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引言载人和不载人的许多宇宙任务是利用在地球上空100～1000哩高处沿轨道飞行的低高度飞船。高度这样低时,由于视距限制,飞船不能与任务控制中心保持连续接触。可以想象,当在地面站目标不能直接可见时,未来的系统将采用高空卫星转发器来传递信息。这样虽能确保复盖的连续性,但目标和转发器之间的线路受到地球反射产生的多径干扰。图1 为典型情况,此处沿高度为h的轨道飞行的目标T经转发器卫星S与地面站G通
Introduction Many of the manned and unmanned missions to the universe are low-altitude spacecraft that take orbit at altitudes of 100 to 1,000 miles above Earth. At such low altitudes, the spacecraft can not maintain continuous contact with the mission control center due to line-of-sight restrictions. It is conceivable that future systems will use altitude satellite transponders to transmit information when the target at the ground station is not directly visible. Although this ensures continuity of coverage, the line between the target and the transponder is subject to multipath interference from the Earth’s reflections. Figure 1 shows a typical case where a target T flying along an orbit of altitude h passes through a transponder satellite S and communicates with a ground station G