Length Variations of European Baselines Derived from VLBI and GPS Observations

Results of VLBI and GPS observations were analyzed with goal to investigate differences in observed baseline length derived from both techniques. VLBI coordinates for European stations were obtained from processing of all available observations collected on European and global VLBI network. Advanced model for antenna thermal deformation was applied to account for change of horizontal component of baseline length. GPS data were obtained from re-processing of the weekly EPN (European Permanent GPS Network) solutions. Systematic differences between results obtained with two techniques including linear drift and seasonal effects are determined.

💡 Research Summary

The paper presents a comparative analysis of baseline length variations derived from Very Long Baseline Interferometry (VLBI) and Global Positioning System (GPS) observations over the period 1996‑2001 for six European stations that operate both techniques (MATE, MEDI, NOTO, NYAL, ONSA, WETT). VLBI data were processed with the OCCAM software using all available 24‑hour sessions from 1983.9 to 2001.5. A state‑of‑the‑art antenna thermal deformation model was applied, correcting both vertical and horizontal components; because most antennas lack real‑time temperature data, a zero‑delay assumption between ambient and structural temperature was adopted, which mainly affects intra‑day offsets but not seasonal signals.

GPS data were taken from the weekly European Permanent GPS Network (EPN) SINEX solutions. The official EPN products impose tight constraints on fiducial stations, leading to network distortion and artificial baseline jumps. To obtain homogeneous time series, the authors de‑constrained the official solutions, applied a six‑parameter Helmert transformation (three rotations, three translations, no scale change) to align with ITRF2000, and thus preserved seasonal geophysical signals.

In addition to the space‑geodetic products, the authors evaluated atmospheric loading effects using three‑dimensional loading time series supplied by H‑G Scherneck. Weekly averages were computed to match the GPS epoch cadence, and baseline length variations induced by loading were derived. The loading analysis revealed that atmospheric pressure variations contribute significantly to horizontal displacements, even for relatively short, coastal‑inland baselines, and that both seasonal and secular components are present.

Statistical results are summarized in Table 1. Linear trends (rates) obtained from VLBI and GPS are generally consistent within one sigma for most baselines, with typical VLBI rate uncertainties of ~0.5 mm yr⁻¹ and GPS uncertainties of ~0.3 mm yr⁻¹. However, amplitudes of the annual and semi‑annual terms differ markedly between the two techniques. The limited number of VLBI sessions and the relatively short observation span reduce the reliability of VLBI seasonal estimates, whereas GPS provides more robust seasonal amplitudes but is still affected by residual network distortions.

Error analysis (Figure 4) shows that GPS rate errors are nearly independent of baseline length, whereas VLBI errors increase for shorter baselines, reflecting the geometry and scheduling constraints of VLBI observations. The authors also note that atmospheric loading contributes appreciably to the observed rates, especially for baselines involving the Wettzell station, which is frequently used in European geodynamic studies.

The paper concludes that VLBI and GPS are complementary for monitoring long‑term (secular) baseline changes, but accurate characterization of seasonal variations requires careful modeling of atmospheric loading, antenna thermal effects, and other geophysical processes such as hydrological loading, snow loading, and post‑glacial rebound. Future work will incorporate additional space‑geodetic techniques (SLR, DORIS), employ updated thermal deformation models, and use higher‑resolution atmospheric and hydrological loading models to achieve a more comprehensive geodynamic interpretation. The authors also plan to extend the analysis to vertical components of station motion, which were not addressed in the present study.