A tracking system includes a global positioning system (GPS) module and a modem for mobile communications both attached to a pet (or other trackee), and a virtual fence (which includes a base station sending a signal to a certain range and a receiver attached to the pet (or other trackee) and receiving the signal sent by a base station when the receiver is within the range of the base station). A portable virtual fence system includes a signal-sending base station, and a signal-receiver worn by a to-be-fenced pet or other trackee. Advantageously, the base station is portable.