自制GPS发射机系列之二 —— 星载设备

2.4 GPS和 GLONASS星载设备
GPS与GLONASS卫星的星载设备基本相似，见图。对于实现导航功能而言，卫星可以更加简单，比如像民用通信卫星一样仅装备一个线性应答机。导航信号可以由地面站网络来生成和同步。然而，GPS和GLONASS都首要考虑军事用途。上行传输是不必要的因为这容易导致阻塞（导致死机）并且地面站容易被破坏。因此GPS和GLONASS都设计成能够完全自主运行和产生信号的。同步主要靠星载原子钟维护，每隔较长时间才与地面站对时。

GPS和GLONASS卫星都装载一台铯原子钟作为主时间/频率基准，精度为10^-12到10^-13。更加小巧的铷原子钟作为备份，不过它的精度比铯原子钟小一个量级。由于稳定的空间环境，原子钟通常比在地面上有更好的性能表现。任何长漂和偏差都很容易通过上载修正系数到星载计算机上补偿。

原子频标的输出驱动一个频率合成器，因此所有载波和调制率拥有同一个相干的参考源。星载计算机生成所谓“导航数据”，包括精确的卫星位置信息——ephermeri，原子钟的偏移和漂移信息和其他卫星的信息——almanac。前两个信息都直接用于建立用户计算机里的导航方程。其他卫星的信息可以用来预测可见卫星避免尝试连接工作不正常或者不存在了的卫星，因此可以加速在适当的GDOP下获得四颗有效卫星信号的过程。

除了发送和接收导航信号，GPS和GLONASS卫星都有遥控和遥测链路。地面指令中心使用遥控链路来上载更新星载计算机的导航数据。这个工作通常每天一次，尽管星载计算机内存足够储存几个星期的数据。除了专门的遥测链路，部分遥测数据也插入到导航数据流中。


原文见下：



2.4. GPS & GLONASS satellite on-board equipment

Since the two systems are similar, GPS and GLONASS satellites carry almost the same on-board equipment as shown on Fi[size=+0]g.06. For the navi[size=+0]gation function alone, the satellites could be much simpler, carrying a simple linear transponder like on civilian communications satellites. The required navigation signals could begenerated and synchronized by a network of ground stations.

                               
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However, both GPS and GLONASS are primarily intended as military systems. Uplinks are undesired since they can be easily jammed and a network of ground stations can be easily destroyed. Therefore, both GPS and GLONASS satellites are desi[size=+0]gned for completely autonomous operation and generation of the required signals. Synchronization is maintained by on-board atomic clocks that are only periodically updated by the ground stations.

Both GPS and GLONASS satellites carry a cesium atomic clock as their primary time/frequency standard, with the accuracy ranging between 10^-12 and 10^-13. Much smaller and lightweight rubidium atomic clocks are used as a backup in the case the main time/frequency standard fails, although rubidium atomic clocks are an order of magnitude less accurate. Due to the stable space environment these atomic clocks usually perform better than their ground-based counterparts and any long-term drifts or offsets can be easily compensated by uploadin the required correction coefficients in the on-board computer.

The output of the atomic time/frequency standard drives a frequency synthesizer so that all the carrier frequencies and modulation rates are derived coherently from the same reference frequency.

The on-board computer generates the so-called navigation data. These include information about the exact location of the satellite, also called precision ephemeris, information about the offset and drift of the on-board atomic clock and information about other satellites in the system, also called almanac. The first two are used directly by the user's computer to assemble the navi[size=+0]gation equations. The almanac data can be used to predict visible satellites and avoid attemptin to use dead, malfunctioning or inexistent satellites, thus speeding-up the acquisition of four valid satellite signals with a reasonable GDOP.

Besides the transmitters for broadcasting navigation signals, GPS and GLONASS satellites also have telecommand and telemetry radio links. In particular, the telecommand link is used by the command stations to regularly upload fresh navigation data into the on-board computer. Usually this is done once per day, although the on-board computer memory can store enough data for several weeks in advance. In addition to dedicated telemetry links, part of the telemetry data is also inserted in the navi[size=+0]gation data stream.

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