A U.S. navy drone wandered into Canadian-controlled airspace this week off the coast of Nova Scotia.
It was over the ocean when it crossed into the Canadian zone about 230 kilometres southwest of Yarmouth.
The U.S. navy's Global Hawk drones have a wingspan of nearly 40 metres, wider than a Boeing 737.
The drone was on its way to Maryland on Tuesday morning after a six-month tour in the Middle East. Bad weather forced it to divert to an airfield in Maine.
"Normally we would receive the flight plan that would indicate its route of flight through Canadian airspace in advance," said Michelle Bishop of Nav Canada, the corporation that operates Canada's civil air navigation system.
She said the aircraft would then be handed off from U.S. air traffic controllers to Canadian controllers before it entered the airspace.
Bishop said the drone came less than three kilometres into the Canadian zone.
She said there was no danger, since the drones, which fly without pilots, cruise at altitudes above 15,240 metres.
Nav Canada has referred the incident to the Royal Canadian Air Force to improve co-ordination and ensure the mistake is not repeated.