In some special scenarios, such as road tunnels or tall buildings, the satellite signal is weak or indiscernible.
而在某些特殊的场景，如隧道或高楼路段，卫星信号较弱甚至无信号。
The deep space exploration has the characteristics of long time delay, high satellite′s velocity and the weak signal.
深空探测具有时延长、运动速度大以及信号强度低的技术难点。
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