Method to calculate a space target orbital elements using its position and velocity parameters acquired by spaceborne radar is discussed in this paper. The initial orbit determination is realized consequently. The impact of the measurement errors in the radar coordinate system upon the object's orbital elements in the conventional celestial coordinate system is also analyzed. One solution of the encounter problem of two space objects is given. The Space Station's orbit is used in the simulation part. The encounter of the space station and an object within a week is predicted by the simulation. The orbit forecast error is given.