[WizWom]

The combat radios in use now were designed by ITT in the 90s; my dad was part of the project.

They use a common frequency pattern for typical communications; their encryption keys are provided on a one-time pad basis, but replacing that pad should units be lost in the field would be prohibitive.

What does this mean? It means that should a radio be compromised, the channel or channels that radio was programmed for would need to be sequestered until the OTP chips could be distributed. So, if a unit commander's set is stolen, then the unit has no secure comm. There are some anti-tamper devices, but, of course, they cannot be too aggresive, or the troops technicians would not be able to do the needed OTP replacement as needed.

As for the frequency-hopping being tracable by direction finding, it's designed to be random noise digital signalling. so to direction find it, you'd need to know the FSK OTP already to differentiate it from the background.

Note that that version has been superceeded in the US military by a cellular type system, since it offers more channels and is less able to be jammed.

[Southack]

"As for the frequency-hopping being tracable by direction finding, it's designed to be random noise digital signalling. so to direction find it, you'd need to know the FSK OTP already to differentiate it from the background."

Nope.

Radio communication contents can be digital if desired, but the physical electromagnetic radio transmission wave itself is always analog.

Look up at the stars tonight. Some of those stars transmit radio waves (which are analog by definition) at seemingly random frequencies and times.

Yet a typical college astronomy student can sweep a spectrum analyzer over any such star and quickly see on an oscilloscope the "random" frequencies on which the star is emitting radio waves.

In fact, the student can sweep the spectrum analyzer over black sky to "discover" stars that aren't even seen.

Now, does it matter if there is data digitally encrypted/encoded on any of those radio frequencies/waves? Not for the purpose of discovering the direction of said transmitter.

...

Pause

...and likewise, such digital encryption matters not to a DF team...said team being interested only in the fact that your antenna is emitting energy.

Energy can be tracked. That's what DF'ing is all about.

And if you are moving, then the directional antenna that is tracking you won't even care about background noise. Won't matter.

Likewise, changing frequencies will change where on the spectrum your energy is detected, but it won't change the fact that the energy you are emitting is being detected.

And if two antenna's detect your radio transmission frequency, then it becomes a simple matter to determine your location.