obtained by weighting the average of the two signals which correspond to the nearest correct landing positions on opposite sides of the erroneous landing position x v y c identifying the two nearest correct landing positions as x 1 y c and x 2 y c having corresponding signals s 1 and s 2 the compensation signal s x v y c for the position x v y c is given by equ1 in the second type of landing error the x coordinate is correct but the y coordinate is erroneous and the method used in the case above may be used again the corrected signal for the erroneous landing point x c y h is obtained by weighting the average of the two signals which correspond to the nearest correct landing positions on opposite sides of x c y h identifying these two nearest correct landing positions as x c y 1 and x c y 2 having corresponding signals s 1 and s 2 the compensation signal s x c y h is given by equ2 the third type of landing error is the most common and is a combination of the two types described above so that both the x and y coordinates are erroneous the compensation signal for the erroneous landing position x v y h is obtained from a weighted average of the two signals which correspond to the nearest correct landing positions on opposite sides of the erroneous position x v y h identifying the two nearest correct landing positions as x 1 y 1 and x 2 y 2 having corresponding signals s 1 and s 2 the compensation signal s x.