The article proposes a technique for determining the linear diameter of a planet by measuring the angular diameters of the planet. Ballistic characteristics of spacecraft motion and phase angle change are used as additional characteristics. The article reflects the general problem of astronomy, space geoinformatics and geodetic astronomy -the determination of linear characteristics through angular measurements. It is recommended to use the concept and model of the information situation in private observations of space bodies. The article offers two options for determining the planet's diameter by angular measurements. The first option is called normal. It describes the situation of a spacecraft moving along a straight line connecting the observation point with the center of the planet. He describes the situation of displacement of the comic apparatus only vertically. The second option for determining the diameter of the planet is called oblique. He describes the situation of displacement of the comic apparatus vertically and horizontally relative to the center of the planet. To solve the problem in the second case, it is recommended to use a change in the phase angle.