”. The IGS will continue to support the IAG’s initiative to coordinate cross-technique global geodesy for the next decade, via the development of the Global Geodetic Observing System (GGOS), which focuses on the needs of global geodesy at the mm-level. IGS activities are fundamental to scientific disciplines related to climate, weather, sea level change, and space weather. The IGS also supports many other applications, including precise navigation, machine automation, and surveying and mapping. This article discusses the IGS Strategic Plan and future directions of the globally-coordinated ~400 station IGS network, tracking data and information products, and outlines the scope of a few of its numerous working groups and pilot projects as the world anticipates a truly multi-system GNSS in the coming decade.

The impact of orbital errors on the estimation of satellite clock errors and PPP

Satellite orbit error and clock bias are the keys to precise point positioning (PPP). The traditional PPP algorithm requires precise satellite products based on worldwide permanent reference stations. Such an algorithm requires considerable work and hardly achieves real-time performance. However, real-time positioning service will be the dominant mode in the future. IGS is providing such an operational service (RTS) and there are also commercial systems like Trimble RTX in operation. On the basis of the regional Continuous Operational Reference System (CORS), a real-time PPP algorithm is proposed to apply the coupling estimation of clock bias and orbit error. The projection of orbit error onto the satellite-receiver range has the same effects on positioning accuracy with clock bias. Therefore, in satellite clock estimation, part of the orbit error can be absorbed by the clock bias and the effects of residual orbit error on positioning accuracy can be weakened by the evenly distributed satellite geometry. In consideration of the simple structure of pseudorange equations and the high precision of carrier-phase equations, the clock bias estimation method coupled with orbit error is also improved. Rovers obtain PPP results by receiving broadcast ephemeris and real-time satellite clock bias coupled with orbit error. By applying the proposed algorithm, the precise orbit products provided by GNSS analysis centers are rendered no longer necessary. On the basis of previous theoretical analysis, a real-time PPP system was developed. Some experiments were then designed to verify this algorithm. Experimental results show that the newly proposed approach performs better than the traditional PPP based on International GNSS Service (IGS) real-time products. The positioning accuracies of the rovers inside and outside the network are improved by 38.8% and 36.1%, respectively. The PPP convergence speeds are improved by up to 61.4% and 65.9%. The new approach can change the traditional PPP mode because of its advantages of independence, high positioning precision, and real-time performance. It could be an alternative solution for regional positioning service before global PPP service comes into operation.

Global Navigation Satellite System (GNSS) precise orbit, clock and observable-specific bias (OSB) products are the prerequisite for precise point positioning with ambiguity solution (PPP-AR). Under the condition of sparse or regional ground network, integrating low Earth orbit (LEO) onboard observations is a potential approach to supplementing GNSS global tracking measurements. In this contribution, a method of integrating ground and LEO onboard observations to generate GNSS precise satellite clock and phase OSB based on square root information filter (SRIF) estimation is proposed. Particularly, a method of phase OSB estimation with a 1.5 h sampling rate is established under regional ground stations and 8 LEO satellites. The results show that integrating 30 global ground stations and 8 LEO satellites, the standard deviation (STD) and convergence time of satellite clocks reach to 0.093 ns and 4.93 h, which reduced by 41.5% and 35.4% compared to ground-only solutions, respectively. Phase OSB accuracy improves from 0.092 to 0.078 cycles, and the product availability increases from 42.2% to 95.1%. The kinematic PPP-AR accuracy of integrated solutions improved by 26%, 17% and 18% on the E, N and U component, and convergence time accelerated by 17%. In addition, phase OSB products generated by 20 European regional ground stations integrating 8 LEO satellites achieve an STD of 0.086 cycles, with the product availability of more than 98%, and kinematic PPP-AR accuracy is close to the solutions with phase OSB generated by 70 ground stations.

The study of climate change is an important field of research. Monitoring of atmospheric variability especially the tropospheric precipitable water vapor (PWV) is a powerful way to investigate climate change. Global navigation satellite systems (GNSS) provide a good tool for studying atmospheric parameters as GNSS signals along its path from the satellites to the ground based receivers suffer a significant delay due to the refractivity of earth’s atmosphere. GNSS signals are not only delayed but also refracted in the neutral atmosphere. The zenith wet delays (ZWD) caused by the troposphere can be estimated during the geodetic processing of GNSS signals. Since the ZWD is tightly correlated to the PWV, GNSS observations can be used to study the changes of PWV. In this study GNSS data from the Egyptian permanent GNSS network (EPGN), International GNSS Service (IGS), EUREF Permanent Network (EPN) and Scripps Orbit and Permanent Array Center (SOPAC), for the years 2013 and 2014, were used. These GNSS stations provided a good geometrical coverage over the respective region. All GNSS data were processed using Bernese V 5.2 software. The needed product from GNSS data processing is the tropospheric zenith total delay (ZTD) over each GNSS site. The ZTD is divided based on physical parameters into zenith hydrostatic delay (ZHD) and zenith wet delay (ZWD). The ZWD is the basic observable used to calculate PWV. The values of the PWV were calculated and its variation over the study area was investigated. The quality of calculated PWV values from GNSS data evaluated against traditional Radiosonde (RS) measurements. The results of GNSS PWV showed good agreement with RS PWV when the PWV values were low, but this agreement became worse at high PWV values as the differences between the two techniques increased. The correlation coefficient between RS PWV and GNSS PWV varied from 0.31 to 0.84. Standard deviation of the differences between RS PWV and GNSS PWV ranged from 2.369 to 5.973 mm. The PWV estimated from GNSS observations had annual cycle, and its cycle in 2013 was different from that in 2014. The PWV differences between the 2 years clarified that the water vapor content over the east Mediterranean in most of 2014 days was higher than that in 2013. Furthermore, PWV variations were noted on both temporal and spatial scales. The highest temporal variation value was 25.41 mm whereas the maximum value of the spatial variation was 19.67 mm. The present study illustrated the importance of using geodetic networks to provide atmospheric information.

<p>Numerous active low Earth orbiters (LEOs) and Global Navigation Satellite System (GNSS) satellites, including the Galileo constellation, are equipped with laser retroreflectors used for Satellite Laser Ranging (SLR). Moreover, most of LEOs are equipped with GNSS receivers for precise orbit determination. SLR measurements to LEOs, GNSS, and geodetic satellites vary in terms of the number of registered normal points (NPs) or registered satellite passes. In 2016-2018, SLR measurements to LEOs constituted 81% of all NPs and 59% of all registered satellite passes, whereas 10% of NPs and 30% of satellite passes, respectively, were assigned to GNSS. The remaining SLR measurements were completed by geodetic satellites, including LAGEOS-1/2, and LARES-1.</p><p>In this study, we show that the SLR observations to Galileo, passive geodetic and active LEO satellites together with precise GNSS-based orbits of LEOs and Galileo, can be used for the determination of global geodetic parameters, such as geocenter coordinates (GCC) and Earth rotation parameters (ERPs), i.e. pole coordinates, and length-of-day parameter.</p><p>GCC are typically determined using SLR observations to passive geodetic satellites, such as LAGEOS-1/2. Also, the SLR observations to LAGEOS-1/2 together with GNSS and Very Long Baseline Interferometry data are used for the determination of ERPs. Here, we use SLR observations to Galileo, LAGEOS-1/2, LARES-1, Sentinel-3A, SWARM-A/B/C, TerraSAR-X, Jason-2, GRACE-A/B satellites to investigate whether they can be applied for the reference frame realization and for deriving high-quality global geodetic parameters.</p><p>We present various types of solutions to investigate the best solution set-up. The studied solutions differ in terms of solution lengths, the combination of different sets of satellites and the relative weights for the variance scaling factors of technique and satellite-specific normal equations. We compare our results with the standard LAGEOS-based solutions, the combined EOP-14-C04 products and show the consistency of the results.</p>

GNSS ile konum belirleme yöntemlerinden mutlak konum belirleme tekniği olan Hassas Nokta Konumlama (PPP) yöntemi tek bir Küresel Konum Belirleme Sistemi (GNSS) alıcısıyla konum bilgisi sağlamaktadır. Bu durum maliyet ve zaman tasarrufu, yüksek konum doğruluğu, kolay ve pratik çözümler sunması son yıllarda geleneksel-PPP ve Hassas Nokta Konumlama-Belirsizlik Çözümü (PPP-AR) yöntemlerine olan ilgiyi arttırmış ve bu sayede çok sayıda yazılım üretilmiştir. Bu çalışmada son yıllarda geliştirilmiş ve kullanıcının hizmetine sunulmuş olan Net_Diff ve PRIDE yazılımları kullanılarak PPP-AR yönteminin konum belirleme performansı incelenmiştir. Bu amaçla ISTA ve MERS Uluslararası GNSS Hizmeti (IGS) istasyonlarının yirmi günlük (29.10.2022-17.11.2022) gözlem verileri dikkate alınmıştır. Değerlendirmelerde GPS, GLONASS, GALILEO ve BeiDou uydu sistemleri kullanılmıştır. Yazılımlardan elde edilen sonuçlar istasyonların referans (IGS günlük çözüm) koordinat değerleriyle karşılaştırılmıştır. Çalışma sonucunda yatay ve düşey yönlerde elde edilen karesel ortalama hata değerlerinden PRIDE yazılımının Net_Diff yazılımına göre daha yüksek nokta konumlandırma doğruluğuna ve daha iyi performansa sahip olduğu tespit edilmiştir.

Long-term performance detection and evaluation of GLONASS onboard satellite clocks

GipsyX/RTGx is the Jet Propulsion Laboratory’s (JPL) next generation software package for positioning, navigation, timing, and Earth science using measurements from three geodetic techniques: Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS); with Very Long Baseline Interferometry (VLBI) under development. The software facilitates combined estimation of geodetic and geophysical parameters using a Kalman filter approach on real or simulated data in both post-processing and in real-time. The estimated parameters include station coordinates and velocities, satellite orbits and clocks, Earth orientation, ionospheric and tropospheric delays. The software is also capable of full realization of a dynamic terrestrial reference through analysis and combination of time series of ground station coordinates.Applying lessons learned from its predecessors, GIPSY-OASIS and Real Time GIPSY (RTG), GipsyX/RTGx was re-designed from the ground up to offer improved precision, accuracy, usability, and operational flexibility. We present some key aspects of its new architecture, and describe some of its major applications, including Real-time orbit determination and ephemeris predictions in the U.S. Air Force Next Generation GPS Operational Control Segment (OCX), as well as in JPL’s Global Differential GPS (GDGPS) System, supporting User Range Error (URE) of <5 cm RMS; precision post-processing GNSS orbit determination, including JPL’s contributions to the International GNSS Service (IGS) with URE in the 2 cm RMS range; Precise point positioning (PPP) with ambiguity resolution, both statically and kinematically, for geodetic applications with 2 mm horizontal, and 6.5 mm vertical repeatability for static positioning; Operational orbit and clock determination for Low Earth Orbiting (LEO) satellites, such as NASA’s Gravity Recovery and Climate Experiment (GRACE) mission with GRACE relative clock alignment at the 20 ps level; calibration of radio occultation data from LEO satellites for weather forecasting and climate studies; Satellite Laser Ranging (SLR) to GNSS and LEO satellites, DORIS-based and multi-technique orbit determination for LEO; production of terrestrial reference frames and Earth rotation parameters in support of JPL’s contribution to the International Terrestrial Reference Frame (ITRF).

&amp;lt;p&amp;gt;&amp;lt;span&amp;gt;&amp;lt;span&amp;gt;Global Navigation Satellite Systems (GNSS) play a critical role for providing real-time positioning and navigation services, and the precise satellite orbit and clock products are essential for the high-precision GNSS applications. The International GNSS Service (IGS) and its Analysis Centers (ACs) have been working on the study on precise GNSS data processing and provision of the precise products. The German Research Center for Geosciences (GFZ), as one of the ACs, also provides the multi-GNSS rapid products: the GBM product. We introduce the GBM data processing strategy, analyze the precision of GBM multi-GNSS orbits from 2015 to 2021, and present the impact of applying the undifferenced ambiguity resolution on satellite orbits. The GPS orbits of GBM products agree with the IGS final orbits at the level of 11-13 mm in the three directions, and the GPS orbit 6-hour prediction precision is around 6 cm. The 6-hour prediction precision of GLONASS is around 12 cm, slightly worse than that of GALILEO, which hold an average value of 10&amp;amp;#160;cm in the same period but shows a significant improvement to around 5&amp;amp;#160;cm after end of 2016. The prediction precision of BDS MEO satellites are around 10 cm, and that of the BDS GEO satellites and QZSS satellites are at the level of 1 to 3 meter. The Satellite Laser Ranging (SLR) residuals show that the orbit precision of GALILEO, GLONASS, and BD3-MEO is 23 mm, 41 mm, and 47 mm, respectively. Moreover, comparing the double-differenced ambiguity resolution, adopting the undifferenced ambiguity resolution improves the 6-hour orbit prediction precision by 9-15%&amp;lt;/span&amp;gt;&amp;lt;/span&amp;gt;&amp;amp;#65292;&amp;lt;span&amp;gt;&amp;lt;span&amp;gt;15-18%&amp;lt;/span&amp;gt;&amp;lt;/span&amp;gt;&amp;amp;#65292;&amp;lt;span&amp;gt;&amp;lt;span&amp;gt;11-13%&amp;lt;/span&amp;gt;&amp;lt;/span&amp;gt;&amp;amp;#65292;&amp;lt;span&amp;gt;&amp;lt;span&amp;gt;6-17%&amp;lt;/span&amp;gt;&amp;lt;/span&amp;gt;&amp;amp;#21644;&amp;lt;span&amp;gt;&amp;lt;span&amp;gt;14-25% for the GPS, GLONASS, GALILEO, BDS-2 and BDS-3 MEO satellites, respectively.&amp;lt;/span&amp;gt;&amp;lt;/span&amp;gt;&amp;lt;/p&amp;gt;