The Definition of J/S Jamming in GPS

By Darla Ferrara

GPS units use satellites to determine a position.
i Jupiterimages/Creatas/Getty Images

A Global Positioning System has many of the same characteristics as a standard radio, which acts as a receiver for signals sent by an external source. Jamming is possible with a GPS just like it was with the old-style radios in World War II. Once you understand the basics of these systems, you will be able to define terms like J/S jamming.

Global Positioning Systems

GPS is a satellite navigation system. The receiver gets signals from satellites and uses them to determine its latitude and longitude at any given moment. The program works with multiple U.S. satellites that generate the signals a receiver interprets. The receivers are passive. All they do is receive the signals, they do not send signals. In a sense, this is similar to your car radio. A broadcaster produces a signal that the stereo receives and translates into the music. By monitoring signals sent by designated satellites, a GPS receiver can triangulate its position.


When you jam a signal, you produce an obstruction between the sender and the recipient. Jamming in GPS means masking the satellite transmissions before the navigational unit can locate them. The process is not that complex. The system that monitors the airwaves for signals fails to receive any even though broadcasting continues. They are still there, the unit just cannot find them. This prevents it from being able to determine a location.

Electronic Countermeasure

Electronic countermeasure is the process of masking a signal going to a device such as a GPS receiver. A jammer works by issuing a signal at the same frequency as the satellites. This confuses the receiver until it cannot separate the satellite transmissions from the masking signal. The purpose of a jammer is to hide the location of the GPS receiver. If you do not want someone to track your cellphone or a positioning device in your car, you could turn on a jammer. This way the receiver cannot determine a location.

J/S Ratio

J/S stands for jamming to signal ratio. Jamming involves two signals, the one the receiver picks up and the one coming from the jamming device as a mask. The "J" is the signal coming from the jamming device. The "S" is the signal the receiver monitors. The J/S ratio is a measure of how strong the jamming signal is compared to the one being produced by a satellite. If the jamming signal is not strong enough, it will not be an effective mask. If you purchase a commercial jammer, the ratio is probably not an issue as it will be preset.