Discovering the Sky at the Longest Wavelength Mission−A Pathfinder for Exploring the Cosmic Dark Ages
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摘要: 低频射电探测任务构想——鸿蒙计划旨在利用多颗卫星绕月编队形成超长波天文观测阵列，在月球背面开展空间低频射电天文探测。其科学目标是高精度测量全天射电频谱，揭示宇宙黑暗时代与黎明的演化历史；实现首次高分辨率超长波巡天，打开最后一个电磁窗口；观测太阳和行星超长波活动，揭示空间环境相互作用规律。该任务将获得超长波频段全天空图像，获取超长波波段天文射电源的强度、频谱、分布等信息。这些科学数据对于探索宇宙黑暗时代和黎明时代、研究银河系星际介质、宇宙线起源与传播、河外射电星系、类星体和星系团的演化、太阳活动与行星磁场等，具有重要的科学价值。Abstract: In this paper we describe a low frequency radio astronomy mission known as the DSL project (also “Hongmeng Project” in Chinese). It is an interferometer array made up of a formation of satellites in a lunar orbit, which makes ultralong wavelength observations in the part of orbit behind the Moon. The scientific objectives include making high precision measurement of the global spectrum, probing the cosmic dawn and dark ages; realizing ultra-long wavelength sky survey with a high resolution for the first time, opening up the last unexplored electromagnetic window; monitoring the radio activity of the Sun and planets, analyzing their interactions in the interplanetary space. The mission will obtain the whole sky map in the ultralong wavelength, and provide information on the intensity, spectrum and distribution of ultralong wavelength radio sources. It has great scientific value for the exploration of the cosmic dark ages and cosmic dawn, and investigations on the interstellar medium, origin and propagation of cosmic rays, extragalactic radio galaxies and quasars, evolution of clusters and groups of galaxies, solar activities, and planetary magnetic field.
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图 1 宇宙历史（a）和对应的21 cm平均谱（b）
Figure 1. Cosmic history (a) and the corresponding 21 cm global spectrum (b)
图 2 ULSA模型预测的10 MHz（a），3 MHz（b）和1 MHz（c）天图
Figure 2. Sky map of 10 MHz (a), 3 MHz (b) and 1 MHz (c) obtained by the ULSA model
图 4 低频干涉成像谱仪系统
Figure 4. Low frequency band interferometric imaging spectrometer system
图 5 星间通信、测距和时钟同步系统
Figure 5. Inter-satellite communication, ranging and clock synchronization system
图 9 低频子星天线展开前后状态
Figure 9. Low frequency daughter satellite before and after antenna deployment
图 12 还原天图与相对误差及球谐相关系数
Figure 12. Reconstructed map, relative error, and spherical harmonic correlation coefficients
图 13 不同观测时间下10′ 分辨率下的天空温度方差
Figure 13. Standard deviation of sky temperature at 10′ angular resolution with different observation time
表 1 有效载荷配置
Table 1. Payloads
卫星 载荷名称 数量 高频子星 高频频谱仪 1 星间通信测距同步系统 1 星间测角系统 1 低频子星 低频干涉成像谱仪 8 星间通信测距同步系统 8 星间测角系统 8 主星 相关处理器 1 定标系统 1 星间通信测距同步系统 1 星间测角系统 1 
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表 2 高频频谱仪技术指标
Table 2. Parameters of the high frequency band spectrometer
技术指标 设计要求 天线 尺寸小、波瓣与频率无关、响应平坦 探测频率 30~120 MHz 探测灵敏度 优于0.1 K @ 80 MHz
（1 MHz带宽10 min积分时间）探测动态范围 80 dB 频谱分辨率 ≤100 kHz EMI/干扰幅度 ≤ 3 nV·Hz1/2 EMI/宽频干扰 30~120 MHz内不存在带宽
≥ 2 MHz的宽频干扰EMI/单频点干扰 30 ~ 120 MHz内的占比≤5%
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表 3 低频干涉成像谱仪技术指标
Table 3. Parameters of the low frequency band interferometric imaging spectrometer
技术指标 设计要求 天线 2.5 m可展开三正交单极子 探测频率 0.1 ~ 30 MHz 空间分辨率 优于0.18° @ 1 MHz，0.012° @ 30 MHz 谱分辨点数 优于8192 探测灵敏度 优于1000 K @ 30 MHz
（1 MHz带宽，10 min积分时间）EMI 优于≤ 3 nV·Hz1/2（0.1~30 MHz，90%观测频段）
优于 9 nV·Hz1/2（0.1~30 MHz，其他频段）
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表 4 星间通信、测距和时钟同步技术指标
Table 4. Parameters of the inter-satellite communication, ranging and clock synchronization
技术指标 设计要求 频率范围 Ka前向链路（22.55~23.55 GHz）
反向链路（25.60~27.22 GHz）多星通信方式 1主9从 接入方式 频分复用 通信距离 100 km 测距精度 优于1 m 时钟同步精度 ≤ 3.3 ns 每星数传速率 ≥ 20 Mbit·s–1
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表 5 星间测角系统技术指标
Table 5. Parameters of inter-satellite direction measurement system
技术指标 设计要求 星空相机视场 9.5° 测角精度 9 μrad 焦距 40 mm 相对孔径 1:1.2 星空相机重量 ≤ 400 g 灯阵发射角 110° 灯阵平均电功率 15 W 灯阵重量 300 g
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表 6 点源探测灵敏度（10 MHz频率，1 MHz带宽）
Table 6. Point source detection sensitivity (10 MHz frequency, 1 MHz bandwidth)
积分时间 1 hour 1 day 1 week 1 month 1 year 流量/Jy 300 61.1 23.1 11.2 3.2
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