PURPOSE: To obtain the stable time matched for the apperance of the video being the actual position assignment by taking the difference between the current time and that time and performing wake tracking from the time of that difference and the wake data indicated manually.

CONSTITUTION: The position assigned by a PPI position indicator 1 is converted to a digital XY signal by an XY signal generating circuit 2. This signal is then outputted to a data synchronizing register 9 and a bearing calculation circuit 7. On the other hand, the output of a clock oscillator 3 is counted with a time counter circuit 4, by which the time is made. This time is outputted to a bearing corresponding time memory circuit 8 and is stored in the bearing corresponding signal time memory circuit 8 at the memory address corresponding to the bearing data of an antenna bearing signal generating circuit 6 in synchronization to the rotation of the antenna. Hnece, the time when the antenna is passed with the position on the PPI assigned by the apparatus 1 as final is obtained as the output of the circuit 8 and is synchronously sampled together with the assigned XY coordinates and the current time in a register 9. When a wake tracking assignment unit 6 is operated, the data are inputted to a tracking calculation circuit 10 and tracking calculation is performed by using the antenna simultaneous time not being the current time.