Yeah that ia definitely one no to stomp on. Vls an tacan are good ones too but they are immediately apparent if you stare at the band. Gps is so quiet you would only know by looking it up. Which you should absolutely do before TXing.
Is the system different for Civilians? It seems like a system that is so fragile would be a pretty big vulnerability for something that is in such high use by the Military.
The military does not have a different system, though - at least in the past, not sure how things are now - they had access to different modes of precision, and it is possible to augment GPS by using ground based reference beacons if you want.
As for the signal levels at the antenna, that is just a function of distance of the transmitters and the crappy antenna on the receivers. The fragility is to some extent overcome by using multiple satellites (more than you need a fix for), but of course these can still be overcome by a jammer.
If you really want a stronger signal you have a couple of options, the first one would be a better antenna, the second to chill the receiver pre-amp.
The best analogy is to imagine someone who is 100 yards away from you shouting at the top of their lungs and then someone who is whispering up close straight in your ear. That's just the physics side of it and no amount of trickery is going to change that in a way that will make the system more robust against jammers. Any radio based system can be jammed like that.
The military has a higher bitrate (and encrypted) signal, which leads to higher positional accuracy if you have the code. It's no less vulnerable to brute force jamming though, which is why military platforms typically solve this by having GPS receivers with high receive selectivity upwards and very low selectivity in the horizontal plane (where hostile platforms carrying jammers are likely to be).
Source: studied weapons engineering in Naval College.
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