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转台式激光载荷和固定式激光载荷的配置原则已在轨应用于两颗宽带节点卫星,卫星部署在高度880 km、倾角86.5°的圆轨道上,两颗卫星位于前后相位差36°的同一轨道面内。各载荷的指向角度范围和安装方式如图 5所示。固定式激光终端服务于同轨道面内相邻两颗卫星之间的激光建链,当卫星入轨建立对地三轴稳定状态,并且前后两颗卫星的相位关系调整到位后,激光的指向角度随即确定,且仅与两颗卫星之间的地心角有关,数值为地心角大小的一半,而与卫星的轨道高度和轨道倾角均无关,卫星的轨道高度仅影响激光链路的通信距离。上述两颗卫星的地心角为36°,固定式激光终端的指向角度即为固定的18°,在卫星总装过程中将固定式激光终端倾斜安装在卫星舱板上,即可使激光收发望远镜相对于卫星有18°的倾斜角。
转台式激光终端服务于异轨道面内相邻两颗卫星之间的激光建链,在卫星本体坐标系下描述异轨相邻卫星运动过程中的角度变化,其结果如图 6所示。从图中可以看出, 激光指向的方位角变化范围为±60°,俯仰角的变化范围为9°~18°,由此可知转台式激光终端2维转台的转动范围,通过使载荷在卫星舱板上倾斜9°安装,可使转台在俯仰方向仅需0°~9°的转动范围。表 1中展示了两种激光终端的指向范围。
表 1 两种激光终端的指向范围
Table 1. Pointing range of two types of laser terminals
terminal type azimuth/(°) elevation/(°) service type fixed laser terminal 0 18 in the same orbital planes rotatable laser terminal -60~60 9~18 in the same orbital planes & across different orbital planes -
卫星入轨后,按着任务规划分别开展了激光终端的开机自检、恒星标校、信标光以及信号光捕获、光轴一致性调整等工作,利用转台式激光终端在低轨卫星上成功实现了星间激光链路的长时间稳定通信,前后共历时13 d,使用了41轨测控资源。
在恒星标校阶段,选取了编号为91262#和102098#的两颗恒星进行标校工作。结合激光终端在轨扫描捕获多颗恒星以及标校过程中的数据分析,激光终端在此次发射任务中经过发射阶段的冲击以及在轨阶段的温度变化后,初始不确定区域在20 mrad左右。地面分析计算并上注修正矩阵后,激光终端再次指向目标恒星的指向不确定区域已小于3 mard,在轨恒星标校后图像如图 7所示。
随后开展了30余次的双向信标光扫描捕获试验,100%成功,平均捕获时间22.6 s。之后通过调整粗跟踪点实现了信号光捕获跟踪,再通过调整精跟踪点和提前量振镜位置实现了光轴一致性调整, 并基于此实现了长时间的稳定跟踪,回放遥测中显示跟踪误差值如图 8所示。计算可得粗跟踪的精度为9.6 μrad,精跟踪的精度为2.3 μrad。图中x, y表示距离中心点位置的大小。
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在前述建立稳定跟踪激光链路中,设置终端进入通信自测模式,分别在1.25 Gbyte/s和2.5 Gbyte/s两个通信速率下开展通信测试,并保持链路稳定,最后通过下传遥测数据判读通信误比特率,最终实现了连续145 min的长时间无误码激光通信。
通信过程中的信噪比是通过误比特率来估计的。在激光终端研制阶段,通过误码测试仪监测不同输入光信号强度下的误比特率,获取误比特率与信噪比的对应关系。设备在轨后,根据自测模式下误比特率获取对应信噪比值,实现信噪比估计,进而转入业务通信。后续可通过带内光信噪比监测技术[21],在正常业务通信模式下实现高精度、高可靠的链路监测。
经在轨实验的验证,激光终端在采取该建链策略后,可以实现30 s内的在轨快速建链操作,且激光通信链路可以保证长时间的持续通信零误码,验证了激光建链策略的有效性,以及激光通信终端设计的指标符合性。
低轨卫星激光载荷配置及快速建链方法的研究
Research on configuration of laser payload for LEO satellites and rapid link establishment method
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摘要: 为了适应未来低轨卫星轻量化和低成本的发展趋势, 满足星间激光快速建链的使用需求, 对星间激光配置原则进行了分析, 采用转台式激光载荷和固定式激光载荷相结合的载荷配置方式, 分别用于异轨道面间和同轨道面内的星间建链, 设计了基于在轨恒星标校、信标光辅助建链和光轴一致性的激光载荷快速星间建链方案, 并介绍了全流程。结果表明, 星间激光载荷建链时间小于30 s, 可实现长时间的星间激光稳定跟踪通信。该研究结果有助于推动星间激光通信的发展和大规模在轨应用。Abstract: In order to adapt to the development trend of lightweight and low-cost low earth orbit (LEO) satellites and meet the demand for rapid laser inter-satellite link establishment, a payload configuration method combining a rotatable laser payload and a fixed laser payload was adopted based on the analysis of inter-satellite laser configuration principles, used for inter-plane link establishment and intra-plane link establishment respectively. A fast inter-satellite link establishment method and process for laser payloads based on in-orbit star calibration, beacon light-assisted link establishment, and optical axis consistency design were introduced. In-orbit experimental data show that the inter-satellite laser payload link establishment time can be less than 30 s, which can achieve stable and continuous inter-satellite laser communication. The study is helpful for promoting the development and large-scale in-orbit application of inter-satellite laser communication.
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Key words:
- optical communication /
- rapid laser linking /
- laser in-orbit calibration /
- beacon light
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表 1 两种激光终端的指向范围
Table 1. Pointing range of two types of laser terminals
terminal type azimuth/(°) elevation/(°) service type fixed laser terminal 0 18 in the same orbital planes rotatable laser terminal -60~60 9~18 in the same orbital planes & across different orbital planes -
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