GNSS Redundancy Currently, there are only limited alternatives available to replace GNSS systems as an accurate source of frequency, phase, and time at a remote location or for an isolated piece of equipment. See "Chapter 3. Synchronization and Timing Concepts", for much more information on the features and capabilities of the various GNSS systems. This lack of an alternative worries experts who are concerned about the vulnerability of GNSS sys- items. That risk can arise from bad actors denying the signal or even normal events such as disrupt- ive space weather conditions or even human error. There are several organizations lobbying for al- ternative methods of timing distribution beyond the current space-based radio systems, one being the Resilient Navigation and Timing Foundation (https://rntfnd.org/). One obvious way to provide backup would be to build another, much more powerful, terrestrial ra- dio signal for positioning, navigation, and timing (PNT). The U.S. Congress has initiatives in this area, although it does not seem to be moving with any great urgency. For further information, please consult the National Timing Resilience and Security Act of 2018 and the U.S. President's Execut- ive Order 13905, "Strengthening National Resilience Through Responsible Use of Positioning, Navigation, and Timing Services." This section is not meant to be a comprehensive guide to every method available to harden a GNSS- based timing system, but here are some of the options available that give the operator a better chance of surviving an event: Use hardened and resilient receivers, with better anti-jamming and anti-spoofing capabilities. These mechanisms are not foolproof, but they help. Previous historical events have shown that many older models of receiver are quite fragile. Incorporate improved antenna technology, an area that does not receive nearly enough consider- ation. Even basic shielding and a more directional antenna can help mitigate the signal from ground level jamming. For example, there is a class of anten