Krzysztof Sośnica

Geocenter Coordinates from GNSS and Combined GNSS-SLR Solutions Using Satellite Co-locations

Satellite Laser Ranging (SLR) data to LAGEOS, ETALON and to Global Navigation Satellite Systems (GNSS) were combined with GNSS microwave data for 5 years. Including SLR data to GNSS satellites and estimating common orbit parameters allows it to connect both space-geodetic techniques using satellite instead of station co-location. We show that only SLR data to the spherical satellites can improve the geocenter estimates, whereas SLR data to the GNSS satellites suffer from the same GNSS orbit modelling deficiencies as in the analysis of microwave data.

Sensitivity of Lageos Orbits to Global Gravity Field Models

Sensitivity of Lageos Orbits to Global Gravity Field Models Precise orbit determination is an essential task when analyzing SLR data. The quality of the satellite orbits strongly depends on the models used for dynamic orbit determination. The global gravity field model used is one of the crucial elements, which has a significant influence on the satellite orbit and its accuracy. We study the impact of different gravity field models on the determination of the LAGEOS-1 and -2 orbits for data of the year 2008. Eleven gravity field models are compared, namely JGM3 and EGM96 based mainly on SLR, terrestrial and altimetry data, AIUB-CHAMP03S based uniquely on GPS-measurements made by CHAMP, AIUB-GRACE03S, ITG-GRACE2010 based on GRACE data, and the combined gravity field models based on different measurement techniques, such as EGM2008, EIGEN-GL04C, EIGEN51C, GOCO02S, GO-CONS-2-DIR-R2, AIUB-SST. The gravity field models are validated using the RMS of the observation residuals of 7-day LAGEOS solutions. The study reveals that GRACE-based models have the smallest RMS values (i.e., about 7.15 mm), despite the fact that no SLR data were used to determine them. The coefficient C20 is not always well estimated in GRACE-only models. There is a significant improvement of the gravity field models based on CHAMP, GRACE and GOCE w.r.t. models of the pre-CHAMP era. The LAGEOS orbits are particularly sensitive to the long wavelength part of the gravity fields. Differences of the estimated orbits due to different gravity field models are noticeable up to degree and order of about 30. The RMS of residuals improves from about 40 mm for degree 8, to about 7 mm for the solutions up to degrees 14 and higher. The quality of the predicted orbits is studied, as well.

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.

Pre-combined GNSS-SLR Solutions: What Could be the Benefit for the ITRF?

In standard combination approaches (e.g., for the International Terrestrial Reference Frame), the space-geodetic techniques are connected by the Earth rotation parameters and by the station coordinates at co-located sites, using the so-called local ties. These local ties are usually derived from terrestrial measurements together with GNSS measurements for linking to a global reference system. The local ties often differ from the coordinate differences derived from space-geodetic observations. However, the sources for the discrepancies are not always clear, and a validation within the combination is difficult as long as the local ties are needed for the combination. We provide an alternative combination method by using the co-location of GNSS and SLR observations onboard GNSS satellites. As the local ties do not need to be applied in this approach, the resulting station coordinates are consistently estimated, but independent of the local ties. This allows us to evaluate the agreement of the terrestrial local ties and the space-geodetic coordinate differences derived from a 12-years solution. The 61 co-locations investigated in a multi-year solution show an agreement better than 1 cm in the horizontal and height components for 41 and 27 co-locations, respectively. Co-locations showing big discrepancies can be explained by the shortness of the data set included, or the rare distribution over the time span. Only the co-location site San Fernando shows unexplained differences of several centimeters. When using satellite co-locations, the counterpart of local ties at stations are space ties at satellites. The offsets of the microwave satellite antenna form one component of the space tie, with the offset of the laser retro-reflector array (LRA) forming the second part. We show that corrections to the space ties can be estimated from combined GNSS-SLR solutions. The corrections to the LRA offsets are only 5–6 mm, whereas the corrections estimated for the microwave antennas can exceed 1 dm, with a mean correction of −86.1 mm and −110.4 mm for GPS and GLONASS, respectively. The corrections to the microwave satellite antennas cause a scale difference for the GNSS ground network of 0.67 ppb. We show that this scale is consistent to the SLR scale, thus, the scale of SLR is properly transferred to the GNSS network via the co-location at the GNSS satellites.

LAGEOS Sensitivity to Ocean Tides

Satellite Laser Ranging (SLR) to LAGEOS has a remarkable contribution to high-precise geodesy and geodynamics through deriving and validating various global geophysical models. This paper validates ocean tide models based on the analysis of satellite altimetry data, coastal tide gauges, and hydrodynamic data, i.e., CSR3.0, TOPEX4.0, CSR4.0A, FES2004, GOT00.2, and the CSRC Schwiderski model. LAGEOS orbits and SLR observation residuals from solutions based on different ocean tide models are compared and examined. It is found that LAGEOS orbits are sensitive to tidal waves larger than 5 mm. The analysis of the aliasing periods of LAGEOS orbits and tidal waves reveals that, in particular, the tidal constituent S2 is not well established in the recent ocean tide models. Some of the models introduce spurious peaks to empirical orbit parameters, which can be associated with S2, Sa, and K2 tidal constituents, and, as a consequence, can be propagated to fundamental parameters derived from LAGEOS observations.

Erratum to: Impact of the arc length on GNSS analysis results

6 Bundesamt für Kartographie und Geodäsie, Richard-Strauss-Allee 11, 60598 Frankfurt am Main, Germany day to day, orbit misclosures in the Earth-fixed system exclusively characterize the difference of the orbits at the day boundaries in one and the same reference frame. In the inertial system the pole misclosures (Eq. 1) affect the orbit misclosures, as well. Subsequently, we uniquely analyze the orbit misclosures in the inertial system.”