  A general approach to the problem of positioning by means of pulsars or other
pulsating sources located at infinity is described. The counting of the pulses
for a set of different sources whose positions in the sky and periods are
assumed to be known, is used to provide null emission, or light, coordinates
for the receiver. The measurement of the proper time intervals between
successive arrivals of the signals from the various sources is used to give the
final localization of the receiver, within an accuracy controlled by the
precision of the onboard clock. The deviation from the flat case is discussed,
separately considering the different possible causes: local gravitational
potential, finiteness of the distance of the source, proper motion of the
source, period decay, proper acceleration due to non-gravitational forces.
Calculations turn out to be simple and the result is highly positive. The
method can also be applied to a constellation of satellites orbiting the Earth.
