Abstract
Objective: The terrestrial reference frame (TRF) constitutes the fundamental basis for global, regional, and national geodetic networks, and it is necessary for the quantitative investigations of Earth system processes. To meet the stringent requirements of geoscientific studies concerning small-scale variations of the Earth system, the global geodetic observing system (GGOS) has established target accuracies of 1 mm in position and 0.1 mm/a in stability for the TRF. As the most accurate realization of the TRF, the international terrestrial reference frame (ITRF) is generated through the integrated processing of multiple space geodetic techniques within GGOS. However, the latest realization, ITRF2020, still falls short of the 1 mm accuracy goal, owing to technique specific systematic errors and uncertainties in local tie measurements at co-location sites. This paper aims to review the current status of TRF establishment within the framework of GGOS, identify opportunities to advance TRF accuracy toward the 1 mm level, and summarize the key scientific and technical challenges facing the next generation GGOS. Methods: We first review the present status of ITRF realization and the architecture of GGOS, analyzing in detail the contributions of the four space geodetic techniques, namely global navigation satellite system (GNSS), satellite laser ranging (SLR), very long baseline interferometry (VLBI), and Doppler orbitography and radiopositioning integrated by satellite (DORIS), to the combined solution. The principal limitations of the current GGOS infrastructure are then examined. On this basis, we discuss the prospective architecture of the next generation GGOS, including low Earth orbit (LEO) satellite constellations, co-location geodetic satellites, and space VLBI (SVLBI) satellites, together with their potential contributions to ITRF realization.Results: The review shows that ITRF2020 has benefited from considerable progress in all four space geodetic techniques, including the expansion of global station networks, advances in instrumentation, and the refinement of data processing models and strategies. Nevertheless, several factors continue to prevent the ITRF from reaching the 1 mm accuracy target, most notably insufficient exploitation of available observations, technique specific systematic errors, and excessive reliance on ground based local ties. The next generation GGOS, represented by space-based observations, can expand the existing observation system and provides an effective means to overcome these limitations. LEO satellite constellations can provide abundant observations linking GNSS satellites with ground stations, thereby strengthening geocenter determination within the GNSS observation. Co-location geodetic satellites enable the integration of GNSS, SLR, and VLBI through onboard space ties, improving the accuracy of key geodetic parameters such as Earth orientation parameters (EOP) and geocenter coordinates relative to any single technique solution. SVLBI satellites can substantially extend baseline lengths within regional VLBI networks. Incorporating SVLBI observations enables the Chinese VLBI network to determine EOP with an accuracy comparable to that of the global network. Conclusions: The ITRF has progressed substantially through the multifaceted development of GGOS, yet reaching the 1 mm accuracy target requires more advancement. The next generation GGOS, built upon multiple complementary space missions, is expected to provide richer observations, more reliable inter technique ties, and a means of overcoming the constraints of ground-based networks, thereby advancing ITRF accuracy toward the 1 mm level. To realize this vision, several urgent challenges must be addressed, including precise orbit determination for large constellations and complex satellite platforms, the development of rigorous theoretical frameworks for multiple technique combination, and the design of high-performance software capable of processing large scale and heterogeneous geodetic observations.
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