Public Safety Services Overreliance on GPS
To date, most actual incidents involving GPS jamming, whether intentional or unintentional, involve in-band or out-of-band harmonic RF transmissions that mask weak GPS spread-spectrum signals.
In recent years, great concern has been expressed about cheap GPS jammers, which have power levels as low as 1 watt and could cause widespread interference to GPS services. Despite these unforeseen circumstances, GNSS outages can and have occurred and result in localized or even widespread outages of radio networks.
Below are a few examples of short-term problems that public safety services have encountered that, thankfully, have not adversely affected public safety.
In one case, the wrong satellite code uploaded to the GPS constellation caused a time standard error. GNSS receivers in many LMR systems see this as a valid failure or error in the timing criteria, and if they are pre-set in a specific way, they will shut themselves down as a precaution.
The problem in this situation for many systems is that all receivers in the system are programmed in the same way, so all GNSS receivers in the system treat the time as a local anomaly and go offline by themselves - assuming the same Other GNSS receivers in the system will take over. The operational impact of this situation, and the short-term panic that followed, is very disturbing to say the least.
To avoid this from happening in the future, changes have been made to GNSS receiver programming, and they have been configured to ignore short-term timing anomalies and marshal for as long as possible, relying on their internal rubidium standard to provide the required synchronization. This is of course sufficient and works in the short term, but if it persists, the system will eventually fall back to a soft failure.
In another case, there was a local outage in the availability of GNSS receivers due to local in-band radio interference. Fortunately, this is a short period of time, and during a holiday like this, system traffic is low and potential disruption and impact are negligible.
The concern with this particular incident is that it occurred in close proximity to a strategic node in the system that is critical to the operation of the system and affected a large number of GNSS receivers, including those that provide time for internal IT networks serving public safety. If it persists and if the source cannot be found, there is a contingency plan to "fail over" to a redundant node - located some distance away, that will be able to provide the required time.
The cause of the disturbance was never discovered, it suddenly disappeared after 6 hours and has not been seen since. It could be as sinister as a known "GPS jammer" or as simple as an unlicensed rogue wireless microphone or cordless phone.