Bathymetry data is an essential element in marine survey and mapping, and plays important roles in researching the Earth and guiding the underwater works. Satellite altimetry is one of the most effective ways to obtain global ocean gravity anomalies and vertical gravity gradients. In this study, the transfer function between gravity anomalies and gravity gradients and seabed topography is derived in spectral domain. Based on the error propagation analysis between altimetry data and gravity filed products, a bathymetry prediction method using different data fusion according to different water depths is proposed. A simulation is carried out in the Mariana Trench Seabed Terrain to verify the proposed method. The results show that vertical gravity gradients data does perform better in shallow water areas than gravity anomaly data does. And when the vertical gravity gradient data is used in the area with water depth shallower than 1000m, and the gravity anomaly data is used in the area with water depth deeper than 1000m, the predicted bathymetry has higher accuracy than that derived from gravity anomalies alone or vertical gravity gradients alone, which improves the RMS 11.35% compared to just using anomalies alone.
With the resolution of spaceborne synthetic aperture radar reaching sub meter level, the mapping bandwidth is only 3km to 5 km. This reduces the effectiveness of aiming and capturing the observed target. Therefore, it is necessary to analyze the influencing factors for aiming accuracy of spaceborne SAR. This article conducts analysis from the aspects of ground telemetry/track and command system, ground operation control system, satellite platform and payload. An error model based on the influencing factors of entire satellite-ground link and on-orbit testing method suitable for any scenario are proposed. The results of theoretical prediction and real test show that 120m orbit height prediction error, 90m target elevation error, 5m slant measurement error, 0.03° satellite attitude pointing error and 0.01° systematic beam pointing error of SAR antenna will bring about 400m aiming accuracy.
KEYWORDS: Satellites, Data transmission, Reflection, Satellite communications, Aerospace engineering, Mathematical optimization, Design and modelling, Data modeling, Receivers, Process modeling
In recent years, with the advancement of aerospace integration and the development of space missions, micro-nano satellites have become a hot research direction in aerospace field. Limited by the performance limitations of single micro-nano satellites, the networking of micro-nano satellite constellation is becoming the main trend. Optimized Link State Routing (OLSR) protocol is widely adopted as the micro-nano satellite networks routing protocol. For the micro- nano satellite constellation networks, frequent changes of the network topology caused by the rapid movement of satellite nodes pose a high challenge to the invulnerability of routing protocols. However, traditional OLSR routing protocol lacks awareness of the satellite node running trajectory and running status, resulting in untimely updates of network topology changes, the invulnerability cannot be guaranteed. To this end, in view of the predictability of the micro-nano satellite constellation orbit, we propose an improved OLSR protocol based on link survival time optimization. By adding the interaction of position and other information in OLSR protocol, link survival time is predicted based on satellite orbit. Extensive simulation experiments show that improved OLSR protocol based on link lifetime optimization has significantly improvement in transmission success rate and invulnerability.
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