The XNAV concept has been analysed in detail. The performances obtained from simulations have been validated successfully using real data from XMM-Newton space telescope.
The performances obtained using real data are not in line with the XNAV performances (100 m for the Crab pulsar) due to limitations in the XMM-Newton instrument operations. We have been forced to use a source much fainter than the Crab pulsar and surrounded by a background much stronger than expected.