Most likely, not very due to the nature of GPS it Broadcasts same signal to every device, thus every device should have the same key to decrypt it. Since the data is very short to begin with,and the GPS system design is decades old,my bet is that it was not hard to either crack the key, or retrieve it from some Mil device. Unless they are using the Public-key crypto to authenticate the transmitter. However, its unlikely, because the system was build long time ago and designed for not very sophisticated receivers.
Probably why it didn’t Self Destruct.
Probably why it didn’t Self Destruct.