Thank you Pegasus 77 and HJ, that is what I suspected. If illegal, then probably dirty signals, high power, and likely harmonics in the GPS part of the spectrum. . .. .On every departure and arrival, we lose GPS within 20 miles of NAP. It is frustrating because we always fly B-RNAV and end up dead reckoning for a few minutes until the GPS comes back on line.. .. .We have not encountered this problem at other Italian airfields, just Naples. It goes with the territory here.