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. It is a reasonable question given that applying the use of GPS in the public safety arena really is quite different from using a Garmin or TomTom device in your car for navigation or finding a Starbucks.
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 being worn by the defendant depends upon it receiving signals from 4 or more of the 24 satellites revolving around the earth at any time. Years ago, these satellites were placed in a constellation by the US Department of Defense and have highly accurate atomic clocks onboard, as well as, predictable orbits and relationships to each other in the sky. The signals from the satellites are constantly broadcasting their position and the exact time the signal was sent. So the GPS device on the monitored person 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 monitored person’s 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 person’s 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. If the tracking data identifies exceptions (potential non-compliance) have occurred within the established schedule and zone rules, it will alert and notify monitoring staff accordingly. Then staff can use the mapping and other system software tools to investigate if the monitored person’s behavior actually represents non-compliance and/or a public safety risk.
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.
As always, please feel free to contact us if you have any questions about GPS or any other form of electronic monitoring.