Impact of Attitude Model, Phase Wind-Up and Phase Center Variation on Precise Orbit and Clock Offset Determination of GRACE-FO and CentiSpace-1

Currently, low Earth orbit (LEO) satellites are attracting great attention in the navigation enhancement field because of their stronger navigation signal and faster elevation variation than medium Earth orbit (MEO) satellites. To meet the need for real-time and precise positioning, navigation and timing (PNT) services, the first and most difficult task is correcting errors in the process of precise LEO orbit and clock offset determination as much as possible. Launched in 29 September 2018, the CentiSpace-1 (CS01) satellite is the first experimental satellite of LEO-based navigation enhancement system constellations developed by Beijing Future Navigation Technology Co. Ltd. To analyze the impact of the attitude model, carrier phase wind-up (PWU) and phase center variation (PCV) on precise LEO orbit and clock offset in an LEO-based navigation system that needs extremely high precision, we not only select the CS01 satellite as a testing spacecraft, but also the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO). First, the dual-frequency global positioning system (GPS) data are collected and the data quality is assessed by analyzing the performance of tracking GPS satellites, multipath errors and signal to noise ratio (SNR) variation. The analysis results show that the data quality of GRACE-FO is slightly better than CS01. With residual analysis and overlapping comparison, a further orbit quality improvement is possible when we further correct the errors of the attitude model, PWU and PCV in this paper. The final three-dimensional (3D) root mean square (RMS) of the overlapping orbit for GRACE-FO and CS01 is 2.08 cm and 1.72 cm, respectively. Meanwhile, errors of the attitude model, PWU and PCV can be absorbed partly in the clock offset and these errors can generate one nonnegligible effect, which can reach 0.02~0.05 ns. The experiment results indicate that processing the errors of the attitude model, PWU and PCV carefully can improve the consistency of precise LEO orbit and clock offset and raise the performance of an LEO-based navigation enhancement system.

Displacement monitoring using multi-technique antenna calibrations in processing GNSS data from multi-frequency low-cost receivers

<p>In the framework of the EU founded GIMS-Project (Geodetic Integrated Monitoring System), a low-cost solution for detecting and measuring ground movements have been developed (https://www.gims-project.eu/). In particular, our focus was the improvement of processing techniques of multi-constellation GNSS data from low-cost devices, culminated in the development of improved algorithms implemented now in the goGPS open-source software (https://gogps-project.github.io). The goGPS engine has been used to analyse the motion of two different landslides in Slovenia. It is currently successfully employed to monitor, in near real-time, numerous landslides and structures.</p><p>During the last year, we developed an advanced technique to improve the on-site calibration of the low-cost antennas that allows a drastic improvement of the stability of sub-daily coordinates. This approach is based on iterative steps designed to filter the multipath affecting phase measurements and the phase center variation of the antennas.</p><p>A first solution using more than 10 days of data is computed with the goal of retrieving phase residuals. Then these residuals are filtered using a combination of Zernike polynomials interpolation and multi-resolution gridding in order to obtain hemi-spherical maps able to correct the observations of longer periods. A final solution is finally obtained applying the stored corrections.</p><p>Eventually the procedure can be iterated to reach convergence and maps can be updated after environmental changes.</p><p>By using these maps in a multi-constellation environment, the monitoring of displacements is feasible with low-cost receivers (single and multi-frequencies) even in challenging conditions. The accuracy improvements are higher than 50% with respect to a scenario without maps.</p><p>In this work, the processing procedure of low-cost receivers is presented with examples of successful monitoring experiences<strong>.</strong></p>

Altitude dependent empirical modeling of the topside ionosphere and plasmasphere using GPS- TEC from Swarm, GRACE-FO and the Sentinel satellites.

<p>Slant Total Electron Content (sTEC) measurements can be obtained by dual-frequency GPS<br>onboard Low Earth Orbiting (LEO) satellites. Within the last few years, a fleet of LEO Satellites at<br>altitudes ranging from 450 km (Swarm A/C) to 815 km (Sentinel 3) became operational. With<br>Swarm B, the recently launched GRACE-FO, and the Sentinel 1 and 2 satellites orbiting at<br>intermediate altitudes, we gain insight into the altitude dependent profile of the topside ionosphere<br>and plasmasphere.<br>We make use of this constellation to estimate a global three dimensional model of the electron<br>density distribution and will also carefully asses the impact of different profile functions, geometry-<br>free phase center variation maps and the P1-P2 receiver biases. Since the absolute value of the P1-<br>P2 biases are generally unknown, we focus on a consistent estimation for the whole LEO<br>constellation.<br>We will present first results for selected months in 2019 and investigate the day to day variability of<br>the topside ionosphere and plasmasphere. We also intend to make use of COSMIC-2 data to<br>improve local time coverage in equatorial regions.</p>

Research on Attitude Models and Antenna Phase Center Correction for Jason-3 Satellite Orbit Determination

We focused on the researches of two models used for Jason-3 precise orbit determination (POD)—Jason-3 attitude modes and receiver phase center variation (PCV) model. A combined attitude mode for the Jason-3 satellite is designed based on experimental analysis used in some special cases, such as in the absence of quaternions or when inconvenient to use. We researched the linking of satellite attitude with antenna phase center. Specially, to verify the validity of the combined attitude, we analyzed the effects of different attitude modes on receiver phase center offset (PCO) estimation, PCO correction and POD. Meanwhile, the difference analysis of PCO correction based on attitude modes also contains the combined attitude modeling processes. The POD results showed that the orbital accuracies with the combined attitude are slightly more stable than those with attitude event file. By introducing receiver PCVs into POD, the mean residuals root-mean-square (RMS) is reduced by 1.9 mm and orbital 3D-RMS position difference is improved by 5.7 mm. The eight schemes were designed to integratedly verify the effectiveness of different attitude modes and receiver PCVs model. The results conclude that the accuracy using the combined attitude is higher than that of event file, which also prove the feasibility of the combined attitude in integrated POD and it can be as a revision of attitude event file. Using all mentioned attitude modes, the orbital accuracy by introducing PCVs can be improved by the millimeter level. The integrated effects of attitude modes and receiver PCVs on POD are almost consistent with the effects of a single variable. The optimal results of Jason-3 POD indicate that orbital mean radial RMS is close to 1 cm, and the 3D-RMS position difference is within 3 cm.