I'm a cryptographer, so I'm inherently curious about secret communication. It just so happens that I'm also very curious about radios, satellite communications, etc.
Do you have a link that provides more information on these "below-noise-floor gold codes"? I tried googling, but I haven't found much technical information that I trust is entirely truthful or relevant.
The principle is that if your ally has a receive antenna with a gain of X, and you use a modulation scheme with a coding gain of Y, and you transmit strong enough that your signal can be received by your ally with a snr margin of Z, then as long as your enemy doesn't have a receive antenna with gain > X+Y-Z, they can't detect your signal from the noise.
Since Y can be increased arbitaraly (at the cost of channel data capacity), it's always possible to transmit some information to your ally without your enemy seeing a transmission, it might just be a few bits per second, but theres a lot you can do with a few bits per second in warfare.
One needs to make sure that the above holds true for all possible frequency windows and time windows, so no carrier waves or high power sync pulses!
I believe the implementation is much harder than the theory - just one crystal oscillator or PLL anywhere in your radio hardware will leak some of that frequency into the output, and since the bandwidth of a crystal oscillator is super low (obviously an ideal oscillator has zero bandwidth), even a tiny leak is detectable by an enemy.
Do you have a link that provides more information on these "below-noise-floor gold codes"? I tried googling, but I haven't found much technical information that I trust is entirely truthful or relevant.