Ionospheric tomography offers several challenges. The tomographic reconstructions are dependent on geometries, algorithms, accuracies, noise, bias, and multipath, among other things. Relative roles of different factors are not clearly understood, and the objectivity of tomographic reconstruction is generally uncertain. A series of simulations has been performed to understand and obtain quantitative descriptions of various factors in tomographic reconstruction. Simulations include different geometries, beam divergences, angular coverage, algorithms, and initial guesses, etc. Simulations have been performed under the most general circumstances, and the results can be used for generalized tomographic reconstruction. On the basis of these simulations we provide guidelines for objective reconstruction of the ionosphere using ground-based and satellite-based receivers. These guidelines allow determination of uniqueness of reconstruction under different situations. ¿ 2001 American Geophysical Union