  Determining range, Doppler and astrometric observables is of crucial interest
for modelling and analyzing space observations. We recall how these observables
can be computed when the travel time of a light ray is known as a function of
the positions of the emitter and the receiver for a given instant of reception
(or emission). For a long time, such a function--called a reception (or
emission) time transfer function--has been almost exclusively calculated by
integrating the null geodesic equations describing the light rays. However,
other methods avoiding such an integration have been considerably developped in
the last twelve years. We give a survey of the analytical results obtained with
these new methods up to the third order in the gravitational constant $G$ for a
mass monopole. We briefly discuss the case of quasi-conjunctions, where
higher-order enhanced terms must be taken into account for correctly
calculating the effects. We summarize the results obtained at the first order
in $G$ when the multipole structure and the motion of an axisymmetric body is
taken into account. We present some applications to on-going or future missions
like Gaia and Juno. We give a short review of the recent works devoted to the
numerical estimates of the time transfer functions and their derivatives.
