As a practical matter, jamming of one GNSS would most likely affect the others as well. Particularly since multi-GNSS receivers usually only support the GNSS th

As a practical matter, jamming of one GNSS would most likely affect the others as well. Particularly since multi-GNSS receivers usually only support the GNSS that share bands (GPS, Galileo, GLONASS) Perhaps the most compelling option for multi-system redundancy has been the reintroduction of a complete terrestrial navigation system. There have been perennial proposals to fund the construction of an "enhanced LORAN" system to cover the United States that have gotten as far as early design, basically ever since the decommissioning of LORAN-C, but it's never quite happened. Somewhat contrary to what a lot of people might expect, the technology does exist to build a completely earth-based radiolocation system with substantial (but not complete) land coverage, it's just a pretty expensive infrastructure project and has a hard time getting any traction at a time when the FAA is slashing the budget for existing navaid technology. The infrastructure that was already "half way there," e.g. FAA VOR/DME amd the Coast Guard DGPS infrastructure built on the back of the cancelled GWEN project, is largely all decommissioned or in a slow process of decommissioning. ASAT warfare has lead to some renewed interest, but not enough to actually get the money on the table.