  At present, tracking data for planetary missions largely consists of radio
observables: range-rate range and angular position. Future planetary missions
may use Interplanetary Laser Ranging (ILR) as a tracking observable. Two-way
ILR will provide range data that are about 2 orders of magnitude more accurate
than radio-based range data. ILR does not produce Doppler data, however. In
this article, we compare the relative strength of radio Doppler and laser range
data for the retrieval of parameters of interest in planetary missions, to
clarify and quantify the science case of ILR, with a focus on geodetic
observables.
  We first provide an overview of the near-term attainable quality of ILR, in
terms of both the realization of the observable and the models used to process
the measurements. Subsequently, we analyze the sensitivity of radio-Doppler and
laser-range measurements in representative mission scenarios. We use both an
analytical approximation and numerical analyses of the relative sensitivity of
ILR and radio Doppler observables for more general cases.
  We show that mm-precise range normal points are feasible for ILR, but
mm-level accuracy and stability is unlikely to be attained, due to a
combination of instrumental and model errors. ILR has the potential for
superior performance in observing signatures in the data with a characteristic
period of greater than 0.33-1.65 hours This indicates that Doppler tracking
will typically remain the method of choice for gravity field determination and
spacecraft orbit determination in planetary missions. Laser ranging data,
however, are shown to have a significant advantage for the retrieval of
rotational and tidal characteristics from landers. Similarly, laser ranging
data will be superior for the construction of planetary ephemerides and the
improvement of solar system tests of gravitation.
