VOA慢速英语:SpaceX赢得制造美国导弹跟踪卫星的合同

track – v. record the progress or development of something over time

sensor – n. a device that detects or senses heat, light, sound, motion, etc., and then reacts to it in a particular way

detect – v. discover or notice something

encrypt – v. to change (information) from one form to another especially to hide its meaning