translation.en: So we had to know the velocity of Rosetta much better than one centimeter per second, and its location in space better than 100 meters at 500 million kilometers from Earth.
translation.zh: 因此我们得将罗塞塔的速度误差 精确到小于1厘米每秒， 而它在太空中远离地球5亿公里的位置， 误差要精确到100米以内。