Sure, we want to back up every GNSS-based system with a $100K Cesium clock with 1x10-14 stability, a $100K 12-element CRPA antenna with a SAASM or M-Code receiver and a high-end INS with 0.01deg accuracy. Sadly, the reality is we almost always can’t afford it, either in terms of cost or in terms of SWaP constraints (Size, Weight and Power). Often, the economics are simply not justified.

GNSS signals are very weak and easily disrupted from potentially miles away by low cost jammers that are widely available on the internet for $50. Individuals and other entities interfere with GNSS signals by jamming them in support of criminal activities or simply to escape tracking of their daily movements.