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We examine the impact of using seasonal and long-period time-variable gravity field (TVG) models on GPS orbit determination, through simulations from 1994 to 2012. The models of time-variable gravity that we test include the GRGS release RL02 GRACE-derived 10-day gravity field models up to degree and order 20 (grgs20x20), a 4 × 4 series of weekly c...
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Earth rotation parameters with a daily and sub-daily resolution were determined using SLR and GPS data. Although both techniques can deiiver highly accurate results the GPS data have clear advantages because of the better data distribution.
Citations
... [23,36]). Other studies have investigated the sensitivity to different force-models, by evaluating absolute values of root-mean-square fits of observations on orbital arcs (e.g., [10,25,30,38,46] the fact that SLR (satellite laser ranging), DORIS (Doppler Orbitography and Radio positioning Integrated by Satellite), PRARE (Precise Range And Range-rate Equipment) and altimeter measurements have proven to be of practical benefit in these previous studies, little attention has been paid to the instantaneous differences to accelerometer measurements. Since space accelerometers can measure the non-gravitational forces acting on a satellite's surface, we present a new approach to evaluate the error committed in force modeling. ...
In satellite geodesy, the motion of artificial satellites has a crucial importance and the satellite's position is the core point in the geodetic network. The simplicity of the broadcast ephemeris model directly determines the rate and efficiency of real-time navigation and positioning. GNSS broadcast message is transmitted in ECEF system either (GLONASS) or are Keplerian-like orbital elements (GPS, BeiDou, Galileo). The main aim of this paper is to describe the magnitude of Keplerian elements and correction coefficients of the navigational satellite changes based on a GPS satellite. The author examines six Keplerian elements and six correction coefficients for three different intervals: 2-hour, 1-day and 5-day intervals for periods of 9 days, a year and 10 years respectively. The analysis shows the varying behaviour of the time-series of the analysed components on the basis of the on-board GPS message, both random-like and repetitive (seasonal) trajectory time series.
The Analysis Centers (ACs) of the International GNSS Service (IGS) have reprocessed a large global network of GPS tracking data from 1994.0 until 2014.0 or later. Each AC product time series was extended uniformly till early 2015 using their weekly operational IGS contributions so that the complete combined product set covers GPS weeks 730 through 1831. Three ACs also included GLONASS data from as early as 2002 but that was insufficient to permit combined GLONASS products. The reprocessed terrestrial frame combination procedures and results have been reported already, and those were incorporated into the ITRF2014 multi-technique global frame released in 2016. This paper describes the orbit and clock submissions and their multi-AC combinations and assessments. These were released to users in early 2017 in time for the adoption of IGS14 for generating the operational IGS products. While the reprocessing goal was to enable homogeneous modeling, consistent with the current operational procedures, to be applied retrospectively to the full history of observation data in order to achieve a more suitable reference for geophysical studies, that objective has only been partially achieved. Ongoing AC analysis changes and a lack of full participation limit the consistency and precision of the finished IG2 products. Quantitative internal measures indicate that the reprocessed orbits are somewhat less precise than current operational orbits or even the later orbits from the first IGS reprocessing campaign. That is even more apparent for the clocks where a lack of robust AC participation means that it was only possible to form combined 5-min clocks but not the 30-s satellite clocks published operationally. Therefore, retrospective precise point positioning solutions by users are not recommended using the orbits and clocks. Nevertheless, the orbits do support long-term stable user solutions when used with network processing with either double differencing or explicit clock estimation. Among the main benefits of the reprocessing effort is a more consistent long product set to analyze for sources of systematic error and accuracy. Work to do that is underway but the reprocessing experience already points to a number of ways future IGS performance and reprocessing campaigns can be improved.
The thermosphere is located above the mesosphere and below the exosphere, and its highly ionized state increases the complexity of its geophysical processes due to the magnetosphere-ionosphere-thermosphere (MIT) coupling. Therefore, accurate measurement of neutral density variations is essential for atmospheric research, Precise Orbit Determination (POD), and radio wave propagation. However, it is difficult to quantify and interpret the upper atmospheric variations due to large uncertainty in the models and lack of measurements from the limited traditional technologies of observation. Nowadays, thermospheric neutral densities estimated from accelerometers and GNSS onboard LEO satellites provide a unique opportunity to study the upper atmosphere with new observational data. In this thesis, thermospheric neutral density variations are inferred and investigated from accelerometers and precise orbits of GRACE (Gravity Recovery and Climate Experiment) for the period 2003-2016. Main results and findings are summarized as follows:
1) A new methodology to derive non-gravitational accelerations is presented through the first derivatives of precise-orbit velocities, which can calibrate the GRACE accelerometers. The conservative-force anomalies derived from analytical time-varying gravity models, accurate orbit solutions, and accelerometer measurements along GRACE orbits are analyzed in space and time through a new technique based in the Principal Component Analysis (PCA). The results reveal intriguing structures at the sub-daily frequency probably related to atmospheric tides.
2) Thermospheric neutral density variations are estimated from accelerometer measurements and GRACE precise orbits for the period 2003-2016, showing good agreement and a better estimation than the NRLMSISE00 empirical model. Furthermore, thermospheric neutral density variations from GRACE measurements are investigated using the new PCA technique, and the resulting modes have been parameterized in terms of Local Solar Time (LST), day-of-year, and solar P10.7 flux and geomagnetic Am indices. The LST parameterization shows an interesting fluctuation controlling a middle-latitude 4-wave pattern. This model is suitable to represent small scale variations including, e.g., Equatorial Mass Anomaly (EMA) and Midnight Density Maximum (MDM), and can be used to improve the current thermosphere modeling.
3) The residuals from the PCA parameterization are further analyzed in the spectral domain, and additional periodic contributions have been found at the frequencies of the radiational tides (P1, K, T2, and R2). In addition, periodic contributions are found at the periods of 83, 93, 152 and 431 days. The 93-day period could be caused by the satellite´s drift of perigee, and the 83 and 152 day periods might be attributed to solar activity. Variations at the free-core nutation frequency (431 day) suggest a possible core-MIT coupling.
4) As for the long term variations, the average distribution shows a clear alignment with the geomagnetic field, with higher values in the southern hemisphere than in the northern hemisphere. Two asymmetric cells are located in the polar caps: a southern-enhancement, and a northern-attenuation, both located in their eastern polar sides. In addition, each polar-cell has a corresponding attenuation (enhancement) in the western polar side. The latitudinal variation shows higher values of density in the southern hemisphere during all time-span, and strongly controlled by the solar radiation.
5) Thermospheric neutral density variations and behaviors following the March 2013 and March 2015 geomagnetic storms are investigated in relation to space weather and geomagnetic indices, showing good correlation with the Dst index and the merging electric field (Em), for low latitude and high-latitude variations, respectively. In March 2015, density enhancements reached maxima deviations of 500 %, and daily-averaged deviations up to 180 %.
A better understanding of global thermospheric neutral density variations is achieved, which validates the suitability of the technique and the resulting empirical model. The new periodicities and dependences in the residuals promise numerous intriguing questions for future research.
