This is a question we seem to get more and more as pretrial programs and law enforcement agencies consider using this technology within the community supervision arena.
First, it’s probably important to distinguish the differences in the actual GPS equipment. The GPS equipment we use in the criminal justice arena includes a more robust design/packaging and carries on-board the necessary cellular communication capabilities, tamper-sensors, schedule/zone regimnens, alert indicators and other security measures. But that is the visible side of the equation and still does not explain how the device really interacts with the GPS satellites. There are many detailed explanations out there for you to find on the web…so we will just offer up a simple summary:
Fixing the position of the GPS device depends upon it receiving signals from 4 or more of the 24 satellites revolving around the earth at any time. The signals from the satellites are constantly broadcasting their position and the exact time the signal was sent. So the GPS device then uses the timing signals it is receiving (and the precise time on earth where it is located) to determine how long it took the satellites’ signals to travel to the GPS device. With that timing information in-hand the GPS device then only needs to determine where the satellites are positioned (in their orbit and in relation to each other) in order to calculate the car exact position through a form of triangulation. Because the GPS device stores on-board all the necessary information regarding the satellites’ positioning at all times, establishing the location is fairly easily achieved.
Naturally, as the monitored cars location is established and constantly updated, the GPS device is constantly registering that movement/tracking data against the schedules and zones that have been established within the monitoring system.
NOTE: It is important to note that there are certain gravitational and ionospheric conditions that can impact the orbits of the satellites and/or the travel-time of signals down to earth. These variations are quickly resolved because the Department of Defense constantly monitors the exact position of the satellites in the constellation and immediately makes any needed adjustments. The ongoing adjustments are communicated to the GPS devices on earth as part of the continuous signal transmissions emanating from the satellites.
