In this paper, we discuss the use of the Electromagnetic Time Reversal (EMTR) method to locate lightning discharges. The main problem of EMTR is that losses are not invariant under time reversal. In this paper, we propose 3 different models of back-propagation to fix this problem. Simulations are made to evaluate the accuracy of the proposed methods. It is shown that by assuming a lossless back-propagation model, the resulting location errors are in the order of some hundreds of meters. It is also shown that a theoretically exact estimate can be obtained considering a back-propagation over a fictitious `inverted-loss' ground.