S4

Over the last decades, Global Navigation Satellite Systems (GNSS) have become one of the standards for positioning applications with highest precision. However, uncertainties in the modeling of f.i. solar radiation pressure (SRP) are still an important limiting effect in orbit modeling and hinder for example the accurate and reliable estimation of the origin of GNSS-based terrestrial reference frames (TRF). Therefore, current developments strive for observation types that can improve the absolute accuracy of the orbit as well as of the derived products. For example, the next generation GNSS, proposed by the German Aerospace Center (DLR) under the name "Kepler" as a concept for a future satellite constellation, involves the use of optical inter-satellite links (OISL), which have demonstrated to hold great potential for improving accuracies and decorrelating solved-for parameters. Building on this concept, we investigate the potential of other possible observation types for a future GNSS constellation using highly realistic simulations. To this end, we use not only OISL, but also accelerometer and attitude data, as well as the synchronization of the satellite clocks via the OISLs between the satellites. We evaluate the potential of these techniques each by itself and in combination in terms of orbital accuracy and formal errors as well as correlations of the solved-for parameters.

How to cite: Schreiner, P., König, R., Glaser, S., Raut, S., Neumayer, K. H., and Schuh, H.: On the potential of future observation types for the next generation GNSS, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-23, https://doi.org/10.5194/iag-comm4-2022-23, 2022.

Ambiguity resolution enabled precise point positioning (PPP-RTK) can provide fast, potentially even instantaneous, centimeter-level positioning results, given that the phase ambiguities are correctly resolved. A main problem for fast and reliable ambiguity resolution are the ionospheric delays in the user’s global navigation satellite systems (GNSS) observations. Without external ionospheric corrections, a time-to-first-fix the ambiguities of around 30 min is often reported for GPS-only solutions. Faster solutions are possible when ionospheric corrections are provided, but these have to be at the level of at most a few centimeters for a clear gain in terms of the convergence time. Such a precision is currently not possible with global ionospheric models but requires corrections from nearby reference stations, which limits the field of applications.

In this contribution we investigate the capabilities of centimeter-level PPP-RTK without any a-priori ionospheric information. The key aspects are 1) the MSE-optimal best integer-equivariant estimator, which does not ‘fix’ the ambiguities to integers but rather weights different candidates, 2) a multi-GNSS solution using GPS, Galileo, BDS, and QZSS, and 3) a proper weighting of the satellite clock and bias corrections in order to obtain realistic observation models. Simulations are used to show that in an area with good visibility of BDS and QZSS, one can expect centimeter-level results with on average just slightly more than two observation epochs already with corrections from only a single reference station. We confirm this result with real GNSS data and show that centimeter-level horizontal positioning errors are reached within one and two epochs in 87.6% and 99.7% of the cases during an exemplary day, thereby demonstrating that almost-instantaneous PPP-RTK without atmospheric corrections is indeed possible with the current constellations.

How to cite: Brack, A., Männel, B., and Schuh, H.: Almost-instantaneous PPP-RTK without atmospheric corrections, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-28, https://doi.org/10.5194/iag-comm4-2022-28, 2022.

Affected by various frequencies, pseudo-random-noise (PRN) codes, and tracking techniques, the pseudo-range observations from different channels of the Global Navigation Satellite Systems (GNSS) contain systematic biases. The International GNSS Service (IGS) releases the latest Differential Signal Biases (DSB) products, which include more signals and multi-GNSS constellations than the Differential Code Biases (DCB) products. Currently, DSB and DCB products are used in parallel, even though the signal classification and the bias values are rather different. We investigate the performance of DCB and DSB products in precise GNSS data processing, including the satellite orbits and clock determination, Uncalibrated Phase Delay (UPD) derivation, and precise point positioning with ambiguity resolution (PPP-AR). We demonstrate: (1) using DCB or DSB in the same signal setting causes a systematic difference up to 0.73 ns, 0.47 cycle, and 0.46 cycle in the satellite clocks, wide-lane (WL), and narrow-lane (NL) UPDs estimates, respectively, although their impact on satellite orbits is only around 3 mm in general, (2) using the same bias product in different signal settings can bring more significant differences up to 5.3 mm, 1.7 ns, 0.47 cycle, and 0.47 cycle for orbit, clock, WL, and NL UPDs, respectively, and (3) using DSB products improves the convergence time of PPP-AR solutions compared with DCB solutions and reduces the systematic biases of the pseudo-range observation residuals.

How to cite: Cui, B., Wang, J., Ge, M., and Schuh, H.: Impact of the differential signal bias of pseudo-range on precise GNSS data processing, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-31, https://doi.org/10.5194/iag-comm4-2022-31, 2022.

Abstract Global Navigation Satellite Systems (GNSS), which are fundamental to Positioning, Navigation, and Timing (PNT), play also a critical role for the determination of ITRF and the monitoring of global geodetic parameters, such as geocenter and Earth Rotation Parameters (ERP), thanks to the globally distributed network. The Low Earth Orbit (LEO) satellites with onboard GNSS receivers as moving stations can significantly improve the observation geometry of a ground tracking network, and thus enhance the solutions. Previous studies have demonstrated that integrated processing of ground GNSS stations and LEO satellites can improve the GNSS satellite orbits and the determination of the geocenter, especially for spare networks. In this study, we further investigate the contribution of having five LEO satellites (two GRACE-FO satellites and three SWARM satellites) to a rather robust ground GNSS network, i.e., 120 stations. We focus not only on the GNSS satellite orbits, and the geocenter and ERP, but also on some other parameters like ambiguities. We further investigate the impact of the LEO orbit modelling and the weighting strategies of the LEO observations in order to properly consider the fact that onboard GNSS observations can be modelled more precisely than that from ground stations due to the differences in station environment and atmosphere.

How to cite: Nie, L., Wang, J., Ge, M., and Schuh, H.: Integrated processing of GNSS observations of LEO satellites and ground tracking network, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-45, https://doi.org/10.5194/iag-comm4-2022-45, 2022.