Global navigation satellite system (GNSS) orbits are traditionally determined by observation data of ground stations, which usually need even global distribution to ensure adequate observation geometry strength. However, good tracking geometry cannot be achieved for all GNSS satellites due to many factors, such as limited ground stations and special stationary characteristics for the geostationary Earth orbit (GEO) satellites in the BeiDou constellation. Fortunately, the onboard observations from low earth orbiters (LEO) can be an important supplement to overcome the weakness in tracking geometry. In this contribution, we perform large LEO constellation-augmented multi-GNSS precise orbit determination (POD) based on simulated GNSS observations. Six LEO constellations with different satellites numbers, orbit types, and altitudes, as well as global and regional ground networks, are designed to assess the influence of different tracking configurations on the integrated POD. Then, onboard and ground-based GNSS observations are simulated, without regard to the observations between LEO satellites and ground stations. The results show that compared with ground-based POD, a remarkable accuracy improvement of over 70% can be observed for all GNSS satellites when the entire LEO constellation is introduced. Particularly, BDS GEO satellites can obtain centimeter-level orbits, with the largest accuracy improvement being 98%. Compared with the 60-LEO and 66-LEO schemes, the 96-LEO scheme yields an improvement in orbit accuracy of about 1 cm for GEO satellites and 1 mm for other satellites because of the increase of LEO satellites, but leading to a steep rise in the computational time. In terms of the orbital types, the sun-synchronous-orbiting constellation can yield a better tracking geometry for GNSS satellites and a stronger augmentation than the polar-orbiting constellation. As for the LEO altitude, there are almost no large-orbit accuracy differences among the 600, 1000, and 1400 km schemes except for BDS GEO satellites. Furthermore, the GNSS orbit is found to have less dependence on ground stations when incorporating a large number of LEO. The orbit accuracy of the integrated POD with 8 global stations is almost comparable to the result of integrated POD with a denser global network of 65 stations. In addition, we also present an analysis concerning the integrated POD with a partial LEO constellation. The result demonstrates that introducing part of a LEO constellation can be an effective way to balance the conflict between the orbit accuracy and computational efficiency.

Earth rotation parameters (ERP) are one of the key parameters in realization of the International Terrestrial Reference Frames (ITRF). At present, the International Laser Ranging Service (ILRS) generates the satellite laser ranging (SLR)-based ERP products only using SLR observations to Laser Geodynamics Satellite (LAGEOS) and Etalon satellites. Apart from these geodetic satellites, many low Earth orbit (LEO) satellites of Earth observation missions are also equipped with laser retroreflector arrays, and produce a large number of SLR observations, which are only used for orbit validation. In this study, we focus on the contribution of multiple LEO satellites to ERP estimation. The SLR and Global Positioning System (GPS) observations of the current seven LEO satellites (Swarm-A/B/C, Gravity Recovery and Climate Experiment (GRACE)-C/D, and Sentinel-3A/B) are used. Several schemes are designed to investigate the impact of LEO orbit improvement, the ERP quality of the single-LEO solutions, and the contribution of multiple LEO combinations. We find that ERP estimation using an ambiguity-fixed orbit can attain a better result than that using ambiguity-float orbit. The introduction of an ambiguity-fixed orbit contributes to an accuracy improvement of 0.5%, 1.1% and 15% for X pole, Y pole and station coordinates, respectively. In the multiple LEO satellite solutions, the quality of ERP and station coordinates can be improved gradually with the increase in the involved LEO satellites. The accuracy of X pole, Y pole and length-of-day (LOD) is improved by 57.5%, 57.6% and 43.8%, respectively, when the LEO number increases from three to seven. Moreover, the combination of multiple LEO satellites is able to weaken the orbit-related signal existing in the single-LEO solution. We also investigate the combination of LEO satellites and LAGEOS satellites in the ERP estimation. Compared to the LAGEOS solution, the combination leads to an accuracy improvement of 0.6445 ms, 0.6288 ms and 0.0276 ms for X pole, Y pole and LOD, respectively. In addition, we explore the feasibility of a one-step method, in which ERP and the orbit parameters are jointly determined, based on SLR and GPS observations, and present a detailed comparison between the one-step solution and two-step solution.

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Abstract. Geological disasters such as landslides and debris flows pose a serious threat to human life and property. To mitigate this risk, monitoring and early warning systems are essential. However, monitoring high-angle landslide areas can be challenging due to the steep and complex terrain, making it difficult to carry out large-scale and refined deformation measurements using existing methods. This paper proposes a method to measure the large-scale deformation of landslide bodies based on nap-of-the-object photogrammetry. The method uses UAVs to acquire high-resolution 3D models of steep and high landslide areas at multiple time periods. Then, 3D model matching is employed to obtain accurate variation information for terrain deformation measurement. To obtain fine 3D models, a terrain-adaptive nap-of-the-object photogrammetric flight planning is applied to design the optimal photographic positions and directions for capturing ultra-high-resolution images. The images are processed using photogrammetric principles and technologies to produce fine 3D models. For terrain deformation measurement, an algorithm is proposed to obtain 3D correspondences by fusing DEM differential and 3D model texture matching. The 3D points variation vectors are then calculated, and the large-scale deformation measurement results of the landslide body can be derived after the vectors are aggregated. Experiments were conducted on the Lijiebeishan landslide in Gansu Province, western China. The results showed that the proposed deformation measurement method was highly effective in accurately detecting areas with displacement greater than 5 cm, and the large-scale deformation trend is consistent with GNSS predictions. In conclusion, the proposed method is an effective way to measure the large-scale deformation of landslide bodies in high-angle landslide areas, providing a valuable tool for monitoring and early warning systems.

<p>Earth rotation parameters (ERP) are one of the key parameters in realization of the International Terrestrial Reference Frames (ITRF). Currently, the ERP products from International Laser Ranging Service (ILRS) are generated based on SLR observations to LAGEOS and Etalon satellites, which account for only about 9% of total SLR observations to Earth satellites. A large amount of SLR observations for the geodetic and oceanographic LEOs are neglected due to relatively degraded orbit caused by imperfect orbit models. However, thanks to the recent refinement of both dynamic and observation models, the quality of LEO orbits has been improved significantly, which makes it worthwhile to investigate the potential of these LEOs in the ERP estimation. In this study, we focus on the contribution of SLR observations from multiple LEO satellites to ERP estimation. The SLR observations of current seven LEO satellites (Swarm-A/B, GRACE-C/D, Sentinel-3A/B and Jason-3) as well as LAGEOS are used. Several strategies are designed to investigate the impact of the LEO orbit altitude, inclination and the number of LEO satellites. We also discuss the contribution of the application of ambiguity-fixed orbits and consider the simultaneous processing of SLR and GPS observations. The three-day solutions are selected and all the results are evaluated by the comparison with IERS Bulletin A.</p><p>The results show that for the single-LEO solutions, there is no evident relationship between the accuracy of ERP and the LEO orbit altitude and inclination. The best consistency with the IERS products is achieved by the Jason-3 solutions, with RMS values of 1.9mas, 1.8mas and 93us for X pole, Y pole and length of day (LOD) respectively. The multi-LEO solution results indicate that the accuracy of ERP can be improved gradually with the increase of LEO satellites. Compared with the single-LEO solution, the accuracy of X pole and Y pole of the 7-LEO solution is improved by 39.27% and 53.84% respectively. This result can be easily understood by the evident increase of SLR observations with the increase of LEO satellites. We also find the ERP estimation can benefit from the application of the ambiguity-fixed orbit.</p><p>In addition, apart from the solutions with LEO orbits fixed (two-step method), we also jointly process the onboard GPS observations and SLR measurements to obtain LEO orbits and ERP simultaneously (one-step method). The result indicates that the ERP of the one-step solution present a better accuracy than that of the two-step solution. Moreover, the LEO orbits can also benefit from the integrated processing.</p>