The Electronic Travel Authorization (eTA) is mandatory for visa-exempt citizens and foreign residents traveling to Canada by air. An eTA is a document electronically linked to a traveler’s passport. It is valid for a period of five years or until the expiry of your passport. However, if you change your passport, you must also have a new eTA.
With a valid eTA, you can travel to Canada anytime you want but for short stays (normally up to six months). It is not necessary to have an eTA to travel within Canadian territory.
An eTA does not guarantee your entry into Canada. Once you arrive, you will find a border services officer who will check your passport and other documents (US green card). To access Canada’s territories, you must convince the agent that you are legitimate and eligible.