  The basic elements of Coll positioning systems (n clocks broadcasting
electromagnetic signals in a n-dimensional space-time) are presented in the
two-dimensional case. This simplified approach allows us to explain and to
analyze the properties and interest of these relativistic positioning systems.
The positioning system defined in flat metric by two geodesic clocks is
analyzed. The interest of the Coll systems in gravimetry is pointed out.

General Relativity and Quantum Cosmology