题目内容：
根据以下材料回答第 1～4 题：A new kind of radar has been developed for spaceship travelers．A working laboratory model of a new system of radar that makes use of a beam of light is said to be ten thousand times more accurate than the best comparable system of radar that used microwaves．
The model has shown that this radar system，known as laser－doppler radar，can measure with absolute precision speeds varying from spaceship orbital injection(进入)velocities(速度)of five miles per second down to virtual stops…speeds of less than one ten thousandth of an inch per second．According to the scientists who are developing this system，such fine measures of cdoctiy are of prime importance in space missions．In a rendezvous(对接)between two spaceships，or in a landing approach by a vehicle onto an orbiting space station，a bump could rip open a ship’s skin，or a nudge could knock the station
out of its orbit．
The light-beam radar，which operates at a frequency of trillions(百万兆)of cycles
per second，could easily detect and measure the movement of a vehicle edging up to a satellite space s tation．A control sy stem using so precise a signal as this would allow a huge vehicle to dock a t a space station as lightly as a feather．
Laster-doppler rada ．
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