Knowing how jammers work, it's also helpful to know how the Global Positioning System (GPS) works

The GPS tracker receives microwave signals from an array of satellite transmitters orbiting the Earth. Once the tracker receives signals from four or more satellites, it will determine its position through a series of time calculations and trilateral measurements.
The receiver relies on these precise and specific satellite signals to determine its position in the world. The GPS tracking device then transmits this location and speed information to the monitored location, usually over a cellular network.