Is it legal to use a personal GPS tracker on another person without their knowledge?

Depending upon your relationship with the person you wish to track, this action can be legal. If you wish to track a spouse or child and you track them by means of a car in which your name is also on the title - this is completely legal. However, if you wish to track someone who is not related to you, it appears that in most cases it will be illegal. (www.time.com)