The only known BACK-UP of the GPS when it malfunctions is your ability to use a military protractor and military compass. The RM Military Protractor is the only protractor with a printed formula and a diagram to align map grid north to match the compass magnetic north. Protractors without the conversion formula will provide a reading that is off the exact grid coordinate position. The GPS will not penetrate sandstorms, rainstorms, snowstorms, smoke, structures, forest canopies, and thick cloud cover. GPS readings are distorted during thunderstorms, electronic interferences and when bombing support is very close on enemy positions. It can be jammed and detected when near or behind enemy lines.