Abstract
We know some problems of position fix cannot be solved. What is the defect in necessary conditions for them? The author studied the theme by analytical geometry. All observations are expressed in equations, loci of which are called position lines. Number of unknown variables is the sum of the number of unknown factors and two (due to lat and long of ship's position). Number of the equations must be equal to the number of unknown variables. Then, it is necessary and sufficient for position fix that the Jacobian (function determinant) of the equations is not zero. Geometrically, the condition means that position lines must have a cross and that the coordinate system must be defined without ambiguity. Some examples are discussed in detail.
