Almost every part of our modern economy depends on the Global Positioning System, or GPS. For example, agriculture, construction, mining, rail transportation, and search and rescue all rely on the accurate position, navigation, and timing (PNT) enabled by GPS. An even broader set of industries – communications networks, banking transactions, financial markets, and power grids – rely on GPS for precise time synchronization, to such an extent that most systems would cease working without it. Alternative navigation (ALTNAV) systems can supplement GPS systems in GPS-denied environments using internal clocks and onboard sensors, and those ALTNAV systems should be protected from cyberattacks as well.