  Starting from the description of space-time as a curved four-dimensional
manifold, null Gaussian coordinates systems as appropriate for relativistic
positioning will be discussed. Different approaches and strategies will be
reviewed, implementing the null coordinates with both continuous and pulsating
electromagnetic signals. In particular, methods based on purely local
measurements of proper time intervals between pulses will be expounded and the
various possible sources of uncertainty will be analyzed. As sources of pulses
both artificial and natural emitters will be considered. The latter will
concentrate on either radio- or X ray-emitting pulsars, discussing advantages
and drawbacks. As for artificial emitters, various solutions will be presented,
from satellites orbiting the Earth to broadcasting devices carried both by
spacecrafts and celestial bodies of the solar system. In general the accuracy
of the positioning is expected to be limited, besides the instabilities and
drift of the sources, by the precision of the local clock, but in any case in
long journeys systematic cumulated errors will tend to become dominant. The
problem can be kept under control properly using a high level of redundancy in
the procedure for the calculation of the coordinates of the receiver and by
mixing a number of different and complementary strategies. Finally various
possibilities for doing fundamental physics experiments by means of space-time
topography techniques will shortly be presented and discussed.
