 disturbed by any local event. Time is then trans- ported with PTP and SyncE between all the facilities that need a synchronization source but have lost their local GNSS signal. Susceptibility to jamming, either intentional or incidental: This is becoming a real problem, although (good) receivers are aware that jamming and signal denial events are underway and can protect themselves. Having multi-constellation receivers might not help, because these jam- ming devices tend to block all GNSS signals, not just the L1 signal from GPS (for example). The main remedy is to have multiple backup PRTC and T-GM sites with sufficient geographical dispersion SO that at least one GNSS source is outside the range of the jammer. Luckily, most jamming events cover a limited area, but it is possible, particularly with a state actor, to suffer a severe denial of signal over a very wide area. In that case, the only remedy might be to call the air force. Some countries have developed ground-based alternatives (such as eLORAN) to rely on in case of interference with space-based signals (see the section "Resilience" in Chapter 3). Spoofing: A bigger threat, increasing all the time, is the ability to "spoof" the GNSS signals to mislead the receivers into receiving the incorrect time. The receivers might not always be able to detect these attacks, especially from sophisticated actors. Multiband and multi-constellation devices can help mitigate this threat for now because this makes spoofing much harder (al- though an attacker could attempt to spoof one GNSS signal while jamming the other constella- tions and bands). As mentioned previously, there are options such as antenna technologies be- coming available to help filter out false signals-which helps in very high-threat environments. GNSS systems are also moving to authentication mechanisms to ensure the signal is valid. One example is the Open Service Navigation Message Authentication (OSNMA) developed by Ga- lileo. It makes use of the Timed Efficient Stream Loss-tolerant Authenticatio