GNSS Errors Impacting Position, Navigation, and Timing (PNT)

GNSS errors and anomalies can be caused by a range of issues. Because real world signals from the satellites do not travel in a vacuum, but pass through the ionosphere and the troposphere, errors are induced in the signal path even under normal operations. This causes the actual mean speed of the signal when traveling from satellite to receiver to vary and be difficult to measure because of signal path uncertainty.

Additionally, normal effects, such as reflections, can cause the satellite-to-user distance to be inaccurately determined. This can give rise to signals from the same SV arriving at the receiver having followed different paths, and therefore introducing signal disparity, a phenomenon known as multipath propagation.

Errors can also be introduced due to issues with the GNSS system itself. Problems with the clocks onboard the satellite as well as mistakes made in uploading the timing information from ground based control stations can be contributors to GNSS failures.

Additionally, GNSS signals are extremely weak and highly vulnerable to jamming. This type of incident causes partial or complete loss of the GPS signal and is commonly the result of interference from nearby RF sources. Jamming devices (or jammers) have become widely available at a low cost. A common incident is for a passing vehicle, which may be using a jamming device to prevent GNSS tracking, to also interrupt a GNSS receiver being used by Critical Infrastructure. More complex jamming incidents can be orchestrated by adversaries to make it more difficult to detect the source of the jamming, but the result is the same.

In such cases, the GNSS receiver fails to acquire and track the GNSS signal. In some cases, there are more sophisticated attempts to disrupt the GNSS signal to take control of critical assets or to deny service to specific systems. This type of incident, effectively the propagation of illegitimate GPS signals, is referred to as GNSS spoofing (or complex jamming). The GPS receiver is tricked into tracking illegitimate GPS-like signals; it continues to operate, but the solution for position and time given by the receiver will be wrong. This type of incident is almost always intentional and can be difficult to detect.

To summarize, there are many types of error, both intentional and unintentional, that can impact GNSS reception. Ultimately, the goal is to ensure the integrity of PNT coming from GNSS given this wide range of susceptible environments.

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