The objective of this paper is to estimate the refractive index (RI) field at the Earth's surface from radar echos collected by multiple radars from ground targets. The technique leverages the work of Fabry et al. (1997, 2002), which exploits the fact that the phase of the radar return from a target is based on the path-integrated refractive index of the atmosphere between the radar and the target. The contributions of this paper are built on a linear mathematical model for the set of radar returns. First, rather than employing the pairwise differencing technique of Fabry, the model is employed to derive a modified least-squares technique for RI field estimation that can be employed with either a single or multiple radars. Next, the model is extended to derive an effective solution to the problem of phase wrapping in the radar returns by jointly estimating the phase wrapping and the RI field. Numerical results indicate significant performance improvements through the proposed techniques.