GPS was relatively immature at the time of the Chinook crash
Spot on Beagle. In fact, the CA Release and RTS stated quite clearly that GPS had not received Initial Operating Clearance from the US DoD, and was not to be relied upon. This, in addition to saying the "Err" (error) indication on RNS252 was "meaningless", but omitting to advise them what to do if it appeared (quite important, given much of the Nav kit had absolutely no clearance whatsoever).
Subsequently, in 1995, there was found to be a howler of a mistake in one of the GPS specs manufacturers worked to. Bottom line was equipment using Time of Day for sync'ing would find it losing sync very quickly.