Nothing is completely foolproof, but LORAN is much, much more resistant to jamming than GPS. GPS satellites orbit over 20,000 km from Earth and transmit at very limited power (less than 50 W). That makes it easy to overpower their faint signals. On top of that, GPS operates at a frequency over 1 GHz which means you can broadcast a jamming signal with a small antenna. LORAN is the opposite. It uses low frequency, extremely high power signals transmitted from stations relatively closer to the receiver. Those design choices make it less precise but vastly more robust.