The present invention relates to a tracker and a surveying apparatus comprising the tracker, which improve the reliability of tracking a target. The tracker comprises a first imaging region having a plurality of pixels for taking a first image of a scene including the target; a second imaging region having a plurality of pixels for taking a second image of a scene including the target; a control unit to receive a timing signal indicating a time duration during which an illumination illuminating the target in the scene is switched on and off, control the first imaging region to take the first image of the scene when the timing signal indicates that the illumination unit is switched on, and control the second imaging region to take the second image when the illumination is switched off; and a read out unit configured to read out the first image from the first imaging region and the second image from the second imaging region and to obtain a difference image.