Mega-constellations have the potential for providing 6G Internet owing to the unique advantage of global coverage. However, current satellite technologies are not omnipotent. There are still many challenging problems that need to be solved for mega-constellations to support 6G, e.g., efficient resource allocation, gratifying mobility management, and large-scale full-time TT&C (tracking, telemetry, and command). This paper starts with a novel definition of LEO mega-constellations and a brief review regarding the current typical mega-constellations, discussing the development direction of the mega-constellation air interface. Then, the key technologies development status of satellite networks is illustrated and analyzed from five aspects: network protocol, multiple access, satellite handover, TT&C, and interference mitigation, especially their adaptability in mega-constellations for 6G global coverage. Finally, considering the features and requirements of 6G, future challenges for mega-constellations and some potential solutions are proposed.

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In recent years, in order to provide worldwide broadband Internet access services, many mega-constellation projects have been proposed. Traditional ground-based and space-based telecommand systems only rely on a single path for data transmission, which is vulnerable once the path fails. This paper utilizes inter-satellite-links (ISLs) in the constellation network to reduce the risk of data transmission failure, and proposes the failure-tolerant and low-latency redundant multi-path routing algorithm (RMPR). RMPR transmits multiple data copies on multiple shortest edge-disjoint paths according to the optimal proportions with the minimum delay. Compared with traditional single-path routing and backup multi-path routing, RMPR ensures both reliability and timeliness in telecommand system.INDEX TERMS Mega-constellations, telecommand, multi-path routing, data redundancy, failure-tolerant, low-latency.

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In this study, the Qingshuitang District from the Shifeng Bridge to the Majiahe Bridge was selected as study areas to assess the contamination status and release amount of heavy metals from the surface sediments of the Xiangjiang River, Hunan Province, China. A sum of five surface sediment samples were collected, and the total contents of Cu,Pb,Zn,Cd,Cr,Mn,As and Hg in these samples were measured by the Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Atom Absorption Spectrophotometer (AAS). Results showed that the average contents of Cd and As in the sediments were higher than the Grade III of the China Environmental Quality Standard for Soils. Moreover, release experiments were carried out to study the effect of flow velocity and pH values on the release of heavy metals from the sediments. The results suggested that increased the flow velocity and decreased the pH of overlying water could promote the release of heavy metals, while the release amount of four heavy metals were too small and the largest release amount of Mn was only 7.50 μg/(g·d). It suggested that the release of heavy metals from the sediments into the overlying water is a slow and long-term process.