The United Kingdom's Royal Academy of Engineering issued a report, Global Navigation Space Systems: Reliance and Vulnerabilities this week that led to news reports The Wall Street Journal Tech Europe Blog called "apocalyptic visions of a cyber-hell."
(The WSJ Blog noted Dr. Martyn Thomas, the report's author, dismissed such reporting as hype.)
Much of the 48-page report is devoted to what can go wrong with GNSS/GPS: system-related vulnerabilities (signals and receivers), propagation channel-related issues (atmospheric and multipath) and interference-related scenarios (accidental or intentional).
If you operate facilities that depend on GPS signals, the Report is likely to be at least a bit disturbing.
Annex A of the Report lists current and planned applications using Global Navigation Space Systems (GNSS—a term encompassing not only the U.S. GPS system but other worldwide satellite systems). Not surprisingly the applications requiring the highest accuracy are air and marine transportation systems. Digital Broadcasting and Communications are listed under "Timing Applications," and shown as requiring "medium" accuracy. Use of a stable clock, such as an oven-controlled crystal oscillator or rubidium standard, allows these systems to retain sufficient accuracy during GNSS interruptions. The length of time that the systems can stay in time depends upon the stability of the equipment's time and frequency standard.
Annex B is sobering—it lists GNSS failure modes and characteristics, including examples where available.
Throughout its history, broadcasting has been the one information source people could rely on, as it gets through even if cable TV and telephone lines were down and cell phone sites jammed or destroyed.
Some time ago, I cautioned broadcasters to make sure they could stay on the air if Internet service was disrupted. It might be worth checking to see what happens if GPS disappears or becomes unreliable.