Often, GNSS users ask the question “Can your GNSS receiver operate effectively in GNSS-denied conditions?”
Apart from the obvious response, “No”, the deeper answers to this question are highly dependent on the scenario and what’s causing the interference. The many categories of interference range from signal blocking by structures (e.g., tall buildings or a tunnel) to unintentional interference from a nearby leaky antenna, to an intentional jamming attack.
In each case, mitigation can be very different and highly dependent on your objectives—there is no “one size fits all” measure to alleviate interference.
For continued performance during periods of signal outage, one can use a heterogeneous sensor. A common approach is to add an inertial measurement unit (IMU), which can also help in transient jamming scenarios. For mission-critical applications or longer periods of jamming, one might also consider employing an anti-jam antenna. If the anticipated problem is with spoofing, then a multi-frequency/constellation GNSS receiver and IMU integration will certainly be useful and an authorized user can of course use receivers that can operate with encrypted signals (currently GPS P (Y) Code, and GPS M-Code and Galileo PRS to follow).
Bottom line, it takes three steps to effectively mitigate GNSS interference: detection (establish that interference is really present), So In characterization (define the interference type) and localization (find it).