For what it's worth, at least in the US there is still alot of navigation that is doable without GPS, and in fact is supposed to be done without GPS. For examp

For what it's worth, at least in the US there is still alot of navigation that is doable without GPS, and in fact is supposed to be done without GPS. For example, have a look at this high-altitude chart, centered on the Falmouth VOR in Kentucky, south-east of Cincinnati: https://skyvector.com/?ll=38.641546637139996,-84.30027471561179&chart=450&zoom=3 https://skyvector.com/?ll=38.641546637139996,-84.30027471561... The triangles are airspace fixes, which are defined one of two ways: 1. The point where two radials (one each from a different VOR) intersect. For example, HICKI (http://www.airnav.com/airspace/fix/HICKI http://www.airnav.com/airspace/fix/HICKI). This requires both VORs be operational, and that you be able to receive both VORs at the same time. If one of the VORs has DME equipment, then the fix also falls into the second type, below. 2. A point on a VOR radial, some DME distance away. For example, PETES (http://www.airnav.com/airspace/fix/PETES http://www.airnav.com/airspace/fix/PETES). This requires a VOR with DME equipment, and an appropriate receiver. The black lines (such as J24) are routes that can be navigated using just VORs. No satellite-based navigation is needed, although fixes do have GPS coordinates published. J routes are used in the US at and above flight level 180 (FL180), which is 18,000 feet and a hard-coded barometric pressure of 29.92". Below that are the V routes, which serve the same purpose, just at lower altitudes. By contrast, have a look at the blue stars on the chart. For example, OCASE (http://www.airnav.com/airspace/fix/OCASE http://www.airnav.com/airspace/fix/OCASE). That is a fix that is _only_ defined by GPS coordinates, with Q routes connecting them (although you will often see a Q-route start or end at a fix or VOR). Q routes are used for high-altitude navigation in the US, with T routes used for lower. My understanding is, for VOR-based fixes, J routes, and V routes, the VOR receiver is supposed to be the source of truth, with GPS acting as a backup. If your flight plan has you going VOR—for example, Charleston to Indianapolis via route HVQ J24 VHP—then your FMS should not react any differently if GPS acts weird. Show an alert, sure, but not cause the plane to freak out. (Of course, that doesn't help for surface uses, like vehicle navigation…)