Global geodetic parameters, such as Geocenter Coordinates (GCCs) and Earth Rotation Parameters (ERPs), are critical for the definition of the International Terrestrial Reference Frame (ITRF) and geophysical studies. Traditional ITRF realization relies heavily on ground-based local ties to combine multiple space geodetic techniques, but these ties suffer from spatial unevenness, high maintenance costs, and infrequent updates. Low Earth Orbit (LEO) satellites equipped with multi-geodetic payloads have emerged as a transformative solution, providing "space ties" that allow inter-technique linkage independent of ground surveys. This study explores the potential of GPS-SLR space ties onboard Swarm LEO satellites for precise parameter determination via observation-level combination. Using ground-based GPS observations from 107 IGS stations, Swarm-borne GPS observations, and SLR-to-Swarm measurements from 36 SLR stations, three solutions (GPS-only, GPS+LEO, GPS+LEO+SLR) were analyzed and compared. Swarm-borne GPS observations markedly improve sensitivity to geocenter motion, decreasing GCC formal errors by about 46.5%, 48.4%, and 36.7% in the X, Y, and Z components, and reducing ERP formal errors by 34.3%, 40.1%, and 31.5% for the X-pole, Y-pole, and LOD, respectively, due to enhanced geometry. Correlation analyses further confirm that Swarm-borne GPS observations effectively decouple the GCC Z-component from GPS orbit model parameters, thereby enhancing solution robustness. The inclusion of SLR-to-Swarm observations yields additional reductions in the horizontal components, improves the temporal stability of geocenter estimates, and further mitigates the β-angle dependence of GCC formal errors in the X and Y components. The annual difference between GPS+LEO+SLR and SLR-60day solutions was 0.6 mm in amplitude and 15° in phase for the GCC Z component. However, due to SLR’s insensitivity to Earth rotation and its sparse temporal sampling, its contribution to ERP estimates is limited, yielding only moderate improvements in polar motion bias and standard deviations, especially in the Y component. This study confirms the value of LEO-based GPS-SLR space ties in advancing global geodetic parameter estimation. In the future, a promising direction is the co-location of GNSS and VLBI payloads onboard LEO satellites to address residual ERP limitations.
Li et al. (Wed,) studied this question.