[mmwelt]

The GPS jamming map linked to in the article[1] discusses this somewhat, in the "About the data" box:

    - ADS-B messages include position information from Global Navigation Satellite Systems (GNSS), like GPS, Galileo, GLONASS, BeiDou, etc.
    - It is not possible to directly measure GNSS interference, but we do calculate the NIC (Navigation integrity category) for ADS-B messages.
    - The NIC value encodes the quality and consistency of navigational data received by the aircraft.
    - Poor NIC values alone might indicate a problem with an aircraft’s equipment or unfavorable positioning. However, when observed in multiple aircraft in close proximity during the same time frame, it suggests the presence of a radio signal interfering with normal GNSS operation.

[1] https://www.flightradar24.com/data/gps-jamming