Susanne Glaser

Real-time retrieval of precipitable water vapor from GPS and BeiDou observations

The rapid development of the Chinese BeiDou Navigation Satellite System (BDS) brings a promising prospect for the real-time retrieval of zenith tropospheric delays (ZTD) and precipitable water vapor (PWV), which is of great benefit for supporting the time-critical meteorological applications such as nowcasting or severe weather event monitoring. In this study, we develop a real-time ZTD/PWV processing method based on Global Positioning System (GPS) and BDS observations. The performance of ZTD and PWV derived from BDS observations using real-time precise point positioning (PPP) technique is carefully investigated. The contribution of combining BDS and GPS for ZTD/PWV retrieving is evaluated as well. GPS and BDS observations of a half-year period for 40 globally distributed stations from the International GNSS Service Multi-GNSS Experiment and BeiDou Experiment Tracking Network are processed. The results show that the real-time BDS-only ZTD series agree well with the GPS-only ZTD series in general: the RMS values are about 11–16 mm (about 2–3 mm in PWV). Furthermore, the real-time ZTD derived from GPS-only, BDS-only, and GPS/BDS combined solutions are compared with those derived from the Very Long Baseline Interferometry. The comparisons show that the BDS can contribute to real-time meteorological applications, slightly less accurately than GPS. More accurate and reliable water vapor estimates, about 1.3–1.8 mm in PWV, can be obtained if the BDS observations are combined with the GPS observations in the real-time PPP data processing. The PWV comparisons with radiosondes further confirm the performance of BDS-derived real-time PWV and the benefit of adding BDS to standard GPS processing.

GGOS-SIM: Simulation of the Reference Frame for the Global Geodetic Observing System

The accuracy and stability requirements for the International Terrestrial Reference Frame (ITRF) postulated by the Global Geodetic Observing System (GGOS) are not met so far. The GGOS–SIM project builds a software tool that by simulating the space geodetic infrastructure allows to assess the impact of technique upgrades, new sites, new satellites, local ties, and space ties on the ITRF accuracy and stability. As also the procedure for the combination of the techniques plays a fundamental role in the generation of an ITRF, we discuss peculiarities of current day approaches and draw conclusions relevant for this project. As the assessment of the accuracy of an ITRF is needed for checking against the GGOS requirements, we compile actual methods and present here a new measure of stability which is exemplarily applied to recent ITRFs.

Validation of Components of Local Ties

Local ties (LTs) at co-located sites are currently used to align different space geodetic techniques for the determination of a global terrestrial reference frame (TRF). However, the currently available LT measurements are typically characterized by an inhomogeneous accuracy, which may cause inconsistencies within the TRF and limit the final TRF accuracy. An alternative strategy is a combination of common parameter types to which the individual geodetic techniques are sensitive. In this study, we combine Global Navigation Satellite Systems (GNSS) and Satellite Laser Ranging (SLR) data without using LTs but by combining the common pole coordinates and by adding proper datum constraints. In addition, we constrain the velocities at co-located sites to be the same for all markers. This allows an independent validation of measured LT components. Our data are based on a homogeneous reprocessing of GPS+GLONASS and SLR to LAGEOS-1 and LAGEOS-2 over 17 years in the time span of 1994–2010. A preliminary analysis including the elimination of outliers and the selection of core datum stations was performed based on the station position time series of the single-technique solutions. Applying our combination approach, the north and height components of the LTs can be directly derived from our combined coordinate solution. The differences of the measured and the estimated LTs remain below 1 cm for 96% in the north component and for 50% in the height component of all co-located sites.

Simulations of VLBI observations of a geodetic satellite providing co-location in space

We performed Monte Carlo simulations of very-long-baseline interferometry (VLBI) observations of Earth-orbiting satellites incorporating co-located space-geodetic instruments in order to study how well the VLBI frame and the spacecraft frame can be tied using such measurements. We simulated observations of spacecraft by VLBI observations, time-of-flight (TOF) measurements using a time-encoded signal in the spacecraft transmission, similar in concept to precise point positioning, and differential VLBI (D-VLBI) observations using angularly nearby quasar calibrators to compare their relative performance. We used the proposed European Geodetic Reference Antenna in Space (E-GRASP) mission as an initial test case for our software. We found that the standard VLBI technique is limited, in part, by the present lack of knowledge of the absolute offset of VLBI time to Coordinated Universal Time at the level of microseconds. TOF measurements are better able to overcome this problem and provide frame ties with uncertainties in translation and scale nearly a factor of three smaller than those yielded from VLBI measurements. If the absolute time offset issue can be resolved by external means, the VLBI results can be significantly improved and can come close to providing 1 mm accuracy in the frame tie parameters. D-VLBI observations with optimum performance assumptions provide roughly a factor of two higher uncertainties for the E-GRASP orbit. We additionally simulated how station and spacecraft position offsets affect the frame tie performance.

Estimating Integrated Water Vapor Trends From VLBI, GPS, and Numerical Weather Models: Sensitivity to Tropospheric Parameterization

In this study, we estimate integrated water vapor (IWV) trends from very long baseline interferometry (VLBI) and global navigation satellite systems (GNSS) data analysis, as well as from numerical weather models (NWMs). We study the impact of modeling and parameterization of the tropospheric delay from VLBI on IWV trends. We address the impact of the meteorological data source utilized to model the hydrostatic delay and the thermal deformation of antennas, as well as the mapping functions employed to project zenith delays to arbitrary directions. To do so, we derive a new mapping function, called Potsdam mapping functions based on NWM data and a new empirical model, GFZ-PT. GFZ-PT differs from previous realizations as it describes diurnal and subdiurnal in addition to long-wavelength variations, it provides harmonic functions of ray tracing-derived gradients, and it features robustly estimated rates. We find that alternating the mapping functions in VLBI data analysis yields no statistically significant differences in the IWV rates, whereas alternating the meteorological data source distorts the trends significantly. Moreover, we explore methods to extract IWV given a NWM. The rigorously estimated IWV rates from the different VLBI setups, GNSS, and ERA-Interim are intercompared, and a good agreement is found. We find a quite good agreement comparing ERA-Interim to VLBI and GNSS, separately, at the level of 75%.

Simulation of VLBI Observations to Determine a Global TRF for GGOS

In this study, we present a global terrestrial reference frame (TRF) from simulated very long baseline interferometry (VLBI) observations. In the time span from 2008 until 2014, 695 standard VLBI rapid turnaround (R1, R4) 24 h-sessions were simulated using a network of 28 globally distributed stations. Within the software VieVS@GFZ, we apply different measurement noise at the observation level and investigate the impact on the TRF and on the Earth rotation parameters. We find that the effect of varying only the noise applied within the simulation is not proportional to the changes in the estimates and their uncertainties. For instance, increasing the noise level from 15 ps to 300 ps increases the uncertainty of the station positions by a factor of 3.5, of station velocities by 5, of polar motion by 3.4, and of UT1-UTC by 1.5. A comparison with the VLBI-TRF derived from real observations within the same time span shows that the solution simulated with a noise level based on the formal errors of real observations is still too optimistic.

Recent Activities of the GGOS Standing Committee on Performance Simulations and Architectural Trade-Offs (PLATO)

The Standing Committee on Performance Simulations and Architectural Trade-Offs (PLATO) was established by the Bureau of Networks and Observations of the Global Geodetic Observing System (GGOS) in order to support – by prior performance analysis – activities to reach the GGOS requirements for the accuracy and stability of the terrestrial reference frame. Based on available data sets and simulated observations for further stations and satellite missions the committee studies the impact of technique-specific improvements, new stations, and additional co-locations in space on reference frame products. Simulation studies carried out so far show the importance of the individual station performance and additional stations for satellite laser ranging, the perspectives for lunar laser ranging assuming additional stations and reflectors, and the significant impact of the new VGOS antennas. Significant progress is achieved in processing VLBI satellite tracking data. New insights in technique-specific error sources were derived based on real data from short baselines. Regarding co-location in space PLATO members confirmed that E-GRASP could fulfill the GGOS requirements with reaching a geocenter and scale accuracy and stability of 1 mm and 0.1 mm/year, respectively.

The Assessment of the Temporal Evolution of Space Geodetic Terrestrial Reference Frames

The assessment of the accuracy and the stability of the global Terrestrial Reference Frames (TRFs) is a matter of great importance for the geodetic community. The classical Helmert transformation plays a crucial role in terms of evaluating the datum related parameters of global TRFs (origin, scale, and orientation, and their associated rates). We discuss a new alternative approach for the assessment of TRFs temporal evolution and we compare it with the Helmert transformation. Our concept relies on the splitting of the velocities into two specified parts. The first one is referred to the reference system effect and the latter one to the deformation, respectively. This separation is done in order to create the necessary mathematical tools for the TRF assessment. The new approach is tested on the single-technique TRFs (VLBI, SLR, GPS and DORIS, respectively) of the DTRF2008. The novelty of the new methodology is its ability to treat individually the systematic errors of each TRF. This feature detects systematic effects that the Helmert transformation cannot. The results reveal that the new approach and the Helmert transformation show almost the same results in terms of rates of the datum parameters with an uncertainty of 0.1–0.3 mm/year for the four space geodetic techniques TRFs. The uncertainty refers to the estimated rate differences standard deviation between the new approach and the Helmert one.