Navigational sextant with an illuminator in a tropical-resistant design of the SSS is designed to measure the heights of celestial bodies in the sea or on the coast and the angles between visible ground objects in order to determine the coordinates of the observer. It can be operated on ships of an unlimited navigation area. The principle of measuring the angle with the sextant between straight lines connecting two distant visible objects with the location of their observer is based on the simultaneous vision of these objects. In the presence of a sextant of 1988 in a complete set!