GPS Satellite Question

[Skvez]

Each GPS satellite transmits two frequencies the ‘civilian’ and ‘military’ frequencies.

The US military have a number of options to degrade or block civilian receivers.

The simplest is the poorly named “selective availability” which just adds random noise to the lowest significant digits of the time signal making the calculated location less accurate. The US military turned off selective availability during desert storm I (as they couldn’t get enough military receivers and many of their soldiers were using civilian units) and haven’t turned it back on again. However it’s there and could be turned on.

The military receivers (at least the early ones) use both the civilian and military frequencies and use the difference in the time signals to infer information about density of the air in the signal path and thus the time distortion. This allows improved accuracy over using just the civilian frequencies but the military receivers need *both* frequencies to work. Turning off the civilian frequency would also stop military receivers. (I can’t think of any reason why a military receiver couldn’t be designed to work on just the military frequency with similar accuracy to a civilian receiver, however this is not how they were intended and not how the early ones work).

Over a local area a transmitter can broadcast noise on the civilian frequency and essentially block any receivers from getting a lock. This is how GPS jammers work. Jamming GPS
Jamming GPS over a local area (perhaps up to 100 miles without too unreasonably large a transmitter) is potentially available to anyone who knows the frequencies to jam, this option is not restricted to the US military.

The satellites get their clocks updated multiple times per day from ground stations (with more accurate much bulkier clocks). In a major event (global pandemic for instance) it’s probable that the ground stations would stop being manned and these clock updates could stop happening. It’s unclear how autonomous the ground stations are or how quickly the accuracy would degrade once the satellites stop getting clock corrections from the ground. Like most clocks some will probably drift more than others so accuracy may depend on chance depending on which satellites are overhead.

Typically my GPS receiver is rarely out more than 10 meters and newer ones (with high gain antenna) are even better.

GPS is a very useful tool but like all tools its good to have a backup and if it tells you you’re in China when you’re pretty sure you’re in Wales then I’d consider it suspect.