AS WE ALL KNOW, GPS SIGNALS ARE WEAK. At a receiver’s antenna, in the open air, their strength is about -160 dBW or 1 Ã— 10-16 watts. Compare this to a cell-phone signal, which might be -60 dBW or 1 Ã— 10-6 watts — 10 billion times stronger! While code correlation in the receiver lifts the GPS signals above the background noise floor, the signals are still relatively fragile, and building walls and other obstructions can significantly attenuate the received signal power so that they cannot be tracked by a conventional receiver.
It is the ratio of the signal power to the noise power per unit bandwidth that determines the trackability of the signal. Accordingly, if the receiver’s noise floor should increase sufficiently, even in an outdoor environment, the signals may also become untrackable. This can happen when the receiver is subjected to intentional or unintentional radio-frequency interference (RFI) by a transmitter operating on or near GPS frequencies. If the interference is strong enough, it can jam the receiver. Although intentional jamming is typically of concern only to military GPS users, unintentional jamming can occur anywhere and anytime and can affect large numbers of users within the range of the jamming transmitter. The jamming incident in San Diego harbor in January 2007, for example, affected all GPS users within a range of about 15 kilometers including a medical services paging network.
Such jamming renders a GPS receiver inoperable. But how do users know that their receivers are being jammed and not suffering some other type of malfunction? Clearly it would be advantageous for users to receive a heads-up when jamming signals are present and, if possible, for the receiver to take corrective action automatically.
In this month’s column, we look at some simple techniques, which can be easily incorporated into the design of a GNSS receiver, to detect, characterize, and actually mitigate RFI. Such receiver enhancements will benefit civilian and military users alike.
There’s an interesting way to detect it too (I love there’s an event called Jamfest!)… If every GPS unit or device with GPS (phones) had a sensor, they could report GPS threats.
Though many details of the JLOC system operation cannot be released, Jim Dalrymple, JLOC lead at Navsys Corp., explains some of its principles of operation. “Modern GPS receivers include the capability to detect GPS interference and can provide reports showing higher than normal signal levels in the GPS band and lower than normal signal/noise ratio. This condition indicates the presence of a GPS threat. The JLOC system allows networked GPS receivers to send reports to the JLOC Master Station of detected interference, acting as JLOC sensors. The JLOC Master Station is designed to collate these reports to provide near real-time situational awareness on GPS threats to military users.” Bruce Bockius, who supports JLOC Master Station operations, indicated that thousands of JLOC sensor reports are now being received daily.
Testing during the GPS Jamfest conducted regularly by the 746th Test Squadron has proven the JLOC system operation under elaborate jamming scenarios. Saffel explained: “By providing the warfighter tools situational awareness on the GPS threats and their predicted effects on military operations, they are able to plan their missions accordingly and also develop tactics to counter the threats where appropriate.”