单项选择题
A．measures the movements of a spaceship by means of light beam B．makes use of microwaves C．makes use of sound waves D．Both A and B
A．accuracy ill speed measurement is essential to spaceships B．the speeds of space vehicles will vary C．inaccuracies in measurement could cause accidents in space missions D．a bump could affect the preciseness of the laser-doppler radar
A．Rendezvous in Space B．Landing at a Space Station C．New Radar for the Space Age D．Space Navigation