Page "LORAN" Paragraph 13

In the case of LORAN, one station remains constant in each application of the principle, the primary, being paired up separately with two other secondary stations.

Given two secondary stations, the time difference ( TD ) between the primary and first secondary identifies one curve, and the time difference between the primary and second secondary identifies another curve, the intersections of which will determine a geographic point in relation to the position of the three stations.

These curves are referred to as TD lines.