The phrases “GPS denied” and “GNSS denied” seem to have gone viral. They have even made their way into official requirements documents. But without qualification and explanation, it is little better than the enigmatic requirement for Position, Navigation and Timing (PNT) of a significant military program that once stated that “There shall be GPS”. This article presents answers to the question “Why would GPS be denied?” as well as examines strategies to mitigate against denial of GPS to ensure users can maintain an actionable PNT.
The primary causes of GPS denial include interference, spoofing, signal blockage and GPS constellation failure.
The GPS and other GNSS signals transmitted via satellite are extremely low power, hidden below the thermal noise floor by the time they reach the surface of the Earth. This low power characteristic makes the GNSS signals susceptible to disruption by even very low power interference. For example, a simple 1 Watt interferer in the GNSS bands can totally deny reception of the signal for many kilometres.
GPS denial via interference can be separated into the two broad categories: unintentional interference and intentional interference (jamming). There have been many documented cases of unintentional interference over the years, ranging from faulty TV receivers to other non-GNSS transmitting sources leaking into the GNSS frequency bands. Jamming however is a more immediate and dangerous concern. People may try to jam GNSS signals in a local area for a number of reasons, including privacy concerns over tracking of their location by others. Although illegal in most jurisdictions, very low power jammers known as “personal privacy devices” can be bought on the Internet and used for this purpose. Recent high profile examples include a personal privacy device in a truck causing intermittent disruptions of GPS for landing of aircraft at Newark Airport, ultimately resulting in a fine of $31,875 for the truck driver. Adversaries may choose to jam GNSS for tactical purposes, and may deploy many small low power jammers or even larger power jammers to disrupt GNSS over wide areas, as in the jamming alleged to come from North Korea .
There are a number of protection techniques, with the level of protection increasing as the number of techniques used increases.