One or more of the satelites (sic) was out of position for a while.
Pretty improbable. How is a big satellite like that (they weigh a ton or so, don't they?) going to get "out of position?". That would require a deliberate and prodigious expenditure of propellant and then same again to replace it in correct position. It can't get out of position unless something gives it a big shove...
Suggest you don't fly under power line in the vicinity of Ferrybridge without a decent dr plot on a topo chart.
Who knows what errors can be created by the magnetic fields under massed power lines in a sensitive unit like a GPS?