Tracker, surveying apparatus and method for tracking a target

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 an image sensor arrangement having an imaging region composed of a plurality of pixels arranged in a matrix of columns and rows. The imaging region is arranged to take an image of a scene including the target. A controller receives or generates a timing signal indicating a time duration during which an illumination unit is switched on and off, controls the imaging region to take an image of the scene when the illumination unit is switched on, and reads out a subgroup of neighboring columns or rows constituting a stripe window of the imaging region so that an image section including the target is obtained, controls the imaging region to take another image of the scene when the illumination unit is switched off, and reads out another subgroup of neighboring columns or rows constituting another stripe window of the imaging region so that another image section including the target is obtained, and calculates a difference image section by determining a difference between the pixel values of the pixels of the image section and the other image section.