1933年卡尔·央斯基第一次发现来自宇宙的电磁波，开启了射电天文学的研究。在长达几十年的研究中，射电天文有了许多重要的发现，如脉冲星、宇宙微波背景辐射等等。为了探索更远的宇宙，获取更精确的探索结果，射电天文技术也取得了巨大的发展。从最初的直径只有几米的抛物面天线，到现在直径达到几百米，射电望远镜的设计有了巨大的改变，使天线的灵敏度和分辨率较以前大幅度提高。甚长基线干涉（VLBI）技术通过联合全世界的望远镜，使整个地球成为一个大口径望远镜，通过基线的拉长进一步提高了射电观测的分辨率。与此同时，综合孔径成像使我们从不同望远镜得到的观测数据可以形成一幅完整的图像。随着技术的发展，不少国家开始组建天线阵列对宇宙进行探索，以获取高分辨率和灵敏度。如美国的甚大阵（VLA）以及全球超过20个国家参与建设，阵列主要分布在澳大利亚和南非的平方公里阵（SKA）。本文所研究的VHF频段天线阵列通过接收宇宙传来的电磁波，并对接收数据进行预处理和存储，实现对宇宙天体的观测。天线阵列主要实现低频观测，中心频率为60MHz。天线通过模拟接收机接收信号后将其传到数据接收与采集系统，转换成数字信号，并在其中进行数据预处理。由于阵列需要两个或多个站的数据进行相干处理，因此需要稳定的时频系统。天线时频系统的功能主要有两个方面：1、采样控制计算机需要外部时钟进行授时，由NTP (Network Time Protocol)时间服务器通过网络接口提供；2、采样电路需要精准的外部参考信号作为采集驱动，由GPSDO时钟产生的10MHz经锁相环倍频至200MHz时钟驱动；其中NTP服务器通过接收GPS信号产生标准时间，并通过局域网授予计算机。通过使用LabView编程获取计算机时间并进行对比，得到不同计算机产生时间差的间隔，得到最佳授时策略。
其他摘要
Karl Jansky first discovered electromagnetic waves from universe, and it opened the study of radio astronomy. There have been many import discovers in decades of research in radio astronomy, such as pulsars, cosmic microwave background radiation and so on. In order to explore the universe further, and to obtain a more accurate search result, radio astronomy technology has been made tremendous development.The diameter of parabolic antenna has only few meters at first, but now the few hundreds diameter antenna has been designed. It has increased the sensitivity and resolution of antenna. Based on the technique of Very long baseline interferometry (VLBI), the whole world will be a big telescope through combine the telescopes separated in all over the world. At the same time we can form a complete image from all single telescopes through the techniques of Synthesis aperture imaging. As technology advances, many countries began to build the antenna array to explore universe, such as the Very Large Array (VLA) in America, Square Kilometer Array (SKA) in South Africa and Australia, which more than 20 countries have took part in the design. The VHF band antenna array studied in this paper will receive electromagnetic waves and preprocessing and store the data. The operation frequency of the antenna in low frequency band, and the center frequency is 60MHz. The receiver of antenna array will receive the microwaves and transmit it to the acquisition system, and then the analog signals will be converted into digital signals. The data stored by different station will conduct the coherent processing at the later step, and antenna array requires a stable time and frequency system.Two functions of the time and frequency system is:(1)NTP (Network Time Protocol) server provides reference time to computer which control data sampling(2)GPSDO generates a 10MHz signal and converted to 200MHz by PLL, as a precise external clock signal of acquisition board.The NTP server will produce standard time through receiving GPS signal, and transmit the time to computers in LAN (Local Area Network). Through programming in LabView can get the time of different computers’ system time deviation occurs and making the time strategy best.
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